ECN Forum Forums General Topics of the Trade General Discussion Area New Romex cable

Seems like an excellent idea. Neutrals are considered current carying? I have been using 12/4 for years for bath heat/vent/lights/nightlights. It has a red, black, blue, white, and ground. I guess you could mark one of the colors white and have the same product. I will use it when it becomes available.

See 310.15 (B) (4) regarding the neutral as a current carrying conductor. Except when carrying only the imbalance from the other conductors, the neutral is a current carrying conductor and must be accounted for.

As far as derating ampacity goes..
110.14(C)(1)stipulates which column to use for ampacities based on conductor size & equipment temp. ratings.
But for derating purposes, look under the appropriate column (310.16) to find the initial ampacity for your conductor type. Then derate.
Example: If you had a 3, 50 amp, 3 phase loads and used #6 THHN in a single raceway. (total 9 #6 THHN in raceway.)
1) Table 310.16(90c column) allows 75 amps for #6.
2) 310.15(B)(2) requires derating to 70%.(9 conductors).
3) .7 x 75 = 52.5 amps.
Note that if you used the 75c column for your derating, your ampacity at 70% would be only 45.5 amps,(7 x 65) and you would need a larger conductor.


Anyone else?


[This message has been edited by Redsy (edited 05-20-2002).]

Got it. Thanks

Redsy,

Where I get confused is whether to derate at the ampacity of the the expected load, or the protected load... For example, do I protect #12 wire at 20A (the amount that it is protected) or the 180W (the amount assumed for a receptacle load)...

That would be a big difference!

66wv,
Well, for #14 through #10, see the asterisk at the bottom of table 310.16.

But, to answer your question, derate the CONDUCTOR.
In other words, if you have a 60 amp load with 2 THHN conductors no derating is required. Use the 75c column(per 110-14(C)(1)) which shows a #6 as good for 65 amps.
If, however you had 4 current carrying conductors and needed to derate to 80% as required by table 310.15(B)(2)(a), you may use the 90c column which shows 75 amps for THHN. 75 x .8 =60 amps. You are still good with #6. If, however you used the 75c column to derate, you would have needed to go to a larger conductor because .8 x 65 =52 amps.
I hope this helps (and I hope I'm correct)

What happend to the new "2002" requirement that any conductor smaller than a #6 has to be permetly marked from one end to the other.
I thought this would sopt us from using the neutrals for switch leg or hot feeds down to a dead end switch?
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Fred The 4 conductor NM will also be handy for wiring those bath heat-vent-light units.
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200.7(C)(1) "If part of a cable assembly and where the insulation is permanently reidentified to indicate its use as an un-grounded conductor, by painting or other effective means at its termination, and at each location where the conductor is visible and accessible".

Redsy,

For the Derating, don't forget the 105ºF attic.

Of course it was not NM cable, but recently seen an installation of 2" RMC on a roof top which had 32 #10 THHN/THWN current carrying conductors on 20A CB's.
The ampacity of the #10 was less than 12 Amperes.
Some loads were greater than 16 amperes and were 24/7/365 loads.

Good point.

Yeah, the attic, the boiler room, etc.
Any time you even pass through an area where the ambient temp. is higher than other portions of the run, you should use the higher temp. for derating purposes. Correct?

Don't know about under the NEC, but that's the way we do it here: Derate according to the highest temperature the cable passes through.