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Cable Sizing

Cable Sizing

Cable Sizing

(OP)
I am tapping off a 6900 volt feeder at the high side of the transformer.  The new tap feeder will be about 800' long.  On the load side of the new tap feeder will be a disconnect fused at 100 Amps.  The upstream 6900 volt circuit breaker protection is at 480 Amps.  Since the disconnect on the tap feeder is rated at 100 amps can I size my cable at 100 amps or do I still need to size it at the upstream protection setting of 480 Amps?

RE: Cable Sizing

What codes govern?

RE: Cable Sizing

(OP)
NEC

RE: Cable Sizing

So, does 240.21(B)(5) apply in your case?

RE: Cable Sizing

(OP)
this installation is all indoors, so that exception would not apply

RE: Cable Sizing

What about 240.92?  And don't forget about 240.100(A) and 240.101.  If those don't apply then you have to have adequate protection at the supply end.

RE: Cable Sizing

(OP)
Thanks.  That's it.  240.100 and 240.101 apply.

RE: Cable Sizing

Don't forget cable short circuit protection.

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