Learning Module 13:
101 Basic Series
Panelboards and Switchboards
Panelboards and Switchboards
What You Will Learn
We’ll step through each of these topics in detail:
Introduction
Definitions
Similarities Between Panelboards and Switchboards
Power Supply Systems
Service Entrance Equipment
Distribution Equipment
Grounding and Ground Fault
Equipment Ratings
4
4
4
5
5
6
7
8
Panelboards
Panelboard vs. Loadcenter
Components of a Panelboard
Circuit Identification
Enclosures
10
10
10
11
12
Review 1
13
Circuit Breakers
Balancing the Load
14
14
Types of Panelboards
Lighting and Appliance Panelboard
Power Distribution Panelboard
14
14
14
Installation and Mounting
NEC Mounting Specifications
16
16
Review 2
17
Switchboards
Components of a Switchboard
18
18
Sections of a Switchboard
Service Section
Pull Section
Distribution Section
IFS Section
Spare, Provision and Blank Space
21
21
21
22
22
23
Helping the Customer
Matching a Panelboard to an Application
Matching a Switchboard to an Application
24
24
25
Review 3
26
Glossary
27
Review 1 Answers
31
Review 2 Answers
31
Review 3 Answers
32
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Panelboards and Switchboards
Welcome
Welcome to Module 13, which is about Panelboards and Switchboards.
In Module 10, we discussed power distribution with a loadcenter. Loadcenters are
used mostly in residential applications. Distributing power in commercial and
industrial facilities is more complex than residential distribution. A typical
industrial distribution system is shown in Figure 1. In additional to distribution
devices such as switchgear, switchboards, transformers, and panelboards, such a
system usually contains metering equipment, main and branch disconnects, protective devices, power switching devices, and conductors.
We will look at switchboards and panelboards in detail in this module. These
expandable devices are used to supply electric service in industrial applications.
Figure 1. Typical Components of a Commercial and/or Industrial Distribution System
Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study
the material carefully, then answer the questions without referring back to what
you’ve just read.
You are the best judge of how well you grasp the material. Review the material as
often as you think necessary. The most important thing is establishing a solid
foundation to build on as you move from topic to topic and module to module.
A Note on Font Styles
Key points are in bold.
Glossary items are italicized and underlined the first time they appear.
Viewing the Glossary
Printed versions have the glossary at the end of the module. You may also browse
the Glossary by clicking on the Glossary bookmark in the left-hand margin.
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Panelboards and Switchboards
Introduction
Both panelboards and switchboards contain the circuit feeder cables and protective devices required for the Branch Circuits. Each, however, do slightly different
jobs.
Definitions
A panelboard provides circuit control and overcurrent protection for light,
heat or other electrical loads. It is used in applications in commercial and industrial facilities.
NEC (National Electrical Code) offers this definition for a panelboard: “A single
panel, or group of panel units designed for assembly in the form of a single panel;
including buses, automatic overcurrent devices, and equipped with or without
switches for the control of light, heat, or power circuits; designed to be placed in a
cabinet or cutout box placed in or against a wall or partition and accessible only
from the front.”
A switchboard divides large blocks of electrical current into smaller blocks
of current used by electrical equipment. Applications can be as small as a
small office building, or as large as a major industrial complex.
NEC offers this definition for a switchboard: “A large single panel, frame, or
assembly of panels on which are mounted, on the face or back, or both, switches
overcurrent and other protective devices, buses and usually instruments.” Switchboards are generally accessible from the rear as well as from the front and are not
intended to be installed in cabinets.
Figure 1 shows the panelboard downstream from the switchboard. Distribution
systems, whether simple or complex, typically include a panelboard. However, a
distribution system does not always need a switchboard.
Similarities Between
Panelboards and
Switchboards
There are several similarities between panelboards and switchboards.
These are:
•
Power supply systems
•
Service entrance equipment / Distribution equipment
•
Main device and main lug only types
•
Grounding
•
Ratings
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Panelboards and Switchboards
Power Supply Systems
Panelboards and switchboards get their power from a variety of sources. Both
pieces of equipment can serve as Service Entrance Equipment, receiving power
directly from the utility transformer. In addition, both can serve solely as distribution points, receiving their power from a panelboard or switchboard upstream.
In any case, power originates at the power company or local generator, and may
be stepped down through transformers for distribution. There are three main
power supply systems in use today for panelboards and switchboards.
These are:
Single-phase, three-wire
system
This system can deliver both 240-volt and 120-volt power. Making a
connection across both hot wires provides 240 volts. Connecting with
either hot and the third wire – neutral – provides 120 volts. This system
is predominantly found in residential applications.
Three-phase, four-wire,
wye-connected
transformer
In a 208Y/120 or 480Y/277 wye-connected system, a connection
across any two of the three hot wires yields 208 or 480 volts,
respectively. Connecting across any hot wire and the neutral provides
120 or 277 volts.
Three-phase, four-wire,
delta-connected
transformer
This system is a bit more complex. Connecting across any two of the
three hot wires yields 240 volts. A connection made from the neutral to
either of the two adjacent hot wires (C or A) provides 120 volts.
Finally, a connection across the neutral to the non-adjacent hot wire
(B) provides 208 volts. This non-adjacent hot wire has to be specially
marked so the electrician does not accidentally connect to it when only
120 volts are desired.
As a result, older delta-connected systems are being replaced by wyeconnected systems.
Figure 2. Power Supply Systems
Service Entrance
Equipment
Sometimes panelboards and switchboards can be used as service entrance
equipment. The Service Entrance is the single point at which electrical service
enters a building. Service entrance equipment enables an operator to control and
cut off the electrical supply to the entire building from one point.
To be classified as service entrance equipment, the panelboard or switchboard
must meet these requirements:
•
Must be approved and labeled “Suitable for use as Service Equipment”
•
Must have a means of disconnection and overcurrent protection
•
Must ground the neutral service conductor
•
Must follow the “six subdivisions of the main” rule:
The service entrance conductors must have a readily accessible means of being
disconnected from the power supply. This is why the NEC has a Six Subdivisions
of the Main Rule. This ruling states that you must be able to throw no more than
six handles into the off position to disconnect electrical service.
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Panelboards and Switchboards
In other words, the service entrance panelboard or switchboard can contain up to
six overcurrent devices without the need for a single Main Disconnect Device. If
more than six branch circuits are required, then a main device must be supplied
upstream to disconnect all the branch circuits at once. For this reason, there are
two ways to configure a panelboard or a switchboard:
•
The Main Breaker or main switch unit has a single main Disconnect Device
that will disconnect power to all equipment being supplied by the service. It
also protects the system from short circuits and overloads (as well as Ground
Faults, if equipped with Ground fault protection).
•
The Main Lug Only (MLO) unit is equipped with up to six devices to disconnect power to all equipment being supplied by the service. It does not have a
single main device. The incoming supply cables are connected directly to the
Bus Bar lugs.
Figure 3. Main Device (on left) Vs. Main Lug Only (on right)
Distribution Equipment
Both switchboards and panelboards can be used as Distribution Equipment. This
is the term given to a panelboard or switchboard used at a point downstream
from the service entrance equipment.
Electrically speaking, service entrance panels and distribution panels or switchboards differ in only two respects:
•
Distribution panels or switchboards may or may not be protected by an
integral main breaker. This means the MLO is utilized for distribution. A
feeder cable from the service entrance equipment supplies the power to the
distribution panels. Therefore, it may be protected by the feeder cable’s Circuit Breaker in the service panel.
•
The service entrance is the only point at which the neutral is connected
to ground. The neutral in any downstream equipment is isolated. The explanation for this is in the next section on grounding.
A switchboard that serves both roles simply has a service entrance section and
one or more distribution sections, as shown below.
Figure 4. Simplified Service Entrance/Distribution Switchboard
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Panelboards and Switchboards
Grounding and Ground
Fault
We have mentioned the concept of grounding a few times throughout this module
but we haven’t really defined it. The National Electrical Code defines ground
as “a conducting connection (intentional or accidental) between an electrical circuit or equipment and the earth, or to some conducting body that
serves in place of the earth.”
Proper grounding of any electrical system is vital, not just for personal safety, but
also for equipment longevity. There are two objectives to the intentional grounding
of electrical equipment. These are to:
•
Reduce the potential for electrical shock by minimizing the potential voltage
differentials between various parts of a system.
•
Minimize the ground path’s impedance. Lower impedance means higher current when a fault occurs. That translates into faster-opening overcurrent protection devices.
Ground fault protection is required per the National Electrical Code® for all service entrance equipment mains of solidly grounded wye systems where the voltage to ground exceeds 150 volts and the overcurrent main is rated 1000 ampere
or greater and where a neutral is provided. This includes all 480Y/277 volt systems.
If the six disconnect rule is used, all overcurrent devices that exceed the requirements above will require ground fault protection. As an example, a 3000 ampere
service entrance switchboard at 480Y/277 volts has six mains. Two mains are
1000 ampere, one main is 800 ampere, and three mains are 200 ampere. The two
1000 ampere mains would require ground fault protection; the other four mains
would not.
As we have already stated, the neutral service conductor on service entrance
equipment must be grounded. The neutral is connected to ground only at the
voltage service. In addition to the service entrance, additional grounding is
required at “separately derived services,” such as distribution transformers. Distribution panelboards and switchboards benefit from that upstream grounding in
case of a short circuit or overcurrent problem. A circuit is grounded only at the service entrance or separately derived services, never at any downstream equipment.
Figure 5. Grounding the Downstream Panel
For example, in Figure 6, the computer has a short circuit. If you trace the thick
line back, you will see how fault current is returned to the source. This is why the
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Panelboards and Switchboards
downstream panelboard contains a branch circuit breaker. It trips, disconnecting
power from the load.
Figure 6. A Short in a Downstream Load
Although properly grounded equipment is of vital importance, ground fault protection is equally important. This type of protection is designed to save lives and protect equipment.
A ground fault can occur when someone is washing down a countertop and
water accidentally reaches an electrical appliance or outlet.
A special circuit breaker called a ground fault circuit interrupter breaker can provide protection against this type of fault. It contains the “normal” thermal magnetic
circuit protection, along with a ground fault sensor.
A ground fault breaker can detect extremely low levels of current leakage to
ground – from four to six milliamps. Standard circuit breakers can’t do that. The
level of four to six milliamps was selected because, above this current level, it
would be difficult for a person to physically let go of a conductor.
When only equipment protection is needed, a level of 30 milliamps is used for
detection.
Equipment Ratings
You must have two key pieces of information about an application before you can
select panelboards, switchboards and overcurrent protection devices. These are:
•
Maximum continuous amps
•
Available fault current
NEC article 110-9 states: “Equipment intended to interrupt current at fault levels
shall have an interrupting rating sufficient for the nominal circuit voltage and the
current that is available at the line terminals of the equipment. Equipment
intended to break current at other than fault levels shall have an interrupting rating
at nominal circuit voltage sufficient for the current that must be interrupted.”
Section 110-9 was changed in the 1999 code by substituting the word interrupt
for the work break in two places.
The interrupting rating of overcurrent protective devices is determined under standard test conditions. It is important that the test conditions match the actual installation needs. Section 110-9 states that all fuses and circuit breakers intended to
interrupt the circuit at fault levels must have an adequate interrupting rating wherever they are used in the electrical system. Fuses or circuit breakers that do not
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Panelboards and Switchboards
have adequate interrupting ratings could rupture while attempting to clear a short
circuit.
There are two ways to meet this requirement: the Full Rating Method and the
Series Rated Method.
The full rating method selects circuit protection devices with ratings equal
to or greater than the available fault current.
Consider a building with 65,000 amps of fault current available at the service
entrance. All downstream circuit protection device must be rated at 65,000
Ampere Interrupt Capacity (AIC). Although switchboards are available with short
circuit Current Ratings up to 200,000 amps, anything over 100,000 AIC starts to
get cost-prohibitive because additional bus bracing is required. In Figure 7, the
main circuit breaker and all branch breakers are rated for 65,000 AIC.
Figure 7. Full Rating Method (on left) Vs. Series Rated Method (on right)
The series rated method states that the main upstream circuit protection
device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in
series can be rated at lower values.
Under fault conditions, both the main device and the downstream device would
open to clear the fault.
Consider a building with 42,000 amps of available fault current. Although the
breaker at the service entrance is rated at 42,000 amps, additional downstream
breakers could be rated at only 22,000 amps.
To receive UL listing, series-rated breaker combinations must first pass testing in
series, then pass tests installed in panelboards and/or switchboards.
There are additional rating terms that need to be understood when selecting panelboards, switchboards and appropriate circuit protection devices. These are:
Current Rating: This is the level of fault current a piece of equipment can withstand without sustaining damage.
Interrupting Rating: This is the current rating a protective device (such as a Fuse
or circuit breaker) can safely interrupt.
Ampere Rating: This is the current a protective device will carry continuously without deteriorating or exceeding temperature rise limits.
Voltage Rating: The voltage rating of a switchboard or panelboard can be higher
than the system voltage, but never lower. For example, a 480 VAC switchboard
could be used on a 240 VAC system. A 240 VAC switchboard could not be used
on a 480 VAC system.
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Panelboards and Switchboards
Panelboards
Now that we have looked at the commonalities between the switchboard and the
panelboard, We will look at their specific features separately. Let’s begin with the
panelboard.
Panelboard vs.
Loadcenter
We looked at the Loadcenter in Module 10 of this series. The loadcenter and the
panelboard perform similar functions. They both serve to protect branch circuits from overloads and short circuits.
A panelboard is for use in commercial and industrial applications, while a
loadcenter is primarily for use in residential applications. This is because the
bus bars in a loadcenter are typically rated at a maximum of 200 amps, while
those of a panelboard can accommodate up to 1200 amps. Along the same lines,
a loadcenter can handle a maximum of 240 volts, but a panelboard can handle up
to 600 volts.
Components of a
Panelboard
Now that you have a basic understanding of the functions a panelboard performs,
let’s take some time to consider the panelboard’s construction.
There are several components to a panelboard: a can, interior, circuit protection
devices, label, dead front and trim, and filler plates.
Figure 8. Typical Panelboard Meeting NEC Definition
Can
The Can, also called the box, is the housing in which the other components
reside. Typically, it is made of galvanized steel. The design of the can provides
protection for both personnel and the internal components. The can’s end panels
are removable, allowing the installer to locate and cut holes for conduit installation. Optionally, the end panels can be provided with stamped Knockouts.
Interior
Inside the can, you will find overcurrent protection devices, bus bars, insulated
Neutral Bars, and other components, depending on the application.
The centerpiece is the set of bus bars. A bus bar is a common conductor, used
as a connection point for multiple circuits. Bus bars are usually aluminum, but
can also be copper. They provide a mechanical means of affixing branch circuit
breakers. The bus bars are mounted on the interior, which is mounted on studs in
the box.
The neutral bar provides the termination point for the neutral wires from
both the incoming service and the load circuits. The neutral bar can be
mounted on studs in the back of the box, or on the panelboard interior.
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Panelboards and Switchboards
Figure 9. Bus Bars and Neutral Bar
Circuit Protection Devices
The circuit protection devices are usually circuit breakers. They are mounted
directly to the bus bars. We will talk more about circuit breakers later.
Label
The panelboard label provides information regarding the unit’s voltage rating and
ampacity.
Dead Front and Trim
These components cover the front surface of the panelboard. A hinged access
door is provided as part of the trim. The dead front provides access to the circuit
breakers themselves, while preventing contact with interior components such as
the bus bars and internal wiring.
Figure 10. Components of a Panelboard
Filler Plates
Filler plates are used to cover any unused Pole spaces not used by a circuit
breaker.
Circuit Identification
All circuits in the panelboard must be clearly identified with a number. This can be
done in two different ways. The NEMA (National Electrical Manufacturers Association) numbering scheme assigns odd numbers to the poles on the panelboard’s
right (your left), and even numbers to the poles on the panelboard’s left (your
right). This is shown in Figure 11 on the left.
The other method simply used a vertical numbering sequence, shown on the right.
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Panelboards and Switchboards
Figure 11. NEMA Numbering (on left) Vs. Vertical Numbering (on right)
Attached to the inside of the door, you will find a label containing a circuit directory.
Enclosures
NEMA has established guidelines for electrical equipment enclosures. Most panelboards are supplied as standard in a NEMA Type 1 enclosure. Descriptions of
the enclosure options are listed below.
NEMA Type 1 General This enclosure type is for general purpose, indoor use. It
Purpose
is suitable for most applications where unusual service
conditions do not exist. It provides protection from
accidental contact with enclosed equipment
NEMA Type 3R
Raintight
This enclosure type is intended for outdoor use. It
provides protection against falling rain and sleet, and
damage from external ice formation. It has a gasketed
cover.
NEMA Type 4X
Corrosion Resistant
This enclosure type is intended for indoor or outdoor
use, where corrosion resistance is required. It is
constructed of stainless steel, polymeric, or fiberglass. It
also provides protection from splashing or hose-directed
water, wind-blown dust or rain, and damage from
external ice formation.
NEMA Type 12
Dusttight
This enclosure type is for indoor use. It provides
protection from dripping non-corrosive liquids, falling
dirt, and dust.
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Panelboards and Switchboards
Review 1
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. There are several similarities between panelboards and switchboards. Name
four areas of similarity.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
2. In your own words, explain the difference between the full rating method and
the series rated method of selecting overcurrent protection devices.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
3. Define the following ratings:
Withstand rating:
_________________________________________________
Interrupting rating:
________________________________________________
Ampere rating:
___________________________________________________
4. Briefly explain the function of each panelboard component listed here:
Can ________________________________________________________
Bus Bars____________________________________________________
Dead Front and Trim __________________________________________
Filler Plate___________________________________________________
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Panelboards and Switchboards
Circuit Breakers
One circuit breaker is installed in a panelboard for each branch circuit the panelboard serves. In this section, we will consider the proper way to install circuit
breakers in a panelboard.
Balancing the Load
In the case of a single phase system, there are two bus bars in the panelboard.
When installing breakers, it is important to balance the load so that both
bars are doing the same amount of work. This prevents strain on the system.
Compare this to loading clothes into a washing machine. It is important to get an
even distribution of clothes all the way around the tub. And it’s not enough just to
have an equal number of items on each side of the tub. Having a pair of jeans and
a sweater on one side and two socks on the other side is NOT a balanced load.
The same is true for the panelboard. It is not enough to make sure there are an
equal number of breakers on each bus bar. The total amperage ratings of
the breakers on each bus bar need to be equal.
Figure 12. Balancing the Load
Types of
Panelboards
Lighting and Appliance
Panelboard
Power Distribution
Panelboard
There are two common types of panelboards:
•
Lighting and Appliance Panelboard
•
Power Panelboard
The NEC definition for lighting and appliance panelboards has three primary elements:
•
A maximum of 42 overcurrent devices (poles) installed in one cabinet
•
At least 10% of the overcurrent devices must be rated at 30 amps or less
•
Neutral connections must be provided
The NEC defines power distribution panelboards as all panelboards that are not
lighting and appliance. The only restriction is physical limitations. It is used to feed
other panels, three-phase motors, and transformers.
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Panelboards and Switchboards
Figure 13. Types of Panelboards
Consider this panelboard as an example:
Number of Circuit
Breakers
4
22
8
Description
Number of
Overcurrent Devices
1-Pole, 30 A
1-Pole, 40 A
2-Pole, 40 A
Total
4
22
16
42
There are only four overcurrent devices which are rated at 30 amps or less. This
panelboard does not qualify as a lighting and appliance panelboard, so it is a
power and distribution panelboard.
In the Workplace
Consider what happens when a light switch is turned on. If the circuit takes 18
amps to power the lights, it will continue to take 18 amps until the lights are turned
off.
Now consider a motor circuit. The start button is pressed and the motor experiences a large inrush of current. This inrush could easily be 6 times the current
needed to run the motor at full rated load at full speed. For example, a one-horsepower motor that requires 21 amps to start will only need 3-1/2 amps when running at full speed.
Because the power panel is designed to supply branch circuit loads such as
this, it cannot be regulated as closely as a lighting and appliance panelboard.
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Panelboards and Switchboards
Installation and
Mounting
A panelboard can be Flush-Mounted or Surface-Mounted against a wall. A flushmounting is recessed into a hole in the wall. A surface-mounting is attached to
(and projects out from) the wall.
Flush Mount vs. Surface
Mount
Many panelboards are flush mounted in commercial, office, school and public buildings. Flush-mounting offers some important benefits, such as:
•
Space savings – Because the box is behind the wall, the room is not robbed
of space.
•
Appearance – This option is more attractive, as the wiring and box are concealed.
•
Safety – Except for the trim, the panel does not jut out to possibly catch clothing or other objects.
Figure 14. Flush-Mount Vs. Surface Mount
Surface-mounted panelboards are generally used in industrial buildings and
basements of office and commercial buildings. Because most of these areas
have walls made of poured concrete, flush-mounting is impractical. Steel columns
are often used for surface mounting.
NEC Mounting
Specifications
The NEC specifies clearances around panelboards. This is to provide access and
working space. There are three basic rules:
•
Headroom in the location must be a minimum of 6-1/2 feet.
•
For systems up to 150 volts, the minimum distance from the panelboard to the
ground shall be 3 feet, 4 feet for 151-600 volt systems.
•
For access, there must be 30 inches minimum space in front of the panelboard, and sufficient space to allow the hinged door to open and rotate 90
degrees.
Figure 15. Panelboard Clearance Requirements
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Panelboards and Switchboards
Review 2
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what you’ve
already read.
1. Underwriter's Laboratory (UL) has a number of stringent guidelines for application of the lighting and appliance panelboard. These include:
A maximum of ______ overcurrent devices installed in one cabinet
At least ____% of the overcurrent devices rated at ____ amps or less
___________ __________ must be provided
2. Does this drawing show a balanced breaker load?
Circle YES or NO.
3. The two mounting styles for panelboards are ___________-mounted and
___________-mounted.
4. There are a number of requirements that the installation location for a 240 volt
panelboard must meet. In your own words, explain these requirements.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
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Panelboards and Switchboards
Switchboards
Now let’s turn our attention to switchboards. Switchboards are low voltage equipment, meaning 600 volts or less. The current range is 400 to 6000 amps.
Again, the purposes of a switchboard are to disconnect loads for safe maintenance, and to protect conductors and equipment against excessive current due to
overloads, short circuits, and ground faults provided ground fault protection is supplied.
The basis for a switchboard consists of a frame, bus, Overcurrent Protective
Devices, instrumentation, and exterior covers.
Figure 16. Typical Switchboard Meeting NEC Definition
Components of a
Switchboard
Frame
The frame is the metal skeleton in which the other components reside.
Bus
Bus is mounted within the frame. Like the bus bars of the panelboard, bus is used
to distribute power. Where bus bars move power from incoming utility power
cables to the branch circuits, bus move power between two or more components or circuits.
Horizontal Bus distribute power to each switchboard section. Vertical Bus distribute power through the circuit protection devices to the branch circuits.
Figure 17. Switchboard Frame, Before and After Installing Bus
NEMA requires that the bus be installed with the phases in sequence when
viewed from the front. In the case of the three-phase switchboard shown in Figure 17, the phases must be in the order A-B-C from left to right (for vertical bus) or
from top to bottom (for horizontal bus).
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Panelboards and Switchboards
If a Neutral Bus is required, it is placed at the end of the sequence. With vertical
buses, it is placed at the left, yielding an N-A-B-C sequence. In the case of horizontal buses, it is placed at the bottom, resulting in an A-B-C-N. (No vertical neutral bus is shown above.)
Protective Devices
Protective devices, such as circuit breakers and disconnect switches, are
mounted to the vertical bus bars from the front of the unit.
Other devices installed at this time include meters, transient voltage surge protection (TVSS), utility compartments, panelboards, transformers, and other equipment.
Figure 18. Installing Protective Devices
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Panelboards and Switchboards
Types of Protective Devices
There are four types of protective devices commonly used in switchboard
applications. These are:
Power Circuit Breaker The term “power” is applied to a large circuit breaker with a wide range
of adjustable magnetic overcurrent and solid state. trips. It does not
have a thermal trip.
It is built on an open framework for easy servicing and adjustment.
It can be manually or electrically operated and is available with ratings
up to 4000 amperes.
Power air circuit breakers can be stationary-mounted or draw-out type.
Draw-out breakers can be tested without removal from the
switchboard.
Low voltage power circuit breakers are covered in Module 7.
Molded Case Circuit This breaker is normally both a thermal and magnetic trip device.
Ratings run 15 – 2,500 amps, with a variety of short circuit interrupting
Breaker
capacities.
Some breakers feature interchangeable trip elements. Others have
solid state trip units.
Molded Case Circuit Breakers are covered in Module 8.
Fusible Switch
This is a hand-operated disconnect switch with a fuse on each pole.
When an overload condition arises, the fuse link melts and opens the
pole, protecting the circuit.
Fuses are available in two styles. The “time delay” fuse is suitable for
load surges, such as motor start-ups. The “instantaneous” fuse is
designed to “clear” in a fraction of a second in the event of a high
current short circuit.
They are typically in rated for 240- 600v and 15-1200 amps.
The main difference between a circuit breaker and a fusible switch is
that the circuit breaker can be reclosed after an overload has been
interrupted and corrected. The fuse needs to be replaced.
Bolted Pressure
Switch
This switch type is primarily used for service entrance and feeder
circuits.
The switch is quick-make/quick-break. When the switch moved to the
ON position, the line contacts are squeezed together under pressure
by the contact locking mechanism. This stored up energy is released
for quick make. The same energy quickly breaks the contacts when
the manual trip button is pushed.
The protective elements are heavy duty, current-limiting high
interrupting capacity fuses. Bolted pressure switches are rated 240480 volts and 800 – 4,000 amps.
Accessories are available to open the switch automatically or
remotely. It can be manual or an electric trip
Instrumentation
Meters can be used in the service section to measure current, voltage, power
usages, peak demands just to name a few. This will be explored in Module 15,
Metering and Communication.
Enclosures
Switchboards are usually only available in a NEMA Type 1 or NEMA Type 3R
enclosure.
Exterior Covers
Once the protective devices are installed, exterior cover panels are put in place on
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Panelboards and Switchboards
the frame. Like the dead front and trim of the panelboard, these covers allow
access to the protective devices while sealing off the buses and wiring from accidental contact. The cover panels also serve as trim for the sake of product
appearance.
An informational is attached to the cover panel, providing information on the
switchboard type, voltage and amperage ratings, and part number.
Figure 19. Installing Dead Front and Trim
Sections of a
Switchboard
Now that you understand how a switchboard is put together, let’s take some time
to consider the function of each section. We have made a few brief references
various sections of the switchboard without really describing them.
There are four main section types that a switchboard may have. Note that all
switchboards do not utilize all four of these section types. They are:
Service Section
•
Service section
•
Pull section
•
Distribution section
•
IFS section
This section always contains a main disconnect and usually also has utility
and/or customer metering equipment.
The service section can be fed directly from the utility company’s transformer to
serve as a service entrance. Service from the utility can enter the service section
in a variety of ways. Cable can be brought into the switchboard from the top or
bottom, giving additional application flexibility. You will need to work with the customer to determine what service entrance needs exist. This decision will affect the
cost.
Figure 20. Bringing Service into the Service Section
Pull Section
The center example in Figure 20 shows the use of a Pull Section. The pull section is a common switchboard component. It is simply a blank enclosure
containing empty space through which cables can be pulled, hence the
name.
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Panelboards and Switchboards
A pull section is used most often with service entrance switchboards where the
utility feed comes up through the floor. This allows the service section to be fed
from the top without any exposed conductors.
The fourth example shows the use of a Pull Box, which is basically a top-mounted
pull section. This is useful if there are extra connections to be made because it
permits more space in which to work.
Distribution Section
Power moves from the service section to the distribution section. Here, it is
divided and sent through branch circuit protection devices, then out to the branch
circuits, to provide power to loads downstream.
In some applications, the size of the service section’s main disconnect device and
associated bus requirements may necessitate the service section cabinet to be
deeper than the distribution section cabinet. Because the rear of all cabinet sections are always aligned (for installation against a wall), the front of the service
section cabinet will protrude. This is referred to as a Rear Alignment.
Figure 21. Front and Rear Alignment Vs. Rear Alignment
If the depth of the service section and distribution sections are the same, the
switchboard installation has Front and Rear Alignment.
Some switchboards may require a deeper distribution cabinet for the circuit protection devices and bus. Or, extra depth may be added as an option. Again, work
with the customer to understand the need. This decision may affect the cost.
IFS Section
An IFS section, or Integrated Facility Switchboard, may include panelboards, dry
type transformers and blank back pans for mounting other equipment.
If panelboards and dry type distribution transformers are used within the same
room as switchboards, it may make sense to consider IFS sections. IFS sections
can reduce the need for lineal wall space required for equipment, thus reducing
the area needed for equipment. Additionally, IFS can also significantly reduce the
installation time while reducing the number of pieces of equipment for handling.
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Panelboards and Switchboards
Spare, Provision and
Blank Space
Customer specifications for protective devices will also affect cost. We have
already covered the types of protection devices (switches and circuit breakers)
commonly used in the switchboard. But there are some extra terms used in this
area pertaining to the protective devices that you should know.
For future expandability, the customer could request a Spare protection device.
This means that the protection device is bolted in place and connected to the bus.
All the customer has to do is connect a branch conductor to it.
This is very different from ordering a provision. With this arrangement, a space is
left in the panel for the future insertion of a protection device. The bus connectors
are in place and ready to use when the customer is ready to expand.
Different still is the specification of a space. It is space on the panelboard/switchboard chassis that is simply covered with a blank panel. There is vertical bus in
back of the panel, but a connector kit would be required to install a breaker in the
future. The space can be defined for a defined overprotective device, in which can
an appropriately sized filler cover will be supplied.
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Panelboards and Switchboards
Helping the
Customer
Throughout our discussion of panelboards and switchboards, we have identified a
number of questions you will need to ask the customer to match a product to the
application.
It may be helpful to spend a moment reviewing the information needed so that you
are more prepared to knowledgeably recommend a product to the customer.
Matching a Panelboard
to an Application
As you talk with the panelboard customer, be sure to obtain the following information:
•
System type (including voltage, number of phases, and number of wires)
•
Required amperage rating
•
AIC rating or short circuit current rating
•
Service entrance equipment or distribution?
•
Main breaker or main lug only?
•
Top- or bottom-fed?
•
Flush- or surface-mount?
•
NEMA Type enclosure
•
Any desired special modifications
•
Required shipping timeframe
•
Shipped assembled or as components?
Armed with this information, you should be able to consult your product catalog
and make a good product recommendation.
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Panelboards and Switchboards
Matching a Switchboard
to an Application
The questions you are required to ask of a switchboard customer are somewhat
different. To save time, we have come up with a questionnaire-style form that
should speed up the interview process. It is reproduced here.
Figure 22. Switchboard Questionnaire
Help the customer to complete this form thoroughly. This information should provide a good start on matching a switchboard to the customer’s application.
With this information in hand, you might then try using the form below to sketch
out the proposed switchboard.
Figure 23. Switchboard Layout Form
This should help you and the customer fine-tune the switchboard’s features to
best serve the application.
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Panelboards and Switchboards
Review 3
Answer the following questions without referring to the material just presented.
1. Like the bus bars of the panelboard, ________ are used to _________
________ in a switchboard.
2. There are four types of protective devices commonly used in switchboard
applications. Name three of them.
____________________________________________________________
____________________________________________________________
____________________________________________________________
3. Switchboards are usually only available in enclosure types _____________
and ____________.
4. The four main switchboard section types are:
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
5. In your own words, explain the difference between a spare and a space.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
Page 26
Panelboards and Switchboards
Glossary
Ampere Interrupt
Capacity (AIC)
Also “Interrupting Rating.” A rating of the amount of
current that a protective device, such as a fuse or circuit
breaker, can safely interrupt.
Ampere Rating
A rating of the amount of current a protective device will
carry continuously without deteriorating or exceeding
temperature rise limits.
Blank Space
A space on a panel of a switchboard that will never be
used for expansion. It is covered with a blank panel and
no bus runs behind it.
Branch Circuit
A circuit that supplies power to the electrical loads in a
building and is terminated at the panelboard or
switchboard.
Bus Bar
A component of a panelboard that serves as an
extension of the main service conductors. Simplifies the
connection of circuit protective devices to the main
service conductors.
Bus
Components of a switchboard that serves as an
extension of the main service conductors. Simplifies the
connection of circuit protective devices to the main
service conductors. Utilized to move power between two
or more components or circuits.
Can
Also “Box.” Component of a panelboard used to contain
the other components. It is designed to provide
component and personnel protection.
Circuit Breaker
A reusable overcurrent protection device. After tripping
to break the circuit, it can be reset to protect the circuit
again.
Current Rating
A rating of the level of fault current a piece of equipment
can withstand without sustaining damage.
Disconnect Device
A blanket term used to describe a number of circuit
protection devices, such as fuses and circuit breakers.
Distribution
Equipment
Also “Distribution/ Power Only Panelboard”. A
panelboard or switchboard used to feed lighting and
appliance panelboards and three-phase motor loads.
Also can power small transformers to provide other
voltage levels.
Flush-Mounted
Recessed into a hole in the wall.
Front and Rear
Alignment
A type of switchboard arrangement where the front and
the rear faces of the switchboard sections are aligned.
Full Rating Method
A method of selecting circuit protection devices where
all device ratings are equal to or greater than the
available fault current.
Fuse
A non-reusable overcurrent protection device. After
tripping to break the circuit, it must be replaced to
restore power to the circuit.
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Panelboards and Switchboards
Ground
A conducting connection (intentional or accidental)
between an electrical circuit or equipment and the earth,
or to some conducting body that serves in place of the
earth.
Ground Fault
An electrical fault where contact is accidentally made
between an electrical circuit or equipment and the earth,
or to some conducting body that serves in place of the
earth.
Horizontal Bus
Distributes power to each section of a switchboard.
Interrupting Rating
Also “Ampere Interrupting Capacity (AIC).” A rating of
the amount of current that a protective device, such as a
fuse or circuit breaker, can safely interrupt.
Knockouts
Circular perforations in the top and bottom panels of a
panelboard that can be removed to provide entrances
and exits for feeder wires enclosed in conduit.
Lighting and
Appliance
Panelboard
A panelboard that supplies electrical power to branch
circuits that use a constant amount of power.
Loadcenter
A device that delivers electricity from a supply source to
loads in light commercial or residential applications.
Main Breaker
Also “Main Circuit Breaker.” A reusable overcurrent
protection device designed to protect an entire
panelboard or switchboard. Power from the mains is fed
through a main breaker to the bus bars.
Main Disconnect
Device
A blanket term used to describe a number of panel
protection devices, such as a main circuit breaker or
main switch.
Main Lug Only (MLO) A panelboard where power from the mains is fed directly
to the bus bars.
Neutral Bars
Provides the termination point for the neutral wires from
both the incoming service and the load circuits in a
panelboard.
Neutral Bus
Provides the termination point for the neutral wires from
both the incoming service and the load circuits in a
switchboard.
Overcurrent
Protective Device
A device such as a circuit breaker or fuse. In the event
of an overload or short circuit, this device will quickly
terminate power to the circuit.
Panelboard
A wall-mounted electrical power distribution device for
use in commercial and industrial applications. It
provides circuit control and overcurrent protection for
light, heat or power circuits.
NEC defines it as a single panel or group of panel units
designed for assembly in the form of a single panel;
including buses, automatic overcurrent devices, and
equipped with or without switches for the control of light,
heat, or power circuits; designed to be placed in a
cabinet or cutout box placed in or against a wall or
partition and accessible only from the front.
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Panelboards and Switchboards
Pole
A space in a panelboard (or switchboard) where a
branch circuit protective device can be attached to
connect a branch circuit to the bus bars (buses) and
protect the branch circuit from overload.
Power Panelboard
Any panelboard that does not qualify as a lighting and
appliance panelboard. It is used to feed lighting and
appliance panelboards and three-phase motor loads.
Also can power small transformers to provide other
voltage levels.
Pull Box
A top-mounted pull section.
Pull Section
A common switchboard component, a blank enclosure
containing empty space through which cables can be
pulled.
Rear Alignment
A type of switchboard arrangement where only the rear
faces of the switchboard sections are aligned.
Series Rated Method A method of selecting circuit protection devices where
the main upstream circuit protection device must have
an interrupting rating equal to or greater than the
available fault current of the system, but downstream
devices connected in series can be rated at lower
values.
Service Entrance
The single point at which electrical power enters a
building.
Service Entrance
Equipment
The term used to describe a panelboard or switchboard
used as a service entrance.
Service Section
This section of the switchboard is where upstream
power enters. It always contains a main disconnect and
usually also has utility and/or customer metering
equipment.
Six Subdivisions of
the Main Rule
An NEC ruling which states that you must be able to
throw no more than six handles into the off position to
disconnect electrical service. If throwing more than six
handles would be required, one upstream disconnect
must be provided to disconnect all the branch circuits at
once.
Space
A specification when ordering a switchboard. Room is
purposely left for a protective device to be installed in a
later expansion of service. Bus bars are in place. The
customer needs to buy and install a protection device
and connect a branch circuit to it.
Spare
A specification when ordering a switchboard. An extra
protection device is bolted in place and connected to the
bus. For future service expansion, all the customer has
to do is connect a branch circuit to it.
Surface-Mounted
Attached to (and projects out from) the wall.
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Panelboards and Switchboards
Switchboard
A floor-standing electrical power distribution device for
use in commercial and industrial applications. It divides
large blocks of electrical current into smaller blocks of
current used by electrical equipment.
NEC defines it as a large single panel, frame, or
assembly of panels on which are mounted, on the face
or back, or both, switches overcurrent and other
protective devices, buses and usually instruments.
Vertical Bus
Distributes power through the circuit protection devices
of a switchboard to the branch circuits.
Voltage Rating
A rating of the voltage at which a piece of equipment is
designed to operate.
Page 30
Panelboards and Switchboards
Review 1 Answers
1. Any four of the following:
Power supply systems
Service entrance equipment / Distribution equipment
Main breaker and main lug only types
Grounding
Ratings
2. Answer should basically say: “The full rating method selects circuit protection
devices with ratings equal to or greater than the available fault current. The
series rated method states that the main upstream circuit protection device
must have an interrupting rating equal to or greater than the available fault
current of the system, but downstream devices connected in series can be
rated at lower values.”
3. level of fault current a piece of equipment can withstand without sustaining
damage; current rating a protective device can safely interrupt; current a protective device will carry continuously without deteriorating or exceeding temperature rise limits
4. Answer should basically say:
houses the other components, designed to provide component and personnel
protection
common connection for two or more circuits, mechanical means of affixing
branch circuit breakers
provide access to the circuit breakers while sealing off the bus bars and internal wiring from contact
cover any unused pole spaces not used by a circuit breaker
5. flush, surface
Review 2 Answers
1. 42
10%; 30 amps
Neutral connections
2. Yes
3. Flush; Surface
4. Answer should basically say: “The minimum distance from the panelboard to
the ground must be 4 feet. There must be at least 30 inches of space in front
of the equipment to allow the hinged door to open 90 degrees. Headroom in
the area must be at least 6-1/2 feet.”
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Panelboards and Switchboards
Review 3 Answers
1. bus; distribute power
2. Any three of the following:
power circuit breaker
molded case circuit breaker
fusible switch
bolted pressure switch
3. NEMA Type 1, NEMA Type 3R
4. service
pull
distribution
IFS
5. Answer should basically say: “A spare protection device is bolted in place and
connected to the bus. All the customer has to do is connect a branch circuit to
it. A space is left in the panel for the future insertion of a protection device.
The bus bars are in place and ready to use when the customer is ready to
expand.”
Page 32