NEC 310 (B)(2)(a) 2008

[dselgra1]

So my question is if you are using 4 wire 3phase and considering your neutral a current carry device per 310(B)(4)(c) 2008 per 310 (B)(2)(a) you would have to derate to 80% your wire. If you are already using an 80% rated breaker doesn't this already make the wire 80% rated. It seems ridiculous to example: 200A 4 wire 3 phase service with a 200A 80% rated MCB making the max load going through the conductor 160A and if you have to derate the conductor 80% per above a 250A conductor that there is 90A wasted.

 

[iwire]

A standard breaker can be loaded to 100% up to 3 hours.

 

[david luchini]

You are looking at it wrong. If you had a 160A continuous load on a feeder, 215.2(A)(1) would require the minimum conductor size to be #3/0, and 215.3 would require the minimum feeder OCPD to be 200A.

If you used a 90deg conductor (3/0 has an ampacity of 225 @ 90deg), then the adjustment factor would leave you with a conductor with an ampacity of 180. This is large enough to carry the load, and is properly protected by the 200A OCPD per 240.4(B). So you would need a 3/0 before AND after your derating.

If you used a 75deg conductor (3/0 has an ampacity of 200 @ 75deg), then the adjustment factor would leave you with a conductor with an ampacity of 160. This is large enough to carry the load, but it would not be properly protected by the 200A OCPD, since 175A is the next standard higher OCPD size.