ECN Forum Forums Code Related Discussion NEC & other Code issues One Neutral For Multiple Circuits

electure,
There are very few places where the code permits oversized neutrals for multiple circuits and this is not one of them. 215-4 and 225-7 are two places that I can think of, there may be one or two more.
Don(resqcapt19)

[This message has been edited by resqcapt19 (edited 09-26-2001).]

pauluk,

Prior to 1985, NM cables had conductors rated for 60 deg. c.(or was it 75?). Romex made since then is rated 90 c. and is called NM-B. This poses problems when customers in homes built prior to 1985 want a new fixture installed, as most new fixtures require 90 c. supply wires.

Romex made since then is rated 90 c. and is called NM-B. This poses problems when customers in homes built prior to 1985 want a new fixture installed, as most new fixtures require 90 c. supply wires.

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NM-B ampacity is based upon 60 deg. C per article 336-26. 90 deg. C is used for derating purposes as long as the final derated ampacity does not exceed the 60 deg. C rating. The 90 deg. C rating is for withstanding temperatures (heat transfer)from light fixtures, etc. My 1969 home has the old 60 deg C. NM romex and it's a real bear dealing with the overly baked and cracked conductor insulation.

it's a romex job, go with Scott T's post. If it had been 208Y120 we could talk...


Scott W; http://www.afcweb.com/table2.html
Scroll down to the super-noodle stuff. It's marketed for that 'harmonic' hype we all recently lived thru.
So your post got me to thinking, the code allows generally for no more nuetral load than the ungrounded conductor, are third , fifth, ninth orders exempt??

Joe,
Thanks for the kind words! And also for picking up on the NM-B mistake I made
I so rarely work with Romex, that I neglected that important temperature ID !!!

Mr. Brooklyn,
Glad we all could help out here! If you are using AC or MC [AKA "BX"], the same goes - 2 or 3 wire with ground.
All the MC cable I have worked with lately uses THHN cu conductors, and of course have an insulated grounding conductor [sized per NEC 250].

Steve,

I know that you know what's up with the oversized common grounded conductor in the MC cables you speak of. This would be the few places where inbalanced and excessive load currents would be accepted, as Don [resqcapt19] mentioned.
The triplen harmonic currents would only be in the common grounded conductor, so the best thing to do is increase the ampacity / size of said conductor.

I have no idea where, or if, an increased size common / neutral could be used on multiwire circuits per the NEC - such as using a #8 THHN common neutral with two multiwire hots [2 Line A, 2 Line B, 1 phase 3 wire system].
This would be where Scott W is going with his message.
It would work, but it sure looks suckee!!! [in my mind, that is].
It would work on simple Linear and balancable loads, but what a nightmare if there was any high level THD!!! Seems that even Squirrel Cage Split Phase Induction Motors on these circuits, running at varying loads, would reflect some high levels of double frequency pulsation and resultant harmonics!
Just my opinion only! Please be kind in flamed responses just tossing out some ideas and stuff to discuss.

Scott SET

P.S. edited some bad spelling

[This message has been edited by Scott35 (edited 09-27-2001).]

Scott,
If you have a handbook, 225-7 has a good explanation of using a single neutral sized for the maximum unbalanced possible. ie. If you run (6) 20 a. ckts, off a single phase panel (3 each bus) you could run a neutral sized for 60 a.
Maybe it would help in keeping the mess minimal if running all ckts. through a lighting contactor. Anyone ever do this?

Tom,

I hope that it is clear to you that much of this discussion has nothing to do with your original question. I strongly second the suggestion that you seek help before doing anything.

This latest discussion about the oversized neutral would only work with individual wires within a special cable, raceway or enclosure. Right Guys?

Bill

It is, and I will Bill, thanks. =TB=

Multi-wire branch circuits are allowed by NEC 210-4.

hmmm,
ok now, just what size noodle would 215-4(a)'s have serving 3 sets of 3-wire feeders??