Cable Sizing

[eelectric]

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?

 

[davidbeach]

What codes govern?

 

[eelectric]

NEC

 

[davidbeach]

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

 

[eelectric]

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

 

[davidbeach]

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.

 

[eelectric]

Thanks. That's it. 240.100 and 240.101 apply.

 

[xxjohnh]

Don't forget cable short circuit protection.