Code Requirements for Short-Circuit Current Ratings

The National Electrical Code, NFPA 79, and OSHA require proper short-circuit protection for control panels and industrial equipment. The following are some of the specific requirements regarding Short-Circuit Current Ratings (SCCRs):

All control panels must be marked with its SCCR, using an approved evaluation method (such as UL 508A) (NEC 409.110).
The available short-circuit current at the installation location of the control panel must be documented and available for inspection (NEC 409.22).
Control panels must not be installed where the available short-circuit current exceeds its short-circuit current rating (NEC 409.22).
Components intended to interrupt a short circuit must have a sufficient interrupting rating for the short-circuit current available at the line terminals (NEC 110.9, OSHA 1910.303(b)(4)).
Components not intended to interrupt a short circuit must be properly paired with a short-circuit protection device so as not to create extensive damage during a short-circuit event (NEC 110.10, OSHA 1910.303(b)(5)).

Calculating the Available Short-Circuit Current

Determining the available short-circuit current requires information about the installation site. Information about the nearest upstream transformer and information about the conductors between the transformer and control panel are needed to make this calculation. Some of this information may be more difficult to attain, but conservative estimates can be applied to help define a conservative calculation of the available short-circuit current at the point of connection for the control panel.

To assist with this calculation, Schneider Electric has developed a fault current calculator available on the mySchneider app (located in "My Account"). After entering the installation site details, the fault current is calculated, and a system diagram and label can be sent via email.

Calculating the fault current using the Fault Current Calculator on the mySchneider app

Determining the Short-Circuit Current Rating of the Control Panel

The short-circuit current rating (SCCR) of a control panel can be determined though testing the entire control panel assembly, or it can be determined using the method described in UL 508A Supplement SB.

NOTE: Using the interrupting rating of the main overcurrent protection device (OCPD) for the panel is not an acceptable practice.

WARNING
Inadequate short circuit Interrupting rating Do not use the interrupting rating of the main OCPD as the SCCR rating for the controller. Failure to follow these instructions can result in death, serious injury, or equipment damage.

UL 508A Supplement SB applies a weak link approach to determine the overall short-circuit current rating for the panel. With a few exceptions, the components in the power circuit are considered in the evaluation.

There are four basic steps to determine the control panel SCCR:

Determine the SCCR of each component.
Determine if the SCCR of a branch component can be raised using a current limiting device.
Determine the interrupting rating of all circuit breakers and fuses.
Choose the lowest interrupting rating or established component SCCR.

View a free webinar that explains how SCCR is determined for control panels in more detail using the links below:

See UL 508A for all requirements and exceptions regarding SCCR determination.

Determining the SCCR for Components

The short-circuit current rating (SCCR) of a component is determined by conducting a short-circuit test of the component using an overcurrent protection device. Devices that are not designed to interrupt a short-circuit (such as a contactor, overload relay, or power distribution block) must be properly paired with an overcurrent protection device (such as a circuit breaker or fuse), and tested to determine its SCCR value. Thus it is critical that these devices be applied with the proper overcurrent protection device to achieve a proper short-circuit current rating.

A device can achieve an SCCR in one of two methods:

Testing a component per its UL standard to achieve a component SCCR (often referred to as High Fault)
Testing a set of components together as a combination motor controller to achieve an SCCR for the combination

An SCCR can be applied to a device using either method. However, there are some differences between the two types:

Differences in the Two Methods for Testing Components

Component (High Fault) SCCR	Combination Motor Controller SCCR
SCCR information is located on the product marking or in the instruction manual. Often is more flexible regarding the required short-circuit protection, typically only limits to a maximum size Applicable to feeder and branch circuits Applicable to motors and other loads	SCCR information is located on the UL web site at www.ul.com/sccr. Specified part numbers must be used as prescribed. Limited to branch circuits, motor loads only Often has additional conditions of acceptability that must be observed, such as minimum enclosure volume or wire size May achieve a higher SCCR than what can be obtained by the component ratings

A summary of short-circuit current ratings of common power circuit devices is provided in the following section of this document for the convenience of the reader.

In the event that an SCCR cannot be determined for a component, a default SCCR value is applied per UL 508A Supplement SB Table SB4.1. These SCCR values are often very low. Below are the default SCCR values for common component types:

Motor Controllers (0–50 hp): 5 kA
Motor Controllers (15–200 hp): 10 kA
Non-GFC1 Receptacle: 10 kA
Switch Unit: 5 kA
Power Distribution Block: 10 kA
Terminal Block: 10 kA
Busbar: 10 kA