ECN Forum Forums Code Related Discussion NEC & other Code issues Derating Application

If you are using the three phase system and a grounded conductor for each circuit the grounded conductor is a current carrying conductor. If you are using 2 phases with a common grounded conductor, the grounded conductor is a current carrying conductor. If you are using a three phases with a common grounded conductor the grounded conductor is not a current carrying conductor if a majority of the load is linear, and is a current carrying conductor if a majority of the load is non-linear.
Don

Caselec, you are correct that it would be 10 conductors but only 8 would be current carrying thus derating would be figured at 70% allowing the use of 20 Amp circuit breakers. If this was a three wire single phase system you would have 11 conductors but only 8 would be current carrying and you could still use 20A olcp.
Ron

By the way, hats off to the inspector that caught this, I know a lot of inspectors and electricians would have missed it.

I'm not very fond of multiwire circuits either, so run another pipe.

And thanks to resqcapt19 for explaining the 3-phase multiwire circuit (3 wire). I hadn't thought about those two phases not being opposing, therefore having current in the neutral. Learn something new every day!

Dave

edited for spelling

[This message has been edited by Dave55 (edited 11-03-2004).]

Any time an inspector sees a lot of wire in any single pipe on his inspection it is a red flag that he should be thinking about fill and derating. You can usually "eyeball" fill after a while and get close enough to see whether you need your calculator but derating requires some deeper investigation, particularly if you don't see any multiwires. They bring up their own issues that require a closer look.