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1
SCCR for Industrial
Control Panels
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Interrupting Rating (of Overcurrent Devices)
• NEC® Article 100 Definition
• Highest current an OVERCURRENT DEVICE (fuse or
circuit breaker) is rated to safely interrupt.
• Self protection rating only
• NEC® 110.9 Interrupting Rating.
• Requires the overcurrent device to have an interrupting
rating not less than the maximum available short-circuit
current.
• The maximum short-circuit current must be calculated
and varies, based on system size/location.
• Similar Requirements in OSHA 1910.303(b)(4)
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Short-Circuit Current Ratings (of Equipment)
• NEC® Article 100 Definition
• The highest current EQUIPMENT can withstand without extensive
damage (fire or shock hazard).
• Combination of equipment/component and overcurrent device
• May be based on a specific type of overcurrent device
• NEC® 110.10 Circuit Impedance, Short-Circuit Current Ratings,
and Other Characteristics.
• Requires the equipment to have a short circuit current rating
not less than the maximum available short-circuit current.
• The maximum short-circuit current must be calculated and
varies based on system size/location.
• Similar Requirements in OSHA 1910.303(b)(5)
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SCCR Marking Requirements
• 430.8 – Marked on motor controllers
• 440.4(B) – Marked on HVAC equipment
• 409.110(4) – Marked on industrial control panels
• 409.22 – Short-circuit current cannot exceed marked SCCR
• 670.3(A)(4) – Marked on industrial machinery control
panel
• 670.5 – Short-circuit current cannot exceed marked SCCR
• Typically HVAC, ICPs and IM is rated only 5kA!
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Short-circuit Current Marking Requirement
• 110.24 Available Fault Current.
• (A) Service equipment in other than dwelling
units must be marked with the maximum
available fault current and date of calculation
• (B) If short-circuit current increases due to
system modification, the marking must be
updated.
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How to Comply?
Engineer – Calculate
All equipment must comply with:
NEC 110.9 (IR) &110.10 (SCCR)
Isc = 60,142 A
Contractor – Label
Max Avail. Fault Current = 58,524 A
Service Equipment
Date Determined/Calculated: 9/2010
Required per NEC 110.24
Isc = 38,525 A
HVAC
SCCR = 40kA
Isc = 42,153 A
Isc = 27,532 A
Industrial Machinery Panel
Industrial Control Panel
SCCR = 65kA
SCCR = 30kA
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Isc = 18,752 A
Motor Controller
SCCR = 25kA
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What is Typical for Industrials?
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How to Determine SCCR?
• Short-Circuit Current Rating (SCCR)
• Can be established by testing during the listing and
labeling process
OR
• Can be determined using an approved (calculation)
method
•
UL 508A Supplement SB is an approved method
(industrial control panels and industrial machinery control
panel)
•
AHJ Approved Method
• NRTL field evaluation can also be used.
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Industrial Control Panel SCCR
An industrial control panel is an assembly that
consists of at least two components. They may be
“power” components such as motor controllers,
overload relays, fused disconnect switches, and
circuit breakers, or they may be “control circuit”
components such as push buttons, pilot lights,
selector switches, timers, switches, and control
relays, or they may be a combination of “power” and
“control circuit” components. The controlled
equipment is not considered to be a part of the
industrial control panel.
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UL 508A Supplement SB
• What Needs to be Analyzed per UL 508A,
Supplement SB?
• All power circuit components
• Feeder and branch circuit components that supply power to
loads (motors, lighting, heating and appliances)
• Includes disconnect switches, fuses, circuit breakers, load
controllers, overload relays, power distribution/terminal blocks,
bus bars, etc.
• The Exception for HVAC equipment rated 60A or less deleted
in 2017 NEC.
• Control circuit components are NOT required to be
analyzed
• Pushbuttons, pilot lights, selector switches, timers, control
relays, etc.
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“2 Sweep” Method
(Based on UL 508A)
• Sweep 1 – Component Rating
• Determine lowest power circuit component SCCR
• Use current-limiting overcurrent protective devices or
transformers in feeder circuit to increase low SCCRs
downstream
• Sweep 2 – Overcurrent Protective Device Rating
• Determine lowest overcurrent device IR or combination motor
controller SCCR
• Lowest value of Sweep 1 and 2 = Assembly SCCR
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UL 508A Supplement SB
• What are the Rules (Sweep 1)?
• Lowest component SCCR limits
assembly SCCR unless:
• Combination ratings can be used to
increase branch circuit component
ratings – check with component
manufacturer
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Combination Ratings - Feeder
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
100kA
• Use combination ratings
with specified/marked
OCPD to fix low rated
feeder components
• Note: PDB must have
feeder circuit spacings
(Listed vs. Recognized)
Combination ratings of overcurrent protective devices and components can be used
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Combination Ratings - PDBs
PDB w/14 AWG Load Side Conductors
SCCR=100kA w/175A Class J fuse or smaller
PDB w/12 AWG Load Side Conductors
SCCR=200kA w/200A Class J fuse or smaller
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Combination Ratings - Branch
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
100kA
• Use combination
ratings with
specified/marked
OCPD to fix low rated
branch components
Combination ratings of overcurrent protective devices and components can be used
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Combination Ratings – Motor Starter &
Contactor
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High Fault Current Substitution with
Current-Limiting Overcurrent Device
SB4.2.3 A high fault short circuit current rating for a motor controller, an overload relay, or a
combination motor controller, as specified in SB4.2.2 (a) or (c), shall only be used as the short
circuit current rating of the component when the specified branch circuit protective device is
provided.
Exceptions:
(1)
CL Fuse with more CL Fuse
(2)
Provided in the field if marked
(3)
CL CB with more CL CB
NEW Exception No. 4: When the specified branch circuit protection related to the high fault shortcircuit current rating is a non-current limiting overcurrent device, a current-limiting overcurrent
device is able to be used at the same high fault rating where the interrupting rating of the currentlimiting overcurrent device is equal to or greater than the specified overcurrent device.
Current limiting overcurrent devices can be used in lieu of a non-current
limiting overcurrent device for protection of motor controllers and still maintain
a high fault short circuit current rating.
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Use of CL Devices - OCPD
• CL Device must be in the feeder circuit
• For CL Fuses or Circuit Breakers
• Use PEAK Let-through, Not RMS Let-Through (more
conservative). Compare peak let-through with component
SCCR
• Fuses - cannot use manufacturer specific fuse data – only
tables (or formulas) based on performance requirements
from UL 248 (conservative - applies to all manufacturers)
• Circuit Breakers - must be Listed and marked “currentlimiting” and use published peak let-thru curves
• Can only raise downstream branch circuit component SCCR
(not fuse IR, not CB IR, not fuse/CB series rating, not
CB/CB series rating, not combination motor controller
SCCR)
• New - can be provided in the field (outside of panel) if
properly marked on industrial control panel.
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Feeder vs. Branch Circuits
Feeder Circuit
Branch
Circuit
Branch
Circuit
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Use of CL Devices - OCPD
Component SCCR
Nameplate
Voltage:
480
SCCR:
15kA
18kA
OCPD I.R.
FC
LT
50kA
12kA
100kA 14kA
200kA 20kA
F
•
Current limiting feeder OCPD
can be used to increase branch
component SCCR.
•
If Component SCCR is less than
let-through current at a given
short-circuit current, raise
component SCCR to that shortcircuit current value.
B
Use of CL OCPD can only fix components
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Current-Limiting Feeder
Protective Devices Installed in the Field
NEW:
SB4.3.4 The specified circuit breaker marked "current limiting" or current-limiting
Class of fuse supplied in the feeder circuit that limits the peak let-through
current available in accordance with SB4.3.2 and SB4.3.3 is able to be
provided in the field when the panel is marked in accordance with SB5.1.3.
SB5.1.3 An industrial control panel marked with a high fault short circuit current
rating and is not provided with the required feeder circuit protective device as
specified in the SB4.3.4 shall be marked with the type and size of feeder circuit
protection required to be installed in the field. This marking shall be included as
part of the marking in SB5.1.1.
•
Allows the use of the let-through to increase branch component SCCR even
if the current-limiting overcurrent device is supplied in the field, provided the
associated required marking is provided including the SCCR marking.
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Use of CL Devices - Transformers
• CL Transformer must be in feeder
• Previously only 5 or 10KVA Transformers
• Now Use Equations or Tables
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Use of CL Devices - Transformers
Component SCCR
OCPD I.R.
Nameplate
Voltage:
480
SCCR:
200kA
10 kVA
• 480-120V, single phase, 2.1
impedance or more
• Max short-circuit from table 3970A
• If OCPD IR/Components SCCR is
higher, then assign entire circuit
I.R. of primary overcurrent device
Use of transformers can increase secondary component/OCPD ratings
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UL 508A Supplement SB
• Reviewing the Rules for Sweep 2
• Lowest overcurrent protective device interrupting
rating (or SCCR for some devices) always limits
assembly SCCR
• All Feeder and branch circuit overcurrent devices
• Tested series combination ratings or series ratings
(fuse-circuit breaker or circuit breaker-circuit breaker)
are NOT allowed.
• Branch circuit overcurrent devices, connected to the feeder
circuit, supplying a control circuit.
• Supplemental protective devices, protecting the control
circuit transformer, in a motor branch circuit.
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Why High SCCR?
• NEC & Equipment standards require the SCCR to
be marked
• NEC requires the marked value to be adequate
for the maximum available short-circuit current
• High SCCR is often preferred
• Flexibility of application
• Multiple installations
• Short-circuit current unknown
• Insufficient SCCR can delay installation
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Fix It - Solutions
• High I.R. Protective
Devices
• Class R
• Class J
• Class J Drive Fuse
• Class T
• Class CC
• CubeFuse
• High Speed
• High IR/CL CBs
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Fix It - Solutions
• High SCCR Components
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