Derating conductors?

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dduffee260

dduffee260

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In another post on here I mentioned derating. I don't have a code book here but I believe it is Table 310-15.
I had an engineer inform me that you count the grounded conductor as a current carrying conductor, even if you ran 3 phase. If all three phases had the same amount of current, the balance on the "neutral" would be zero amps or so little it would generate a very small amount of heat or is seems. Alot of times we run 3/4" home run conduits with #12 thhn wires of 2-blacks, 2-reds, 2-blues, 2 whites and 1-green wire. This seems like it would be 6 current carrying conductors to me. Since the #12 Thhn is rated 25 amps, you can derate the 80% and make the wire good for 21 amps. On a 20 amp breaker you are in code by 1 amp.
I have always seemed kinda fuzzy on this subject. How do some of you guys see this? Have we been cheating all along?
I know some guys run a "trunk line" which is usually a 1" or 1 1/4" conduit stuffed with about 15 conductors and they don't derate the wires. They say this saves them conduit. It looks terrible when they have a 4 square box with 3 extension rings so you can fit all the wire in. One of my pet peeves is having to fight for wire space in a 4 square. We always pull home runs in a deep 4 11/16" box, this makes room plus lets us know it is a home run box.
 
The answers you seek are here.

(4) Neutral Conductor.

(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).

(b) In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).

(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
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Bob has summed up the requirements for counting or not counting the grounded conductor. Using #12 THHN would allow you to use the 90 degree conductor rating, which is 30 amps, for derating purposes. So 6-#12 THHN CCC's would have a derated value of 24 amps. If the grounded conductors were also CCC's based of the information that Bob posted, than you would derate the 8-#12 THHN conductor based on 70% which gives you 21 amps.
 
So Bob, I guess what you are saying is that if we install home runs for lay-in lights, we would have to include the neutral. But for general convenience receptacles, we do not include the neutral?
 
dduffee260 said:
So Bob, I guess what you are saying is that if we install home runs for lay-in lights, we would have to include the neutral. But for general convenience receptacles, we do not include the neutral?
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Yes.

But I don't think Bob is saying it, the NEC is saying it. Bob (as usual) just knows where to find it.

BTW, thanks Bob for all of your work here.
 
hardworkingstiff said:
If you had 3 phases and a grounded conductor feeding harmonic generating loads could you put your $100 amp-meter on the neutral and pick up the harmonics?
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That would depend on what type of ammeter you're using. If it's a true RMS ammeter than yes, it would tell you the harmonic current. An averaging type ammeter would give you a lower inaccurate amp reading.
 
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