White paper
Determining short circuit current ratings in industrial
control panels
Steve Zbytowski
Technology Manager-Control Products, ABB
Standards documents can be quite intimidating. Often
lengthy and written in detailed, technical language, they
do not make for particularly enjoyable reading. Little
wonder people are seldom eager to dig into a detailed
review of standards.
Of course these standards aren’t intended to entertain. They
contain critical information related to crucial safety practices.
Such is the case with UL 508A, the UL Standard for safety for
industrial control panels. It includes instructions for calculating
a panel’s short circuit current rating (SCCR), but many people
clearly have trouble working through that process. Steve
Zbytowski, Technology Manager-Control Products, ABB, says
he frequently makes presentations on the topic, and they are
always packed with people eager to better understand it.
“It’s a critical figure to determine,” Zbytowski says. “That
number is essential to adequately protecting the people
who work on or around power equipment. A panel with an
incorrectly calculated SCCR could actually explode or cause
an arc flash, with the potential to cause series injury or death,
as well as significant damage to the facility.”
While people refer to “calculating” the SCCR of a panel, that’s
really not accurate because there aren’t any true calculations.
It requires only that you investigate the fault capacity of the
relevant components and then identify the component with
the lowest capacity ... the weakest link in the circuit. The
SCCR of that component is the SCCR of the entire panel
assembly.
This white paper provides a brief background on SCCR, then
goes on to detail the process for determining the SCCR of an
industrial control panel.
What is SCCR?
Before 2005, the NEC required industrial machinery electrical
panels to be marked only with the interrupting rating of the
main overcurrent-protective device. This, however, did not
ensure the electrical panel was adequately protected against
short circuits. The new standard includes the entire combined
power circuit in determining the SCCR requirements.
Many people confuse SCCRs with interrupting ratings, but
there is big a difference. The interrupting rating pertains
only to an overcurrent protective device like a fuse or circuit
breaker. It is the maximum current a protective device can
safely interrupt. Components and equipment whose function
isn’t to interrupt fault currents are rated with SCCRs, when
used with a specified current protective device.
It is critical that the overcurrent-protective devices and the
combination have an adequate interrupting rating for the
available short circuit current. In fact, that rating is required
by NEC 110.9 which says that the interrupting rating must be
equal to or greater than the maximum available fault current.
But this alone doesn’t provide adequate protection from
damage or injury due to short circuits.
Determining a panel’s SCCR
Following is more detailed information about each step.
Step 1: Determine the short circuit current rating of each
component in the power circuit.
Circuit protection falls in three categories:
− No circuit protection (referred to as non-combination).
− Feeder circuit: Similar to power distribution, it the main
power entering or within the enclosure.
− Branch circuit: A internal starter panel circuit that is protected by a fuse or circuit breaker within the panel.
How is the SCCR calculated?
There are four steps in the process:
Step 1: Determine the short circuit current rating
(SCCR) of each component or combination in the
power circuit. (SB4.2 in UL 508A)
Step 2: Determine whether feeder circuit
components limit fault current (SB4.3 in UL 508A)
circuit-protective devices, like the fuse.
Step 3: Determine overall short circuit current rating
for industrial control panel (SB4.4 in UL 508A).
Step 4: List the SCCR marking on the control
panel nameplate (SB5.1 in UL 508A).
The first step is to determine the SCCR of each component
or combination, which is usually on the component label or its
instruction manual. You don’t need to include the SCCR for
power transformers.
Another source of SCCR information is the Assumed
Maximum Short Circuit Current Rating For Unmarked
Components, Table SB4.1 in the UL 508A Standard. This is
also referred to as the standard fault. All components must
have a standard fault current rating, and it is typically very low.
Finally, you can refer to information in manufacturers’
procedures based on ratings from recognized component
reports or determined from product testing. The application
data for a recognized component can be found on the
UL website, in the component’s UL file, or manufacturer’s
installation instructions. These components typically must be
used with another component to get the desired rating.
Feeder-circuit components that modify fault current include:
− Power transformers
− Current-limiting circuit breakers
− Current-limiting fuses
You need to locate these parts and include them in your
consideration of the SCCR.
On the branch circuit, you need to consider transformer
ratings. For transformers rated 10kVA or less, the transformer
secondary is assigned an available current of 5kA, and all
secondary side components in the power circuit must have a
SCCR of 5kA or greater. On the primary side, only the primary
overcurrent protection is relative to the overall panel SCCR.
An example is Class CC fuses used on the primary side of the
transformer, which have a SCCR of 100kA.
The branch circuits must have a SCCR equal to or higher than
the let-through current of the feeder circuit. If they don’t, the
overall rating for the panel is the lower rating of the panel or
the branch circuit.
Step 2: Determine whether feeder circuit components
limit fault current.
Once you’ve identified the SCCR for the components, the
next step is to determine whether feeder-circuit components,
specifically circuit-protective devices like fuses, limit the fault
current.
Circuit breakers must be marked “Current Limiting” to use
SB4.3.2. The breaker let-through current will not exceed a
defined value. One of two conditions apply:
− If the devices on the load side of this breaker have a higher
SCCR than the interrupt capacity of the breaker, then you
can use the interrupt capacity. These may also be a combination that was tested by the manufacturer or panel shop.
− If the devices have a lower SCCR than the interrupt capacity of the circuit breaker, the SCCR for this circuit is the
lower value.
2 Determining short circuit current ratings in industrial control panels | ABB white paper
The maximum let-through for a circuit breaker is determined
by the manufacturer. For fuses it is determined by a standard,
allowing you to use a Table SB4, “Peak let through currents,
IP and clearing I2T for fuses”.
In determining the panel SCCR, the SCCR on the line side of
any current-limiting circuit breaker can’t exceed the SCCR
of any branch-circuit protection or the interrupt capacity
of the circuit breaker. The peak current let-through cannot
exceed the SCCR for any branch circuit on the load side. This
basically states that the device on the load side of this breaker
can withstand the let-through breaker energy and current.
Know your panel
Most people fear the unknown. When it comes to working
with power-control equipment, that’s a prudent approach.
The people who buy, operate, and maintain industrial powercontrol panels need to understand their capability to provide
protection for their employees and facility from short circuit
current.
The short circuit current rating provides that key piece of
information. Panel makers rely on the steps laid out in the
UL 508A standard to calculate/determine the SCCR of their
products and provide this information.
For fuses, use the values in Table SB4.2 “Peak let-through
currents, IP and clearing, I2T for fuses” to get the I2T and IP
for the fuse used in the combination circuit. Any fuse with a
lower value for both the I2T and IP can be used. If your fuse
size is not shown, use the next larger value in the table.
Step 3: Determine overall short circuit current rating.
With the component research complete, you have the
information needed to determine the panel’s SCCR. You do
this by determining three different SCCRs. The lowest of the
three is the panel SCCR.
The three values to determine are:
− For each protected branch circuit in the panel, determine
the smallest SCCR for the power circuit components on the
load side of a branch circuit protective device. (SB4.4.1)
− Determine the lowest SCCR of all feeder components.
− If current-limiting components are supplied in the feeder
circuit, determine the modified SCCR for the feeder component and all branch circuit(s) [from A) above] connected
to load side. (SB4.3), see step 2 above.
Compare these values from this panel. The SCCR is the
lowest of the three.
Step 4: List the SCCR marking on the control panel
nameplate (SB5.1).
The value from step 3 above must be listed on the panel
rating or name plate label. The name plate marking must
include the SCCR in kAmps rms symmetrical, at the rated
voltage.
ABB white paper | Determining short circuit current ratings in industrial control panels 3