ECN Forum Forums Code Related Discussion NEC & other Code issues Conductor Size

What is the smallest size feeder and neutral that I can use in this situation?

I have a fat storage tank that has a Control panel that feeds a 4500 watt / 240 volt heating element,
7 1/2 HP 3 phase motor 240 V (20.5 amps), fractional hp circulating pump 1/2 amp,
convenience outlet and light. Roughly 40 amps, with the nameplate of the fat tank control panel at 65 amps. ( The tank company stated that the 125% of FLA was listed on the FLA
sticker) The control circuit is fused down to 15 amps 120 volt.

We have 480 Volt 3 phase, so I set a 30 kva transformer feeding a 100 amp 240 Volt disconnect.
Can I feed this control panel with # 6 THHN fused at 70 amps and reduce the neutral to a # 12,or does it have to be a # 6?. Or do I have to run # 4s THHN to this unit?

[This message has been edited by Sparky_1 (edited 01-19-2006).]

If I was answering a question on a test I would say the #6 THHN fused at 70a (using the round up rule) with a #12 neutral serving 1 or 2 single pole 20a breakers on different phases is the minimum. I can't say that is the best design decision.

Everything worked out today, we ran # 6s with a separate 15 amp control circuit.
I threw a amp meter on the unit, the FLA was 41.
The inspector wants a sticker stating that is has two separate disconnects.