SCCR Codes and Standards
Including:
Basic Principles
NEC® 409 and UL 508A
How to Determine the SCCR of an Industrial Enclosure
Agenda
Introduction and Basics
National Electric Code 2005
Underwriters Laboratories
Determining SCCR
How to Increase SCCR
Mersen Tools
2
Definitions
Short Circuit Current
– An overcurrent usually defined as being in excess of ten times normal
continuous rating usually caused by insulation breakdown or wiring
error
Short Circuit Current Rating (SCCR)
– The prospective symmetrical fault current at a nominal voltage to
which an apparatus or system is able to be connected without
sustaining damage exceeding the defined acceptance criteria
Available Fault Current
– The maximum short circuit current that could flow in an unprotected
circuit
3
Q
Why is SCCR important?
&
A
SCCR is now an important factor when considering UL 508A and
the NEC. SCCR is important to you because:
– By knowing about SCCR and available fault currents you will create
safer working conditions for the personnel who work on or around
electrical equipment
– You will be able to ensure compliance with the NEC and UL
– You can simplify your inspection approval process
4
Q
&
A
Who will be affected?
Everybody
Industrial Facilities
Manufacturing Facilities
Contractors
Panel Builders
System Integrators
OEMs
Etc…
5
Q
&
A
When will the changes be enforced?
The changes are already in place
NEC® Article 409 – Industrial Control Panels
– Introduced in the 2005 NEC, “Industrial Control Panels”
require panels to be clearly marked with a short circuit
current rating, SCCR
UL 508A – Industrial Control Panels
– Listed panels will require the SCCR to be clearly
marked on the panel effective April 25, 2006
6
National Electric Code – 2011
The National Electric Code is a standard for the safe installation
of electrical wiring and equipment
Part of the NFPA (National Fire Protection Association)
Not a U.S. Law but is mandated by local or state officials
Articles Related to SCCR
– Article 230
– Article 409
– Article 670
7
NEC Article 230 - Services
230.1 - Scope
– This article covers service conductors and equipment for control
and protection of services and their installation requirements
230.82– Equipment Connected to the Supply Side of Service
Disconnect
– Only the following equipment shall be permitted to be connected to
the supply side of the service disconnecting means:
• (3) – Meter disconnect switches nominally rated not in excess of 600
volts that have a short circuit current rating equal to or greater than
the available short circuit current,…
8
NEC Article 409 – Industrial Control Panels
409.1 – Scope
– This article covers industrial control panels intended for general use
and operating at 600 volts or less
409.110 – Marking
– An industrial control panel shall be marked with the following
information that is plainly visible after installation:
• (4) Short circuit current rating of the industrial control panel based on
one of the following:
– a. Short circuit current rating of a listed and labeled assembly
– b. Short circuit current rating established utilizing an approved method
9
NEC Article 430 – Motors, Motor Circuits,
and Controllers
430.1 – Scope
– This article covers motors, motor branch circuit and feeder
conductors and their protection, motor overload protection, motor
control circuits, motor controllers, and motor control centers
430.7(D) – Multimotor and Combination Load Equipment
430.8 – Markings
– A controller shall be marked with the manufacturer’s name or
identification, the voltage, the current or horsepower rating, the
short circuit current rating, and other necessary data to properly
indicate the applications for which it is suitable
10
NEC Article 670 – Industrial Machinery
670.1 – Scope
– This article covers the definition of, the nameplate data for, and the
size and overcurrent protection of supply conductors to industrial
machinery
670.3 – Machine Nameplate Data
– (A) Permanent Nameplate. A permanent nameplate shall be
attached to the control equipment enclosure or machine and
shall be plainly visible after installation. The nameplate shall include
the following information: (1)…(5)
• (4) Short circuit current rating of the machine industrial control panel
based on one of the following:
– a. Short circuit current rating of a listed and labeled machine control
enclosure or assembly
– b. Short circuit current rating established utilizing and approved method
11
Underwriters Laboratories
Independent Testing Company
Offer 3rd Party Certification
Consistent with NEC
Relative Articles
–
–
–
–
–
–
UL 98 Disconnect Switch
UL 248 Low Voltage Fuses
UL 508 Industrial Control Equipment
UL 512 Fuse Blocks
UL 1059 Terminal Blocks (PDB)
UL 1449 Surge Protective Devices
12
UL 508A – Industrial Control Panels
Guideline for construction and general design
Manufacturers that adhere to the requirements are eligible for
UL508A Listing of their product
Four Parts of UL 508A
– Part 1: General Use
– Part 2: Specific Use
• Enclosures, Elevator Control, Industrial Machinery, Flame Control,
Crane Control, Marine Use, Service Equipment Use, AC and
Refrigeration
– Part 3: Specific Component Requirements
– Part 4: Short Circuit Current Ratings
13
UL 508A – Industrial Control Panels
Industrial control panel assemblies:
– May include motor controllers, overload relays, fused disconnects,
circuit breakers, pushbuttons, switches, timers, indicators, wiring,
terminals, enclosures, etc…
– Will be rated 600V or less
– Where ambient temperatures do not exceed 40°C
UL 508A does not include panels for Hazardous Locations
“NRBX”, which are covered under UL 698A
14
Q
&
A
How do you determine the SCCR of an
industrial control panel?
Four Step Process
15
It’s as easy as 1…2…3
Step 1: Assign a SCCR to each component in the power circuit
Step 2: Assign a SCCR to each branch circuit
Step 3: Assign a SCCR to the feeder circuit
Step 4: Consider the current-limiting effects of the feeder overcurrent protection devices (OCPD)
Before we apply the standard:
Power Electrical Symbols
Power Circuit vs. Control Circuit
Branch Circuit vs. Feeder Circuit
16
Power Electrical Symbols
NO
Fuse
Fused Disconnect
Switch
Power
Transformer
NC
Relay
NC
NO
Circuit Breaker
Motor Starter
17
Electric Motor
Pushbutton
Control Circuit vs. Power Circuit
Control
Circuit
Control
Circuit
Power Circuit
Power Circuit
18
Branch Circuit vs. Feeder Circuit
Feeder
Circuit
Branch Circuit #1
19
Branch Circuit #2
Applying the Standard
Let’s analyze an example circuit
20
Example Circuit
21
Step 1: Assign a SCCR to each component
in the power circuit
SCCR of a component shall be established by one of the
following:
1. SCCR marked on the component or on instructions provided with
the component
2. SCCR can be determined by the voltage rating of the component
and the assumed short circuit current from Table SB4.1
3. SCCR for a load controller, motor overload relay, or combination
motor controller can be determined by type testing under the
procedure specified by UL508 and as described in the
manufacturers procedure
22
Q
&
What Components Do I Look For?
All Power Circuit Components
A
Including disconnect switches, branch circuit protective devices,
branch circuit fuse holders, load controllers, motor overload
relays, terminal blocks, and bus bars, shall have a short circuit
current rating expressed in amperes or kilo amperes and volts
Exception No. 1: Power transformers, reactors, current
transformers, dry-type capacitors, resistors, varistors, and
voltmeters are not required to have a short circuit current rating
23
Q
&
A
Where do I find the SCCR of my
components?
On the product label or in the product
manual
In today’s industry short circuit current ratings are usually listed
in kilo-amperes, or kA
24
If the component SCCR is not listed on the product label or in the product
manual use Table SB4.1
25
Mersen Resources for Determining SCCR
26
200kA
100kA
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
27
Unknown
12A (5HP = 3.7kW) Motor Starter does not have a labeled SCCR and is also
not listed in product manual
28
200kA
100kA
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
29
SB4.1
5kA
Step 2: Assign a SCCR to each Branch
Circuit
Make a pass of each branch circuit
– The lowest component SCCR value will be equal to the branch
SCCR
In this case we have three branch circuits to analyze
30
200kA
Branch 1
100kA
Branch 2
Branch 3
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
31
SB4.1
5kA
Branch 1 First Pass SCCR =
10kA
Branch 1
200kA
10kA
32
Branch 2 First Pass SCCR =
5kA
Branch 2
200kA
NA
200kA
5kA
33
Control
Circuit
Branch 3 First Pass SCCR =
5kA
Branch 3
65kA
SB4.1
5kA
34
Step 3: Assign a SCCR to the Feeder Circuit
Make a pass of the feeder circuit and determine the lowest
SCCR value
35
Feeder Circuit SCCR =
100kA
Feeder
200kA
200kA
100kA
36
Q
What if we stopped now?
&
A
The panel SCCR would be equal to that of the
component with the lowest SCCR; in this
case 5kA (Electronic Motor Controller & 12A
Motor Starter)
Step 4 is pivotal when attempting to increase panel SCCR
UL 508A, SB4.3 – Feeder Components that limit the short circuit
current available
– Power Transformer
– Circuit Breaker
– Fuse
Compare the Ip of the feeder OCPD to the SCCR of the feeder and
each branch circuit
– If the Ip of the feeder OCPD is not greater than the SCCR of the
feeder or branch then the feeder or branch will have an SCCR equal
to the appropriate Ip column on SB4.2
37
Step 4: Consider the current-limiting effects
of the feeder over-current protection devices
Power Transformer
– Use SB4.3.1
Circuit Breaker
– The panel builder must select the current limiting breaker based on
the published peak let-thru curve data provided by the breaker
manufacturer. Listed breakers rated 15A or 20A, 600V or less, also
labeled “current limiting” are able to limit the current to 5kA and
10kA respectively.
Fuse
– Refer to UL 508A Table SB4.2 to determine the Ip of the fuse
– Class CC, G, J, L, RK1, RK5 or T
38
Feeder OCPD
In this example the feeder
overcurrent protection device
is an AJT60
–
–
–
–
Class J
60A
600V
Time Delay
200kA
Feeder
200kA
100kA
39
Class J 60A
– 50kA peak  Ip = 8kA
– 100kA peak  Ip = 10kA
– 200kA peak  Ip = 16kA
40
Ip = 10kA @ 100kA
200kA
Branch 2
= 5kA
Branch 1
= 10kA
Branch 3
= 5kA
100kA
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
41
SB4.1
5kA
Branch 1 Second Pass SCCR =
100kA
Branch 1
=
=100kA
10kA
Ip = 10kA @ 100kA
200kA
10kA
42
Ip = 10kA @ 100kA
200kA
Branch 1
= 100kA
Branch 2
= 5kA
Branch 3
= 5kA
100kA
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
43
SB4.1
5kA
Panel SCCR is still 5kA
Summary
–
–
–
–
Feeder SCCR = 100kA
Branch 1 SCCR = 100kA (was 10kA)
Branch 2 SCCR = 5kA
Branch 3 SCCR = 5kA
Branches 2 and 3 are limiting our panel SCCR
Lets examine Branch 2 in more detail…
44
Branch 2
In this case the Power
Transformer is a
component that limits short
circuit current available
Using UL508A SB4.3.1 we
can determine the current
limiting capabilities of the
power transformer
Branch 2
= 5kA
200kA
NA
200kA
Power Transformer Specs
– 5KVA
– 480V – Pri
– 120V – Sec
45
5kA
Control
Circuit
The current limiting effects of a Power
Transformer
UL508A SB4.3.1
– For branch circuit supplied by a power transformer with an isolated
secondary winding, the short circuit current rating on the line side of
the transformer shall be one of the following:
XFMR Rating
less than or
= 10kVA
less than or
= 5kVA
greater than
10kVA
Secondary Voltage
Secondary Devices
SCCR on line side of transformer
not specified
greater than or = 5kA
Use rating on primary overcurrent device
120V maximum
greater than or = 2kA
Use rating on primary overcurrent device
-
-
Use lowest SCCR of secondary current
components
46
Branch 2 Second Pass SCCR =
200kA
According to SB4.3.1 our
branch power transformer
will limit the let-thru current
to 2kA
Branch SCCR is equal to
our primary overcurrent
device
Branch 2
200kA
= 5kA
200kA
NA
200kA
– ATQR25
5kA
47
Control
Circuit
Panel SCCR is 5kA
Ip = 10kA @ 100kA
Limiting Factor is
Branch 3
200kA
Branch 2
= 200kA
Branch 1
= 100kA
Branch 3
= 5kA
100kA
200kA
NA
200kA
200kA
10kA
65kA
Control
Circuit
200kA
5kA
48
SB4.1
5kA
Panel SCCR is still 5kA
Summary
–
–
–
–
Feeder SCCR = 100kA
Branch 1 SCCR = 100kA (was 10kA)
Branch 2 SCCR = 200kA (was 5kA)
Branch 3 SCCR = 5kA
Branch 3 is the limiting factor in our panel!
49
Q
1.
2.
3.
4.
&
A
What can I do to increase my panel
SCCR?
There are four methods that could be
utilized
Upgrade to components with higher SCCR
Add sub-feeder fuses
Upgrade feeder overcurrent protection device with a higher
current limiting device
Consider type tested components, i.e., components that have
been tested by the manufacturer under specific conditions and
witnessed by UL
50
Option 1: Upgrade to components with
higher SCCR
Solution
Branch 3
5kA
==65kA
– Replace the un-labeled motor starter
with one that has a SCCR of 10kA
65kA
Now we can utilize the current
limiting capabilities of the feeder
OCPD
Ip = 10kA @ 100kA
SB4.1
10kA
5kA
51
Option 2: Add a sub-feeder fuse
By adding a sub-feeder fuse,
the standard allows you to
use this fuse as your current
limiting feeder protection
Branch 3
= 5kA
65kA
In this case lets add a Class
CC 20A Fuse
– ATDR20
SB4.1
5kA
52
Class CC 20A
– 50kA peak  Ip = 3kA
– 100kA peak  Ip = 4kA
– 200kA peak  Ip = 5kA
53
Option 2: Add a sub-feeder fuse
Class CC, 20A
Branch 3
=
=65kA
5kA
– ATDR20
– Ip = 4kA @ 100kA Fault
65kA
This sub-feeder fuse will allow
the panel SCCR to be
increased to 65kA because
once again the MCCB
becomes the limiting device
SB4.1
5kA
54
Option 3: Upgrade feeder OCPD
200kA
Feeder
Not applicable for this example.
We are using the most current
limiting device
Use option 3 if you are using a
circuit breaker as your feeder
overcurrent protective device
200kA
100kA
55
Option 4: Consider type tested components
Some manufacturers “type test”
fuses or circuit breakers with their
motor starters to achieve a higher
SCCR rating when used in
combination
Each major controller manufacturer
type tests their components on an
annual basis. Results are available
on UL.com
Branch 3
= 5kA
65kA
SB4.1
5kA
56
Option 4: Consider type tested components
Example:
– 12A Motor Starter and 15A MCCB are both Schneider products
(Square D, Telemecanique)
– 12A IEC Contactor
• Part # LC1D12
– OVLD Relay, 5.5–8A
• Part # LRD12
– 15A Circuit Breaker
• Part # GJL36015
57
UL provides links to each manufacturers website displaying the SCCR testing
results. (www.ul.com/controlequipment/shortcircuit.html)
Com ponent
Type
Com ponent
Manufacturer
ICB
MC
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
Square D
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
GJL36015M03
LC1D12
LRD10
LRD12
LRD14
LRD16
LRD1510
LRD1512
LRD1514
LRD1516
F
MC
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
OLR
Any
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Telemecanique
Class CC
LC1D12
LRD01
LRD02
LRD03
LRD04
LRD05
LRD06
LRD07
LRD08
LRD10
LRD12
LRD14
LRD16
LRD1508
LRD1510
LRD1512
LRD1514
LRD1516
58
Com ponent Catalog Designation
Com ponent
kA
Com ponent
Am ps
Com bination Com bination
kA
Volts
5
5
5
5
5
5
5
5
5
15
12
4-6
5.5 - 8
7 - 10
9 - 13
4-6
5.5 - 8
7 - 10
9 - 13
65
480
200
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
30
12
0.10 - 0.16
0.16 - 0.25
0.25 - 0.40
0.40 - 0.63
0.63 - 1
1 - 1.6
1.6 - 2.5
2.5 - 4
4-6
5.5 - 8
7 - 10
9 - 13
2.5 - 4
4-6
5.5 - 8
7 - 10
9 - 13
100
480
This information is also available by viewing the Mersen USFM vs. Circuit
Breaker Comparison Sheet available on our website
59
Option 4: Consider using type tested
components
Replace the 15A circuit breaker
with the USFMCCI and the
ATDR20
Branch 3
==100kA
5kA
200kA
The USFMCCI + ATDR25
combined with the Telemecanique
Motor Starter yields a SCCR of
100kA
5kA
60
Panel SCCR is now 100kA
Summary
–
–
–
–
Feeder SCCR = 100kA
Branch 1 SCCR = 100kA (was 10kA)
Branch 2 SCCR = 200kA (was 5kA)
Branch 3 SCCR = 100kA (was 5kA)
By using option 4 we were able to remove the circuit breaker in
Branch 3 and replace it with fusing in order to increase our
panel SCCR
61
Questions and Answers
62