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Fault Current Fundamentals - Understanding SCCR, Common Issues & NEC Compliance
From the archives—Thursday, Oct. 3, 2024
We have an RTU with a short-circuit current rating (SCCR) of 5 kA. During the inspection, the AHJ asked for the available fault current study, and we determined it is almost 8 kA at the RTU. The inspector says we must make the correction, even though we did not provide the RTU. Is this the EC’s responsibility? If so, what options do I have to correct this? Why does HVAC equipment only have a 5kA SCCR rating?
Sections 440.4(B) and 440.10(A)-(B) provide guidance on the requirements for SCCRs and markings, available fault-current calculations and required documentation. The responsibility for the repair would be determined by project contractual obligations and the local jurisdiction’s permitting requirements. One option for compliance would be to modify the installation to reduce the available fault-current at the RTU.
Potential options that could reduce the available fault-current could be (1) installing a transformer ahead of the RTU, (2) increasing the length of conductors supplying the RTU or (3) installing current-limiting fuses that work in series with the OCPD to provide adequate protection from fault-current, which will likely require documentation from the fuse manufacturer and AHJ approval.
The RTU could be modified by the manufacturer to provide a higher SCCR and then a field evaluation could be done on the RTU to confirm the modification and provide field labeling of the equipment.
From the archives—Thursday, Nov. 7, 2024
Regarding the 10/3/24 question and response, this equipment SCCR issue is a constant struggle for contractors and engineers. We try to work with manufacturers to get properly rated equipment, but inadequately rated equipment occasionally slips through or may be the only available choice. Even some equipment that operates at 60A or more at 480V is available with 5 kA ratings, and I can’t imagine any location where these are installed safely. Are you aware of any pending resolution by NFPA or UL proposing minimum short-circuit ratings based on voltage/ampacity for equipment?
Several factors impact the available incident energy, including the available fault current and the clearing time of the OCPD. A longer distance from the source to the equipment/load may result in a lower available fault-current; however, a lower available fault current may result in the OCPD taking more time to operate/open. Therefore, a larger distance does not always equate to lower incident energy.
2024 NFPA 70E, Section 130.5(C), Informational Note No. 3, provides information on this topic. Informational Annex D of NFPA 70E provides guidance on the methods for calculating incident energy.
Post sponsored by Eaton Corp.
Ensuring electrical equipment is applied within its short-circuit current rating (SCCR) is critical for both safety and compliance. Equipment SCCR must be sufficient for the available short-circuit current at its point of connection. Equipment that is misapplied in this manner could result in an arc flashes, explosion and fire. The secret to proper application lies in three simple steps:
- Calculate the available fault current at the equipment location
- Determine the SCCR of the equipment being installed
- Ensure equipment SCCR is equal to or greater than the available fault current
Changes to a facility’s electrical system, such as transformer upgrades or equipment relocation, can increase fault current levels. It is crucial to verify SCCR compliance regularly.
This video demonstrates the power of fault current when equipment SCCR ratings are exceeded. The equipment achieves an unintended rapid disassembly in the field.
Video Link
NEC 2023 requirements related to SCCR include the following:
- Available fault current required to be marked or documented: 110.24, 408.6, 409.22, 430.99, 440.10, 620.51, 670.5
- SCCR is required to be marked on the equipment: 409.110, 430.8, 430.98, 440.4, 620.16, 670.3, 700.5, 701.5, 702.5, 708.54
- SCCR required to exceed available fault current: 110.10, 409.22, 430.99, 440.10, 620.16, 670.5
Eaton Solutions that help with SCCR compliance
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