NEC® Article 409 and UL 508A
Short-Circuit Current Ratings
(SCCR)
1
Using Fuses to Increase Your SCCR
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What is a SCCR?
• A Short Circuit Current Rating is the maximum current a
device or panel can safely withstand for 3 AC electrical
cycles (50 msec.) or while an overcurrent protective device
operates.
• SCCR is usually expressed in RMS kA at the system
voltage. Example: 18 kA at 480 VAC
• RMS is the Root Mean Squared AC current.
• The SCCR of a fuse or circuit breaker is the same as its
Interrupting Rating (IR).
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NEC® Article 409 - Industrial Control Panels
• Article 409 was added to the NEC® in 2005
• Most states and Authorities Having Jurisdiction (AHJs)
have adopted the 2005 NEC.
• Article 409 requires all industrial control panels to be
marked with an SCCR.
National Electrical Code and NEC are registered trademarks of
the National Fire Protection Association (NFPA), Quincy, MA.
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NEC SCCR Requirements
• The 2005 NEC requires SCCR markings on all:
ƒ
Industrial Control Panels - NEC Article 409
ƒ
Motor controllers - NEC Article 430.8
ƒ
HVAC controllers - NEC Article 440.4(B)
ƒ
Industrial machinery - NEC Article 670.3
ƒ
Meter disconnects - NEC Article 230.82(3)
National Electrical Code and NEC are registered trademarks of
the National Fire Protection Association (NFPA), Quincy, MA.
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SCCR of Industrial Control Panels
• The SCCR of a panel is not necessarily equal to the
interrupting rating of the main overcurrent protective
device (fuse or circuit breaker).
• The SCCR of a panel is dependent on the “weakest link”
rating within the panel.
• The SCCR of the Industrial Control Panel must be
greater than the maximum possible fault current at
the panel.
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How Do You Determine SCCR?
• NEC Article 409.110 states that the SCCR of an
industrial control panel is the:
• SCCR of a Listed and labeled assembly, or
• SCCR based on use of an “approved” method
ƒ
ƒ
UL 508A Supplement SB is given as an example of an approved
method
– As of April 25, 2006, all UL 508A Listed panels must be
marked with SCCR
Method approved by local electrical inspector (AHJ)
– Engineer approved Apparent RMS method.
National Electrical Code and NEC are registered trademarks of
the National Fire Protection Association (NFPA), Quincy, MA.
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UL 508A Supplement SB
•
Provides method to determine SCCR of panel based on the types
and ratings of components.
• Does not require UL testing.
ƒ
However, as of April 25, 2006, UL 508A Listed panels must use as default
method.
• SCCR is based on SCCR of lowest rated component, or
• If current-limiting devices (i.e. fuses) are used in the feeder circuit, the
SCCR rating of the panel may be higher than the lowest rated
component.
Current-limiting fuses
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UL 508A Supplement SB SCCR Method
UL 508A Supplement SB describes three steps to
determine the SCCR of industrial control panels:
1. Determine the SCCR of all individual power components and UL
2.
3.
approved series ratings.
Modify the SCCR of certain series combinations if current-limiting
devices are used in the feeder circuit.
The SCCR of the panel is equal to the lowest rated SCCR of any:
–
–
–
–
Overcurrent protective device
UL Approved series SCCR rating
Modified combinations from Step 2
Power component not affected by Step 2
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8
Example
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UL 508A Supplement SB SCCR Method
UL 508A Supplement SB describes three steps to
determine the SCCR of industrial control panels
1. Determine the SCCR of all individual power components
and UL approved series ratings.
2. Modify the SCCR of certain series combinations if current limiting
3.
devices are used in the feeder circuit.
The SCCR of the panel is equal to the lowest rated SCCR of any:
–
–
–
–
Overcurrent protective device
UL Approved series SCCR rating
Modified combinations from Step 2
Power component not affected by Step 2
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10
UL 508A Supplement SB – Step 1 of 3
Step 1 - Determine the SCCR of all power components and
UL approved series ratings.
There are 4 sub-steps to Step 1:
a. Examine all individual components and record their
SCCRs.
b. If SCCR is not marked or known, use UL508A default
Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each
branch circuit.
Let’s begin with Step 1a…
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UL 508A Supplement SB – Step 1 of 3
Step 1 - Determine the SCCR of all power components and
UL approved series ratings.
There are 4 sub-steps to Step 1:
a. Examine all individual components and record their
SCCRs.
b. If SCCR is not marked or known, use UL508A default
Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each
branch circuit.
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UL 508A Supplement SB – Step 1 of 3
Step 1a.
Examine all individual
components and
record their SCCRs.
Branch Circuit #1
Branch Circuit #2
Branch Circuit #3
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13
UL 508A Supplement SB – Step 1 of 3
Step 1 - Determine the SCCR of all power components and
UL approved series ratings.
There are 4 sub-steps to Step 1:
a. Examine all individual components and record their
SCCRs.
b. If SCCR is not marked or known, use UL508A default
Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each
branch circuit.
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UL 508A Supplement SB – Table SB4.1
Step 1b.
If SCCR is not
marked or known,
use UL508A default
Table SB4.1
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UL 508A Supplement SB – Step 1 of 3
Step 1b.
If SCCR is not
marked or known,
use UL508A default
Table SB4.1
10 kA
Branch Circuit #1
Branch Circuit #2
Branch Circuit #3
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UL 508A Supplement SB – Step 1 of 3
Step 1 - Determine the SCCR of all power components and
UL approved series ratings.
There are 4 sub-steps to Step 1:
a. Examine all individual components and record their
SCCRs.
b. If SCCR is not marked or known, use UL508A default
Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each
branch circuit.
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UL 508A Supplement SB – Step 1 of 3
Step 1c. - Check and record any UL certified and approved
series ratings.
•
The contactor in Branch Circuit #2
with a marked SCCR of 5 kA is
UL Listed with a SCCR of 100 kA
and provides “Type 2” no damage
coordination when protected by
UL Class J or CC fuses.
Branch Circuit #2
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UL 508A Supplement SB – Step 1 of 3
Step 1 - Determine the SCCR of all power components and
UL approved series ratings.
There are 4 sub-steps to Step 1:
a. Examine all individual components and record their
SCCR’s.
b. If SCCR is not marked or known, use UL508A default
Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each
branch circuit.
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19
Step 1d. - Feeder Circuit
before Step 1
after Step 1
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20
Step 1d. - Branch Circuit #1
before Step 1
after Step 1
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21
Step 1d. - Branch Circuit #2
before Step 1
after Step 1
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22
Step 1d. - Branch Circuit #3
before Step 1
after Step 1
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Example – after Step 1
Step 1d.
Note the minimum
SCCR in the feeder
circuit and each
branch circuit.
Branch Circuit #1
Branch Circuit #2
Branch Circuit #3
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24
UL 508A Supplement SB SCCR Method
UL 508A Supplement SB describes three steps to determine the
SCCR of industrial control panels
1. Determine the SCCR of all individual power components and UL
approved series ratings.
2. Modify the SCCR of certain series combinations if
current limiting devices are used in the feeder circuit.
3. The SCCR of the panel is equal to the lowest rated SCCR of any:
–
–
–
–
Overcurrent protective device
UL approved series SCCR rating
Modified combinations from Step 2
Power component not affected by Step 2
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25
UL 508A Supplement SB – Step 2 of 3
Step 2 - Modify the SCCR of certain series combinations if
current-limiting devices are used in the Feeder Circuit.
• If current limiting fuses are used in the feeder
circuit, compare the Ipeak of the fuse from UL 508A
Table SB4.2 to the lowest rated SCCR component
or minimum SCCR in each branch circuit.
• The Ipeak of the fuse must be less than or equal to
the lowest rated SCCR of any component or SCCR
in each branch circuit.
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UL 508A Supplement SB – Table SB4.2
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UL 508A Supplement SB – Step 2 of 3
Step 2 - Modify the SCCR of certain series combinations if
current-limiting devices are used in the Feeder Circuit.
Question: What is the Ipeak of the 60 Ampere current limiting
Class J fuses used the feeder circuit?
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UL 508A Supplement SB – Table SB4.2
So for our Example…
Available
RMS fault
current
Maximum
Ipeak
See UL 508A Supplement SB Table SB4.2 for complete listing of all Current Limiting Fuses
Ip x 103 (or Ipeak) values listed in Table SB4.2 represent available RMS fault currents.
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UL 508A Supplement SB – Step 2 of 3
Step 2 - Modify the SCCR of certain series combinations if
current-limiting devices are used in the Feeder Circuit.
Question: What is the Ipeak of the 60 Ampere current limiting
Class J fuses?
Answer:
• The maximum Ipeak of the 60 Amp Class J fuses at an
available RMS fault current of 200 kA is 16 kA
• The maximum Ipeak of the 60 Amp Class J fuses at an
available RMS fault current of 100 kA is 10 kA
• The maximum Ipeak of the 60 Amp Class J fuses at an
available RMS fault current of 50 kA is 8 kA
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30
Step 2 continued
Step 2 - Modify the SCCR of certain series combinations if
current-limiting devices are used in the Feeder Circuit.
Question: What is greatest available fault current the 60 Ampere
Class J fuses will limit the Ipeak to 10 kA or less?
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Step 2 continued
Step 2 - Modify the SCCR of certain series combinations if
current-limiting devices are used in the Feeder Circuit.
Question: What is greatest available fault current the 60 Ampere
Class J fuses will limit the Ipeak to 10 kA or less?
Answer: The 60 Ampere Class J fuses will limit the maximum Ipeak
to 10 kA when the available fault current is 100 kA.
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Step 2 continued
•
Therefore, the modified SCCR of the power distribution block with
60 Amp Class J fuses is 100 kA.
before Step 2
after Step 2
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As a result of Step 2…
After Step 2, the
minimum SCCR of
the Feeder and
Branch Circuits are
now as shown on
this circuit diagram.
Branch Circuit #1
Branch Circuit #2
Branch Circuit #3
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34
UL 508A Supplement SB SCCR Method
UL 508A Supplement SB describes three steps to determine the
SCCR of industrial control panels
1. Determine the SCCR of all individual power components and UL
2.
approved series ratings.
Modify the SCCR of certain series combinations if current limiting
devices are used in the feeder circuit.
3. The SCCR of the panel is equal to the lowest rated
SCCR of any:
–
–
–
–
Overcurrent protective device
UL approved series SCCR rating
Modified combinations from Step 2
Power component not affected by Step 2
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As a result of Step 3…
Since Branch
Circuit #1 contains
the lowest rated
SCCR, the SCCR
of the entire panel
is 14 kA.
Branch Circuit #1
Branch Circuit #2
Branch Circuit #3
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36
SCCR Label for our Example
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37
How to Increase Short-Circuit Current Ratings
1.
2.
3.
Use components and approved combinations with
higher rated SCCRs.
Use current limiting Class J, RK1, or Class CC
fuses whenever possible.
Replace low interrupting rated overcurrent
protection with higher rated fuses and fusible
switches.
• According to UL 508A Articles SB3.2 and SB4.4, the
SCCR of the panel cannot be greater than the interrupting
rating of any overcurrent protective device in branch
circuits or in the primary of control circuits.
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How to Increase Short-Circuit Current Ratings
• In the example given, if the MCCB in Branch Circuit #1
was replaced with a fusible disconnect switch such as
Littelfuse LFFS030CC with Littelfuse Class CC fuses,
the minimum SCCR of Branch Circuit #1 would
increase to 50 kA, and the SCCR of the entire panel
would increase to 50 kA.
LFFS030CC
Disconnect Switch
UL Class CC fuses
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How to Increase Short-Circuit Current Ratings
• If UL 508A Supplement SB method does not yield
desired results,
• Consider Listing the panel by a NRTL (Nationally Recognized
Testing Laboratories)
• If acceptable to the AHJ, consider approval using the
equivalent available RMS (up-over-and-down) method by a
registered PE.
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Important Safety Reminder
• OSHA 1910.132(d) requires employers to identify electrical
hazards in the workplace, inform and train workers how to
avoid hazards, and provide them with personal protective
equipment.
• Fault current studies must be performed regularly and
compared with the SCCR’s of all industrial control panels
and equipment…
• When industrial control panels are installed, moved, or modified.
• When Arc-Flash Hazard assessments are performed to meet OSHA
and NFPA regulations and standards.
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For More Information…
Please contact Littelfuse POWR-GARD Engineering Services
for more information on:
z
z
z
z
NEC Article 409 Requirements
UL 508A SCCR Consultation Services
Arc-Flash & Electrical Safety
Safety Audits
z
z
z
Engineering Services
Technical Support
Littelfuse Products
1. Contact our Technical Hotline at:
800-TEC-FUSE
-or- techline@littelfuse.com
2. Visit the POWR-GARD Technical Center website at:
www.littelfuse.com/technicalcenter
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Questions
&
Answers
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