Neutral query

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TimWA

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In the example page 378/379 of MH 2005 Exam Prep, a maximum unbalanced load of 200A comes out to 2/0 cu neutral per 310.15(B(6). No problem there.

The example then goes on to say "in addition, 250.24(C) requires the neutral to be sized no smaller than 2/0 based on the 4/0 service conductors in accordance with Table 250.66

I don't see how 250.66 specifies this.

[ October 31, 2005, 12:19 PM: Message edited by: TimWA ]
 
Re: Neutral query

I don't have the book, but I am guessing the 4/0 are parelleled to the equivalent of at least 600 kcmil? Or, was it supposed to be #2 AWG instead of 2/0 AWG? Did you check the errata for this edition book?
 
Re: Neutral query

250.66 specifies the minimum size GEC, but 250.24(C)(1) requires the grounded conductor to be no smaller than the required GEC.
 
Re: Neutral query

Further examination, I think this example has typos. The problem is a 240V/120V calculated load of 375A, of which 175A is 240V loads. So 375A phase 200A neutral. The answer is given as two 400kcmil ungrounded and a 2/0 grounded into the service. No dispute with that.

Further down the example states "in addition, 250.24(C) requires the neutral to be sized no smaller than 2/0 based on the 4/0 service conductors in accordance with Table 250.66" So we've gone from 400kcmil to 4/0.

Anyway, nothing is mentioned about paralleled 400kcmils being equivalent to 600kcmil. In any case, neither 4/0, 400kcmil or 600kcmil comes out to 2/0 according to 250.66 from what I can see. Quite perplexing.

[ October 31, 2005, 12:43 PM: Message edited by: TimWA ]
 
Re: Neutral query

Good, you figured it out. 2-400 kcmil ungrounded conductors is equivalent to 1 - 800 kcmil conductor therefore the Table minimum is met.
 
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