Size of Neutrals

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osaengineers

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When there are alot of non-lenear loads then the neutral shall be counted as a current carrying conductor. I would think that the cables have to be derated, example, a 500KCM 4 wire feed in a rgs conduit to a panel service office space would be rated for 304 ampere and not 380 ampere, since you have to derate the cable 20%. Which means if I had 310 ampere load I would have to go to to a 600 KCM cable.

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Thanks
 
Your assumption is correct. Four current carrying conductors would require derating, however you may have derated incorrectly. You should be derating from the 90 degree column of 310.16 if you're using a 90 degree conductor such as THHN. 430(80%)=344 amps. Also I would note that the neutral would only count as a CCC when more than 50% of the load contained harmonics.
 
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