Specing a Disconnect per NFPA 79 1

RAndres · Jul 10, 2023

I need to call out a disconnect for a 480V 3 phase enclosure. I've been reading my way through NFPA 79, but have some questions. The device will be moved to different plants, not just within a plant. In other words, I don't know what the supply short circuit and fault currents could be. That makes it hard to figure out how to address warning signage and disconnect ratings. Can I put a max current in the requirements to the plant, and use that value? Is there a better, more focused way to get a handle on this than working through the standard?

jraef · Jul 11, 2023

If you use a Fused Disconnect, you are unlikely to need to worry about the disconnect itself, most will have at LEAST a 100kA SCCR by themselves with fuses installed and very few places have more than that available at a typical machine location.

Non-fused is a different story. Non-fused disconnects are generally limited to 10kA, unless you can prove their is a specific fuse (or possibly breaker) upstream of it that it is series listed with. That makes it problematic for your application since you will not know. I would stick with a fused disconnect.

Remember though, that will ONLY cover the disconnect itself, your machine control panel will need an SCCR label too and that has to match or exceed the available fault current at the installed site. If you use components inside of that panel that are series listed with the fuse in your disconnect, you may be able to easily get 65kA SCCR, but it takes careful consideration of all of your components and fuse selection.

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Specing a Disconnect per NFPA 79 1

RAndres

Electrical

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jraef

Electrical

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