101 BASICS SERIES
LEARNING MODULE 10:
LOADCENTERS
Cutler-Hammer
LOADCENTERS
WELCOME
Welcome to Module 10, which is about loadcenters. A loadcenter is a device that
delivers electricity from a supply source to loads in residential and light
commercial/industrial applications.
FIGURE 1. TYPICAL LOADCENTER
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 terms are underlined and italicized the first time they appear.
Viewing the
Glossary
You may view definitions of glossary items by clicking on terms and words that are
underlined and italicized in the text. You may also browse the Glossary by clicking
on the Glossary bookmark in the left-hand margin.
1
LOADCENTERS
WHAT YOU
WILL LEARN
We’ll step through each of these topics in detail:
Section Title
•
What Does a Loadcenter Do?
•
•
•
3
Applications
4
Basic Circuitry And Wiring
5
•
Residential Service Entrance Panel
5
•
Residential Distribution Panel
6
•
Commercial/ Industrial Service Entrance Panel
7
•
Commercial/ Industrial Distribution Panel
7
Grounding Issues
8
•
Grounding the Service Entrance Panel
8
•
Equipment Grounding Bus
9
•
Grounding Downstream Loadcenters
10
•
Ground Fault
12
•
Review 1
13
•
Loadcenter Components and Installation
14
•
•
•
2
Page Number
•
Anatomy of a Loadcenter
14
•
Enclosure
15
•
Installation and Mounting
16
Circuit Breakers
18
•
Installing Circuit Breakers
18
•
How Many Circuit Breakers Fit in a Loadcenter?
20
•
Interchangeable vs. Non-Interchangeable
Breakers
21
•
Ratings
21
Loadcenter Types
23
•
Main Breaker Loadcenter
23
•
Main Lug Only Loadcenter
24
•
Convertible Loadcenter
24
Selecting a Loadcenter
25
•
Required Application Information
25
•
Sample Application
26
•
Review 2
27
•
Glossary
28
•
Review Answers
31
LOADCENTERS
WHAT
DOES A
LOADCENTER
DO?
Every residential and light commercial/industrial building utilizes electricity for
lighting, receptacles, and/or appliance loads. Conductors are used to bring power
from the power lines to the building. Then a device is needed to divide this power
for branch circuits and to protect these branch circuits from overloads and
short circuits. This device is called a loadcenter.
Electricity moves from the supply source into the building to the loadcenter, and is
then distributed through the building’s branch circuits to the loads. Each branch
circuit is connected, or terminated, at the loadcenter.
Each branch circuit is protected by a circuit breaker housed in the loadcenter. In the
event of a short circuit or an overload on a branch circuit, the circuit breaker
cuts the power before any property damage or personal injury can occur.
NOTE: New electrical installations use circuit breakers in the loadcenter, so we will
not discuss fuses in this training module.
SERVICE
ENTRANCE
LIGHTING
APPLIANCES
SWITCHES
RECEPTACLES
LOADCENTER
GROUNDING
FIGURE 2. ELECTRICAL DISTRIBUTION TO LOADS
3
LOADCENTERS
Applications
There are two main applications for a loadcenter:
•
As a service entrance panel. This is the term for a loadcenter used at the
point at where the power enters a building. There can only be one service
entrance panel per building.
•
As a distribution panel. A distribution panel is simply a loadcenter used at a
point beyond the building’s service entrance. This can be useful when
adding additional electrical service to an existing building. The power to the
distribution panel usually comes from a branch circuit of the service entrance
panel.
POWER
FROM
UTILITY
SERVICE
ENTRANCE
PANEL
DISTRIBUTION
PANEL
FIGURE 3. A SERVICE ENTRANCE PANEL AND A DISTRIBUTION PANEL
4
LOADCENTERS
BASIC
CIRCUITRY
AND WIRING
Residential
Service Entrance
Panel
Figure 4 shows a typical residential application. Loadcenters are typically rated 225
amps or less, and a maximum of 240 volts. Most homes have 200 amp service
and a 120/240-volt, single-phase, three-wire system. Three conductors run from
the distribution transformer to the service entrance panel. This transformer converts
the utility voltage into a voltage and current supply suitable for use in most
residential applications. Two of the main service conductors (or mains) are
ungrounded (“hot”), and the third is the neutral. The “hots” are connected to the
main circuit breaker in the loadcenter. The neutral is attached to the neutral bar.
If a voltmeter reading is taken between the two hot conductors (“A” and “B”), it will
measure 240 volts. If a reading is taken between a hot conductor and the neutral
(“N”), it will measure 120 volts.
DISTRIBUTION
VOLTAGE
DISTRIBUTION
TRANSFORMER
A
N
B
GROUND
120
120
240
LOADCENTER
NEUTRAL
L
MOTOR
LOAD
LIGHTING
LOAD
M
BUS BARS
FIGURE 4. TYPICAL SINGLE-PHASE, THREE-WIRE LOADCENTER
This is convenient because 240 volts are needed to power central air conditioners,
electric clothes dryers, and electric range tops. At the same time, 120 volts are
required to service lighting, small appliances, and receptacle loads.
5
LOADCENTERS
Residential
Distribution
Panel
Now, let’s consider the case of a residential distribution panel. Remember that a
distribution panel is a panel used at a point beyond the service entrance. This
means that the distribution panel is typically fed by a service entrance panel.
You can add on to a loadcenter in two ways. The first way is to use a branch circuit
breaker. The second way is to use feed-through lugs.
MAIN
BREAKER
N
A
N
A
B
TRANSFORMER
SERVICE
ENTRANCE
PANEL
B
FEED-THROUGH
LUGS
MAIN LUGS
N
A
B
DISTRIBUTION PANEL
FIGURE 5. A SERVICE ENTRANCE PANEL FEEDING A DISTRIBUTION PANEL
In the example shown here, the service conductors (the two “hots” and the neutral)
run from the feed-through lugs of the service entrance panel to the main lugs of the
distribution panel. In this way, the distribution panel itself is protected by the
main circuit breaker in the service entrance panel. The neutral conductor is
isolated. The importance of this will be discussed later.
Usually, a residential distribution panel is used to supply a washer, dryer, airconditioner and/or hot tub.
6
LOADCENTERS
Commercial/
Industrial
Service Entrance
Panel
The commercial/industrial service entrance panel is supplied with a different power
system than the residential one. A distribution transformer provides a 120/208
volt, three-phase, four-wire system to the panel. Four conductors go to the
service entrance. The three mains are ungrounded (“hot”), and the fourth is the
neutral. The “hots” are connected to the main breaker in the loadcenter. The neutral
is attached to the neutral bar and is connected the same way as in the residential
application.
If a voltmeter reading is taken between any two hot conductors (“A,” “B” and “C”), it
will measure 208 volts. If a reading between any hot conductor and the neutral (“N”)
is taken, it will measure 120 volts.
DISTRIBUTION
VOLTAGE
DISTRIBUTION
TRANSFORMER
A
N
C
B
GROUND
120
120
208
120
208
208
LOADCENTER
NEUT
L
MOTOR
LOAD
LIGHTING
LOAD
M
BUS BARS
FIGURE 6. TYPICAL THREE-PHASE, FOUR-WIRE LOADCENTER
This system can handle single-phase or three-phase applications. The 120/208
volt single-phase power is used the same way as in the residential application. The
three-phase, 208-volt power is rarely used. For this reason, we will not discuss
three-phase any further in this module.
Commercial/
Industrial
Distribution
Panel
The distribution panel is treated the same as the residential distribution panel. It is
typically fed by the service entrance panel and it can be single phase or three
phase. Again, it is important to remember that the neutral is isolated.
7
LOADCENTERS
GROUNDING
ISSUES
Grounding is an important aspect of any electrical equipment and must be
considered carefully. The National Electrical Code (NEC) defines ground as a
conducting connection, whether intentional or accidental, between an electrical
circuit or equipment and the earth, or to some conducting body that serves in place
of the earth. There are two objectives to the intentional grounding of electrical
equipment:
•
Keep voltage differentials between different parts of a system at a minimum
which reduces shock hazard.
•
Keep impedance of the ground path to a minimum. The lower the impedance
the greater the current is in the event of a fault. The greater the current the
faster an overcurrent device will open.
In the service entrance panel shown in Figure 7, the neutral from the transformer is
Grounding the
Service Entrance connected to the neutral bar in the loadcenter. The neutral bar is then grounded to
the earth.
Panel
TRANSFORMER
N
A
B
GROUNDING
ROD
NEUTRAL
BONDED
TO BOX
NEUTRAL
BUS
FIGURE 7. GROUNDING A SERVICE ENTRANCE PANEL
8
LOADCENTERS
Grounding the
Service Entrance
Panel
(continued)
This is accomplished by running a conductor from the loadcenter’s neutral bar to a
cold water pipe or copper rod buried in the ground (or other suitable grounding
electrode.) Then, a bonding screw or strap is used to physically “bond” the neutral
bar to the metal enclosure.
In this way, the neutral bar, the neutral of the transformer, and the enclosure are
grounded. When the neutral is grounded in this manner, the equipment ground from
the branch circuits is sometimes allowed to connect to the neutral.
Equipment
Grounding Bus
The equipment grounding bus. is connected directly to the loadcenter enclosure.
For safety, all equipment (on both the feeder and branch circuits) is connected to
the equipment grounding bus. This requirement keeps the connected equipment at
the same potential as the enclosure itself.
EQUIPMENT
GROUNDING BUS
FIGURE 8. EQUIPMENT GROUNDING BUS
One additional point for service entrance applications: this bonded and grounded
neutral can also be used as the point of connection for equipment grounding.
9
LOADCENTERS
Grounding
Downstream
Loadcenters
It is important to remember that the service entrance panel is the only ground
connection point for the neutral. The neutral is both insulated and isolated at any
downstream panels.
As shown in Figure 9, the downstream panel’s enclosure is grounded with a
grounding conductor running back to the service entrance panel.
N
A
B
TRANSFORMER
INSULATED
NEUTRAL
SERVICE
ENTRANCE
PANEL
EQUIPMENT
GROUND
N
N
A
A
B
B
DISTRIBUTION
PANEL
FIGURE 9. GROUNDING THE DOWNSTREAM LOADCENTER
10
LOADCENTERS
Grounding
Downstream
Loadcenters
(continued)
In Figure 10, the table lamp 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 downstream
loadcenter contains a branch circuit breaker. It trips, disconnecting power from the
load.
N
A
B
SERVICE
ENTRANCE
PANEL
TRANSFORMER
EQUIPMENT
GROUND
INSULATED
NEUTRAL
N
N
A
A
B
B
DISTRIBUTION
PANEL
FIGURE 10. A SHORT IN A DOWNSTREAM LOAD
11
LOADCENTERS
Ground Fault
In most cases, NEC also requires equipment to have an additional grounding
conductor. This conductor connects the metal housing of an appliance to the
system ground to reduce the possibility of a shock due to a ground fault.
A ground fault is simply current leakage from an ungrounded conductor to the
grounding path in an electrical system. Consider Figure 11. A piece of insulation
covering a conductor at an electrical outlet has flaked off. This could allow current
to pass from the conductor to the metal outlet box. The box is part of the system
ground. If properly grounded, the current should follow the grounding path to
ground, instead of an unfortunate person’s finger that touched the box.
BARE CONDUCTOR IN
CONTACT WITH
METAL OUTLET BOX
FIGURE 11. GROUND FAULT
Since a distribution panel takes its power from the service entrance panel (Figure
9), the neutral needs to be isolated from the ground. It is considered a branch circuit
of the main service panel, and needs to have its own ground to prevent a ground
fault.
If all the loads in a panel are connected evenly between A-N and B-N, no current
will flow in the neutral. However, loads are rarely balanced. As a result, some
current usually flows through the neutral. The amount current flowing in the neutral
conductor at any one time is the difference between the current flowing through leg
A and leg B. This is why a path to ground is needed.
12
LOADCENTERS
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. Loadcenters have two main applications. These are:
•
_________________________________________________________
•
_________________________________________________________
2. In which two market segments are loadcenters typically used?
•
_________________________________________________________
•
________________________ / _______________________________
3. For safety reasons, the National Electrical Code (NEC) requires the service
entrance panel’s __________ to be grounded. Beyond the service entrance
equipment, it is always ____________ and ___________.
4. In your own words, explain under what conditions no current will flow through
the neutral. Then explain how to figure the amount of current flowing through
the neutral.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
5. In your own words, explain the main purpose of a loadcenter.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
13
LOADCENTERS
LOADCENTER Before we go any further in our exploration of the loadcenter, let’s take a moment to
consider the main components and features of a typical loadcenter.
COMPONENTS AND
INSTALLATION
Anatomy of a
Loadcenter
FIGURE 12. ANATOMY OF A TYPICAL LOADCENTER
14
1.
Front Cover
Provides a neat, protective face for the product. Usually includes a
hinged door to allow access to the circuit breaker handles.
2.
Inner Cover or
Dead Front
Serves as a protective barrier to prevent contact with energized (“hot”)
parts within the unit.
3/4.
Inner Cover
Adjustment
Screws
Used to secure the inner cover tightly against the tops of the circuit
breakers. This eliminates any space that might allow access to “hot”
parts inside.
5.
Twistouts
Removable perforated metal spaces that can be twisted out, to allow
the protrusion of the circuit breaker handles.
6.
Wiring Diagram
Lists the specifics of the loadcenter, including: circuit breaker
application, UL information, short circuit ratings, series ratings, wire
connection torque values, and unit accessories.
7.
Sliding Latch
Provided on indoor loadcenters to allow easy opening of the loadcenter
door.
8.
Door
The hinged part of the front cover which allows access to the circuit
breaker handles.
9/19.
Backpan Top
and Bottom
Mounting
Screws
Allows for easy removal of the loadcenter interior assembly.
LOADCENTERS
Enclosure
10/18
.
Enclosure
Mounting
Keyholes
Located at top center and bottom center. Whether surface-mounted
right-side up or inverted, the unit can be easily centered on the wall by
hanging the enclosure with the uppermost keyhole.
11.
Drywall Scribe
Markings
Serve as a guide for mounting the loadcenter flush with the drywall and
between the wall studs.
12.
Surface
Mounting Holes
(4)
Once centered for surface mounting, the loadcenter is affixed with
screws through these four holes.
13.
Main Lugs or
Main Breaker
Provide the means for the termination of the power cables coming from
the electric meter. Main lugs simply provide a safe mechanical
connection to the bus bars. A main breaker provides thermal magnetic
protection for the loadcenter.
14.
Backpan
The solid steel piece that supports the loadcenter bus bar assembly. It
also provides the means to affix branch circuit breakers.
15.
Bonding Strap
Used to bond the neutral to the enclosure (when required by code).
16.
Bus Bars (2)
An extension of the incoming power cables. Provide a mechanical
means of affixing branch circuit breakers. Notches are added to (or
omitted from) the bus bars to limit the number of branch circuits
allowed.
17.
Knockouts
Circular indentations in the top, bottom, sides, and back of the
enclosure. When removed, the electrician can bring wire into and out of
the loadcenter without compromising safety.
20.
Neutral Bar
Provides the termination point for the neutral wires from both the
incoming service and the load circuits. Most loadcenters have a twin
neutral design, positioning terminations on both sides of the unit. This
is a wiring convenience for the electrician.
The loadcenter enclosure is typically constructed of cold rolled or galvanized steel.
It serves to house the other loadcenter components. It is designed to provide
component and personnel protection.
The National Electrical Manufacturers Association (NEMA) and UL have established
guidelines for electrical enclosures. They are as follows:
NEMA Type 1
General
Purpose
This enclosure type is for general purpose, indoor use. It 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 gasket cover.
15
LOADCENTERS
Installation and
Mounting
A loadcenter 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.
Practically all loadcenters are flush mounted, which offers some important
benefits such as:
•
Space savings – Since 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 13. FLUSH-MOUNT VS. SURFACE-MOUNT
Surface-mounted loadcenters are generally used in industrial buildings, and in
basements of office and commercial buildings. Since most of these areas have
walls made of poured concrete, flush-mounting is impractical. Steel columns are
often used for surface mounting.
16
LOADCENTERS
Installation and
Mounting
(continued)
The NEC specifies clearances around loadcenters. 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 loadcenter to the
ground shall be 3 feet.
•
For access, there must be 30 inches minimum space in front of the loadcenter,
and sufficient space to allow the hinged door to open and rotate 90 degrees.
6 ½ FEET
HEADROOM
30 INCHES IN FRONT
OF DOOR
SPACE ALLOWANCE
OF 90 DEGREES FOR
DOOR TO OPEN
FIGURE 14. LOADCENTER CLEARANCE REQUIREMENTS
17
LOADCENTERS
CIRCUIT
BREAKERS
Installing Circuit
Breakers
Now, let’s take some time to look at one of the main components of a loadcenter:
the circuit breaker.
Each branch circuit in the loadcenter is protected by a branch circuit breaker (better
known as a miniature circuit breaker). Miniature circuit breakers are covered in
much more detail in Module 9, Miniature Circuit Breakers.
Miniature circuit breakers are also called plug-in breakers because they are
connected by plugging them into the bus bar stabs.
The bus bars provide a convenient means of providing electricity to the various
loads in a building. The circuit breakers attach to the bus bars on vertical stabs
that alternate from each “hot” source.
Single pole and double pole circuit breakers are the most commonly used breakers
in a loadcenter. If a double pole breaker is plugged onto two adjacent stabs,
240 volts are drawn. A single pole breaker is plugged onto a single bus stab
and feeds a 120-volt circuit. To understand how this is possible, consider the
physical shape of the bus bars.
BUS BARS
DOUBLE
POLE
BREAKER
DOUBLE POLE BREAKER
PLUGGED ONTO A STAB
FROM EACH BUS BAR
DRAWS 240 VOLTS
SINGLE POLE BREAKER
PLUGGED ONTO ONLY ONE
STAB
DRAWS 120 VOLTS
SINGLE
POLE
BREAKER
FIGURE 15. GETTING 120V AND 240V POWER FROM THE BUS BARS
18
LOADCENTERS
Now, to reinforce the point, consider Figure 16.
DISTRIBUTION
VOLTAGE
DISTRIBUTION
TRANSFORMER
A
N
B
NEUTRAL
120
120
240
LOADCENTER
NEUTRAL
L
MOTOR
LOAD
LIGHTING
LOAD
M
BUS BARS
FIGURE 16. TYPICAL SINGLE-PHASE, THREE-WIRE LOADCENTER
Trace the 240-volt “M” circuit. Current comes in on conductor “A,” flows through the
left bus bar and branch circuit breaker, and enters the motor load’s branch circuit.
From there, current passes through another branch breaker and out on conductor
“B.” The circuit is made across both ungrounded legs of the transformer.
Trace the 120-volt “L” circuit. Current comes in on conductor “A,” flows through the
left bus bar and branch circuit breaker, and enters the lighting load’s branch circuit.
From there, current passes out on the neutral “N.” The circuit is made from an
ungrounded leg of the transformer to the transformer’s grounded leg.
A 240-volt circuit typically consists of four wires: two "hots", one neutral, and one
ground. A 120-volt circuit consists of three wires: one hot, one neutral, and one
ground. Usually, hot wires are both color-coded black, although the second hot
could be red. Neutral wires are always white. Ground wires are either green or bare
(uninsulated) copper.
19
LOADCENTERS
How Many
Circuit Breakers
Fit in a
Loadcenter?
The number of single-pole circuit breakers that can be installed in a loadcenter
is limited by the number of stabs on the bus bars.
A breaker can be applied to each side (right and left) of a stab. Therefore, a
loadcenter with six stabs would accommodate twelve single-pole circuit breakers.
To balance the loads, the breakers should be evenly applied to each side of the
stab.
BRANCH CIRCUIT
BREAKER
STAB –
NOTCHED TAB ALLOWS
FOR ADDITONAL
BREAKER INSTALLATION
ON LEFT SIDE
BUS BARS
FIGURE 17. BREAKER INSTALLATION (VIEWED FROM ABOVE)
If the stab is notched, then half-size branch circuit breakers can be used. Half-sized
branch circuit breakers allow for two single-pole circuit breakers to be
installed in one space. Each unit is typically only 1/2” wide. These breakers are
every bit as functional as the full-size units and allow the added convenience of a
more compact installation.
Some local inspectors will not allow the use of half-size branch circuit
breakers. This is because there is a possibility of overloading the loadcenter and/or
unbalancing the system.
FIGURE 18. SOLID AND NOTCHED AND STABS
The number of notched and unnotched stabs in a particular loadcenter – and
therefore the number of breakers that may be installed – is limited by UL
requirements. This is to prevent a panel from being overloaded.
Regardless of ampere ratings, a “lighting and appliance” type of loadcenter is
limited to 42 circuits in one enclosure. If the specifications require more than 42
circuits, two or more loadcenters will be required.
20
LOADCENTERS
Interchangeable
vs. NonInterchangeable
Breakers
Branch circuit breakers are available from many manufacturers and have evolved
over the years. Those referred to as interchangeable have 1” wide frames, which
means that a single pole, interchangeable breaker is 1” wide, or takes up a 1” space
in the loadcenter.
While these units, in many cases, will physically fit in another interchangeable
manufacturer’s loadcenter, it is not approved by the NEC, UL, or the manufacturer
of the panel.
Non-interchangeable branch circuit breakers typically have a 3/4” frame width for a
single-pole unit and are unique to a single manufacturer’s loadcenter. These units
cannot be installed into another manufacturer’s loadcenter because they will not
physically fit.
Ratings
Every circuit breaker has a specific ampere, voltage, and fault current
interruption rating.
Ampere rating –The ampere rating defines the maximum current a circuit breaker
can carry without tripping. The typical for miniature circuit breakers is ratings from
15-125 amps.
Voltage rating – In residential applications, single pole breakers protect 120 volt
branch circuits and two-pole breakers protect 240 volt branch circuits. The rating of
a circuit breaker can be higher than the circuit voltage, but never lower.
Short circuit interrupting rating – This is the maximum available fault current that a
breaker is designed to interrupt. Typical ratings is from 10,000 amps to 65,000
amps.
21
LOADCENTERS
Ratings
(continued)
When selecting loadcenters and overcurrent protection devices, it is extremely
important to know both the maximum continuous amperes and available fault
current. 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 service entrance with has 22,000 amps of fault current
available. All downstream circuit protection devices must be rated at 22,000 amps.
The requirements of the series-rated method are somewhat looser. The building’s
main circuit protection device must have an interrupting rating which is at
least equal to the system’s available fault current. However, the all downstream,
series-connected panels can have lower ratings.
Consider the same building, still with 22,000 amps of available fault current. While
the breaker at the service entrance is rated at 22,000 amps, additional downstream
breakers could be rated at only 10,000 amps.
BRANCH BREAKERS
22,000 AMPS
MAIN BREAKER
22,000 AMPS
BRANCH BREAKERS
10,000 AMPS
MAIN BREAKER
22,000 AMPS
FIGURE 19. FULL RATING METHOD (AT LEFT) VS. SERIES-RATED METHOD (AT RIGHT)
In the typical single family home, available fault current is normally 10,000 amps or
less.
22
LOADCENTERS
LOADCENTER There are three different types of loadcenters made today. These are:
TYPES
• Main Breaker
•
Main Lug Only
•
Convertible
MAIN BREAKER
MAIN LUG
CONVERTIBLE
FIGURE 20. MAIN CONFIGURATIONS
Main Breaker
Loadcenter
The incoming supply cables of a main breaker loadcenter are connected to the
main breaker, which in turn feeds power to the loadcenter and its branch
circuits. Power from the mains is fed through the main breaker to the bus bars,
which are an extension of the utility power cables.
With the main circuit breaker in the “off” position, no power will flow to the bus bars
for the branch circuit breakers. This serves as a quick and simple way to disconnect
power to the entire building.
MAINS
MAIN BREAKER
BRANCH CIRCUIT
BREAKERS
BRANCH CIRCUITS
(TO LIGHTS, ETC.)
BUS BARS
FIGURE 21. SIMPLIFIED MAIN BREAKER TYPE LOADCENTER
With the main breaker in the “on” position, power is allowed to flow from the
bus bars to the branch circuits. Branch circuits run throughout the building to
supply power for lighting and appliances. Because of these features, the main
breaker is normally used as a service entrance panel.
23
LOADCENTERS
Main Lug Only
Loadcenter
The main lug only (MLO) loadcenter does not contain a main circuit breaker to
protect the loadcenter itself. Instead, it is protected by a breaker back in an
upstream panel. The supply cables from the upstream panel are connected to
the main lugs and bus bars. This type of loadcenter is used primarily used in a
distribution panel.
The MLO is often called an “add-on,” “secondary,” or “downstream” panel. It is most
often used when the main breaker loadcenter’s circuit slots are full, or to provide
power at a remote point.
Convertible
Loadcenter
The convertible loadcenter is supplied without lugs or breakers. Once the
customer decides to use main lugs or main breakers, the appropriate kit is
installed by an electrician.
IN THE WORKPLACE
In this home, a main breaker
loadcenter supplies power to a
main lug only loadcenter in the
home’s workshop.
Main lug only loadcenters can
also be fed from metering
equipment when used in
apartment installations.
MAIN BREAKER
LOADCENTER
MAIN LUG ONLY
LOADCENTER
USING A MAIN LUG ONLY LOADCENTER
24
LOADCENTERS
SELECTING A When assisting a customer in selecting a loadcenter for an application, it is best to
LOADCENTER start by interviewing the customer to determine the application’s requirements.
Required
Application
Information
1. Ask whether this application will involve an indoor or outdoor installation. This
will determine the enclosure type for the loadcenter.
NEMA Type 1
General
Purpose
This enclosure type is for general purpose, indoor use. It 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.
If it is an indoor installation, ask whether it will be flush-mounted or surface
mounted.
FIGURE 22. OUTDOOR, FLUSH AND SURFACE MOUNTING TYPES
2. Ask whether the loadcenter is to serve as a service entrance panel or a
distribution panel.
3. Ask whether the application involves a single-phase, three-wire system or a
three-phase, four-wire system.
4. Ask what type of main, either a Main Lugs Only or a Main Circuit Breaker.
5. Ask for the main ampere rating.
6. Ask what type and rating main breaker is required, if applicable.
7. Ask for the number of breakers required, and for their ratings.
8. Ask how many spaces are required. The customer may want to leave extra
spaces in the loadcenter for future needs.
Once this information is obtained, go to the product catalog. These questions
should cover everything needed to recommend products that fit the customer’s
application needs.
25
LOADCENTERS
Sample
Application
To assist you in visualizing the product selection process, let’s consider a simple,
commonplace customer application.
Suppose a customer comes to you and wants to add two branch circuit breakers to
an existing loadcenter for a newly remodeled kitchen. How would you go about
sizing the breaker?
First, interview the customer to find out what loads will be on the branch circuits.
Let’s say that one will be dedicated to a dishwasher, and the other will be dedicated
to a garbage disposal.
Next, obtain the amperage requirements of the loads from the customer. Suppose
the dishwasher runs on 12.5 amps and the garbage disposal runs on 9.8 amps.
Based on these numbers alone, you might think that it would be a good idea to
recommend a 15 amp breaker for the dishwasher circuit, and a 10 amp breaker for
the garbage disposal circuit. But this would result in a lot of nuisance tripping.
So, we need to do a little simple math. Figure in a 25% safety margin by multiplying
the amp ratings by 1.25.
Dishwasher
12.5 amps x 1.25 = 15.625 amps
Garbage Disposal
9.8 amps x 1.25 = 12.25 amps
This will be sufficient to eliminate nuisance tripping, but the breakers will still trip in
the case of a true overload condition.
Based on these calculations, you would recommend a 20 amp breaker for the
dishwasher circuit, and a 15 amp breaker for the garbage disposal circuit.
26
LOADCENTERS
REVIEW 2
Answer the following questions without referring to the material just presented.
1. List the three main loadcenter types made today.
•
____________________________________________________________
•
____________________________________________________________
•
____________________________________________________________
2. In your own words, explain why a double pole breaker draws twice the voltage
of a single pole breaker installed in the same panel.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
3. Regardless of ampere ratings, a “lighting and appliance” type of loadcenter is
limited to _____ circuits in one enclosure.
4. List four application questions to ask when working with a customer.
•
____________________________________________________________
•
____________________________________________________________
•
____________________________________________________________
•
____________________________________________________________
27
LOADCENTERS
GLOSSARY
28
Bonding
The permanent joining of metallic parts to form an
electrically conductive path that will ensure electrical
continuity to ground.
Branch Circuit
A circuit that supplies power to a load in a building and is
terminated at the loadcenter.
Branch Circuit
Breaker
Also “Miniature Circuit Breaker.” A reusable overcurrent
protection device, used to protect a branch circuit. After
tripping to break the circuit, it can be reset to protect the
branch circuit again.
Bus Bars
A component of a loadcenter that serves as an extension of
the main service conductors. Simplifies the connection of
branch circuit breakers to the main service conductors.
Circuit Breaker
A reusable overcurrent protection device. After tripping to
break the circuit, it can be reset to protect the circuit again.
Convertible
Loadcenter
A loadcenter supplied without main lugs or a main breaker.
Once in the field, it is up to the electrician to install the
appropriate device.
Distribution Panel
A loadcenter used at a point beyond the building’s service
entrance. It is not supplied with a main breaker. This type of
panel can be useful when adding additional electrical
service to an existing building. It has an isolated neutral.
Distribution
Transformer
A device that converts utility voltage into a voltage and
current supply suitable for use in most residential
applications.
Double Pole
Term used to describe a breaker that draws power from
both poles of a loadcenter.
Equipment
Grounding Bus
A bus connected directly to the enclosure. Used for
grounding all feeder and branch circuit equipment.
Feed-Through
Lugs
A set of lugs in a service entrance panel where feeder
cables for a distribution panel are terminated.
Flush-Mounted
A style of mounting a loadcenter in which the loadcenter is
recessed into a hole in the wall.
Full Rating
Method
A method of selecting circuit protection devices for use in a
loadcenter. All devices must have ratings equal to or
greater than the available fault current.
LOADCENTERS
Fuse
A non-reusable overcurrent protection device. After tripping
to break the circuit, it must be replaced to restore power to
the circuit.
Ground Fault
Current leakage from an ungrounded conductor to the
grounding path in an electrical system
Grounding Path
A solid conducting path for electricity to follow to ground.
Half-Size Branch
Circuit Breakers
A specialized reusable overcurrent protection device
designed to take up only half as much space in a
loadcenter as a normal branch circuit breaker. Can only be
installed in loadcenters equipped with notched stabs.
Interchangeable
A type of branch circuit breaker that is standardized in size
to fit in many different manufacturers’ loadcenters.
Loadcenter
A device that delivers electricity from a supply source to
loads in light commercial or residential applications.
Mains
Also “Main Service Conductors.” The conductors that bring
electricity into a building from the power source.
Main Circuit
Breaker
Also “Main Breaker.” A reusable overcurrent protection
device designed to protect an entire loadcenter.
Main Lugs
Component of a loadcenter not equipped with a main
breaker. Termination point for a feeder cable from another
loadcenter.
Main Lug Only
(MLO) Loadcenter
A loadcenter where power from the mains is fed directly to
the bus bars.
Main Service
Conductors
Also “Mains.” The conductors that bring electricity into a
building from the power source.
Miniature Circuit
Breaker
Also “Branch Circuit Breaker.” A reusable overcurrent
protection device, used to protect a branch circuit. After
tripping to break the circuit, it can be reset to protect the
branch circuit again.
29
LOADCENTERS
30
NonInterchangeable
A type of branch circuit breaker that is unique to a single
manufacturer’s loadcenters. This type of breaker cannot be
installed into another manufacturer’s loadcenter because it
will not physically fit.
Plug-In
A style of miniature circuit breaker, so named for the
method of installation into the loadcenter. It is literally
plugged into the bus bar stabs.
Series-Rated
Method
A method of selecting circuit protection devices for use in a
loadcenter. The main upstream circuit protection device
must have an interrupting rating equal to or greater than the
available fault current of the system. Downstream devices
connected in series can be rated at lower values.
Service Entrance
Panel
The term used to describe a loadcenter used as a service
entrance.
Single Pole
Term used to describe a breaker that draws power from
only one pole of a loadcenter.
Stab
A protrusion on the bus bars of a loadcenter which accepts
a miniature circuit breaker.
Surface-Mounted
A style of mounting a loadcenter in which the loadcenter is
attached to and projects out from the wall.
Terminated
The end connection of a circuit. For example, branch
circuits are terminated at the service entrance panel.
Voltmeter
A device used to find the potential voltage between two
points.
LOADCENTERS
REVIEW 1
ANSWERS
1.
•
•
distribution panel
service entrance panel
•
•
residential
commercial / light industrial
2.
3. neutral, insulated, isolated
4. Answer should basically say “If all the loads in a panel are connected exactly
evenly between A-N and B-N, no current will flow in the neutral. The amount
current flowing in the neutral conductor at any one time is the difference
between the current flowing through leg A and leg B.”
5. Answer should basically say “To supply power to lights, receptacles and loads
like dryers, washers, and air conditioners.”
REVIEW 2
ANSWERS
1.
•
•
•
Main Breaker
Main Lug Only
Convertible
2. Answer should basically say “A double pole breaker is plugged onto two
adjacent bus stabs. The circuit is made across both ungrounded legs of the
transformer. A single pole breaker is plugged onto a single bus stab. The circuit
is made from an ungrounded leg of the transformer to the transformer’s
grounded leg.”
3. 42
4. Any four of the following:
• Indoor or outdoor installation?
• If indoor, flush-mount or surface-mount?
• Service entrance panel or a distribution panel?
• Single-phase, three-wire system or a three-phase, four-wire system?
• What type of main, Main Lugs Only or Main Circuit Breaker?
• Main ampere rating?
• What type and rating main breaker is required? (if applicable)
• How many branch breakers required, and the rating of each?
• How many spaces are required?
31
Cutler-Hammer
Milwaukee, Wisconsin U.S.A.
Publication No. TR.31.01.T.E
February 1999
Printed in U.S.A. (GSP)
101 Basics Series and 201 Advanced Series are trademarks of Cutler-Hammer University, Cutler-Hammer and Eaton Corp.
©1999, Eaton Corp.