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.
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
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.