4 Steps to calculate short circuit current ratings in industrial control panels

short circuit current ratings

Frank Basciano
Global Product Manager
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This is an update of a 2020 article written by Marcelo E. Valdes, PE, IEEE Fellow Applications Eng. Manager
ABB Electrification Business

Short circuit current ratings (SCCR) are a critical specification when designing industrial control panels. Determining the appropriate SCCR actually requires no calculations. Instead, there is a simple, four-step process to follow.

The UL Standard for Safety for Industrial Control Panels, UL 508A, includes instructions for calculating a panel’s short circuit current rating (SCCR), but many people have trouble working through that process. Determining an accurate SCCR is essential to ensuring the safety of people who work on or around power equipment. A panel with an incorrectly calculated SCCR could fail or cause an arc flash, with the potential to cause serious injury or death, as well as significant damage to the facility.

People refer to “calculating” the SCCR of a panel, but there actually are no calculations required. Rather, identifying an SCCR requires only that you investigate the fault capacity of the components in the panel circuit. With the list of those values in hand, you need to identify the component with the lowest capacity, which is literally the weakest link in the circuit. The SCCR of the entire panel assembly is the capacity of that component.

Panels must be rated for the available fault current at the time they are installed and for future potential needs, if they may be higher at some future date.

What is SCCR?

SCCR is a rating assigned to electrical components or entire panels indicating their ability to withstand an abnormal current event caused by a failure of downstream components, devices, or assemblies. While components like fuses and circuit breakers have SCCR ratings, transformers influence the panel’s available short circuit currents but don’t have their own SCCR.

Before 2005, the National Electric Code (NEC) required industrial machinery electrical panels to be marked only with the interrupting rating of the main overcurrent-protective device. To help better protect the electrical/control panel against short circuits, NEC editions after 2005 require evaluation and determination of the SCCR for the entire control panel’s combined power circuit.

SCCR values for components are typically found printed on product labels or in manufacturer literature, having been established through testing by listing agencies like UL or ETL according to relevant component standards.

Determining a panel’s SCCR

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)
Step 2 – Determine whether feeder circuit components limit fault current (SB4.3) circuit-protective devices, like the fuse or circuit breaker.
Step 3 – Determine overall short circuit current rating for industrial control panel (SB4.4.).
Step 4 – List the SCCR marking on the control panel nameplate (SB5.1).

Following is more detailed information about each step.

Step 1 – Determine the short circuit current rating of each component in the power circuit
The first step is to determine the SCCR of each component or combination of components, 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.

There are resources available that provide device ratings for recognized components, which include the component’s UL file and manufacturer’s installation instructions. Also, the UL website includes a table of Short Circuit Current Ratings for Combination Motor Controller Components. These components typically must be used with another component to get the desired rating.

Feeder-circuit components that limit and/or modify available fault currents include:

Power transformers (refer to UL 508A SB4.3 for determining available short circuit current from 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 10 kVA or less, the transformer secondary is assigned an available current of 5 kA, and all secondary side components in the power circuit must have a SCCR of 5 kA 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 100 kA.

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 of the circuit breaker. 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.

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 SB4.2, “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.

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 ratings
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) 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.

Know your panel

The people who design and build industrial power-control panels need to understand the required level of short-circuit-current protection for the people who own, operate and maintain those panels. The short circuit current rating provides a key piece of information in ensuring the appropriate level of protection. 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.

Coordinating SCCR

Once you have determined the SCCR of your panel using the four steps outlined above, it’s important to consider how this rating applies to the individual devices within the panel. These devices, including contactors, surge protective devices (SPDs), transformers, and circuit breakers, all play a role in determining the overall panel design.

To be within compliance of UL, the SCCR of each device needs to be coordinated with the overall panel SCCR. In simpler terms, no device in the panel should have a lower SCCR rating than the panel itself. If a device does have a lower SCCR, it would be the weak link in the circuit and could fail during a short circuit event, potentially causing serious injury or damage.

Therefore, after determining the panel SCCR, consult the manufacturer’s data for each device to verify its SCCR rating. If a device has a lower SCCR rating than the panel, you will need to take corrective actions. These actions could include replacing the device with one that has a higher SCCR rating or modifying the panel design to limit the fault current reaching the device.

This proactive approach not only enhances safety but also contributes to the durability and reliability of the electrical infrastructure.

References

[1] 2008, PanelBoard and Switchboard Short Circuit Current Ratings, Underwriters Labratories, https://legacy-uploads.ul.com/wp-content/uploads/2014/04/ul_PanelboardShortCircuitRatings.pdf
[2] UL 508A, Third Edition, Standard for Industrial Control Panels