Short Circuit Current Rating (SCCR) is a critical metric for electrical and fire safety. We are familiar with SCCR as it is a UL 508A specification on power components such as fuses and circuit breakers. To better understand these metrics, let’s explore SCCR from the perspective of the upstream feeder—specifically the transformer.
The transformer shown in Figure 1 was installed for a small wood shop. It was an easy decision because the three-phase power line abutted the property. The new three-phase system allowed the operator to use the smaller, more efficient, and less expensive three-phase motors. The owner stated that someday they may want to add a 10 to 20 hp motor.
As the transformer was installed, the electrician asked the owner, “is your equipment designed to handle the SCCR of the new transformer?”
CAUTION: This narrative addresses safety standards associated with high-powered electronic systems. While the material has been prepared with care, it may contain unintentional errors or misinterpretation of the standards. Refer to DigiKey’s Terms of Service for official guidance. We appreciate any feedback you can provide to improve the narrative and factual content.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Certify It → SCCR
Difficulty: 
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Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 06 Mar 2026
Figure 1: Image of a three-phase 208Y/120 VAC 45 kVA transformer.
SCCR captures a relationship between feeder and control panel
For an order-of-magnitude estimation, we look to UL 508A Table SB4.4 which describes the fault current available for a three-phase transformer. Based on the table, the featured transformer may deliver 7.44 kA. Consequently, the shop’s service entrance, in coordination with the equipment’s control panel must meet or exceed this value. For a worst case situation, consider this industrial accident where forklift driver Elmer causes a phase-to-phase short.
To what SCCR level should a UL 508A listed panel be built?
The workshop’s equipment, inclusive of the wall-mounted breaker panel and machine, must be designed to handle the feeder’s fault current. The control panel must not be the weakest link.
For example, the control panel will likely contain a motor starter for a 15 hp motor. The starter may be protected with a matching Motor Starter Protector (MSP) designed for an SCCR of 75 kA on a 208 VAC system. This would provide a good level of protection, provided the remainder of the machine is designed for the same UL 508A-compliant 75 kA SCCR.
We recognize that the transformer shown in Figure 1 is relatively small. The machine’s safety margin is eroded if it is installed in commercial setting that includes a 100 kVA plus transformer and a high-power load center. In all cases, the system is designed so that the downstream devices are protected from the maximum fault current capability of the feeder.
Parting thoughts
Industrial control panels designed to the UL 508A standard embody a wonderfully complex environment. Where else can a designer work with big wires, little wires, programming, and code compliance—all with a single control panel?
Best wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.