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Hello, can you speak to the code/UL acceptability of using current-limiting fuses to permit use of low-SCCR equipment in a system with higher short circuit availability? My understanding is that it's not permitted for engineers/installers to use this method but, it may be implemented by the equipment manufacturer (who is intimately familiar with the equipment and the ratings of its parts) to raise the SCCR of their equipment. A manufacturer might base their SCCR on the use of current-limiting fuses ahead of their equipment, and require their installation by others, but an engineer/EC can't do it on their own. Low-SCCR equipment is a major issue on our projects with a great deal of equipment that is only rated for 5,000A. I'd love to hear your take on this.
John, thanks for your question. This brings up a common issue especially with HVAC equipment.
Section 110.9 of the NEC® requires electrical equipment like fuses or circuit breakers, to be rated for the level of fault current they are intended to interrupt.
Section 110.10 of the NEC® requires electrical equipment like panelboards, switchboards, and busways etc. to survive a fault condition without suffering extensive damage. The fault current or short circuit current rating of the equipment can be used to determine if it can withstand a fault condition without damage. Additionally, circuit protective devices, can be selected and coordinated to provide protection of the equipment as well. Field installed current limiting fuses are often used as a means for complying with this section, when low-SCCR equipment like a Roof Top Unit with a 5kA rating is installed. This type of solution will often require the involvement and information from the current limiting fuse manufacturer, to ensure the solution is being applied correctly. Often, the current limiting fuses alone are not the solution, but the fuses in series with another OCPD is often required.
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