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#98066 - 05/03/06 01:11 PM Multiwire circuit neutrals  
kyelectric  Offline
Member
Joined: Aug 2005
Posts: 64
Lets say you run 12/4 w/ground and use 3 as hots, 1 as neutral and 1 as ground. The three hots all go to 20 amp breakers. Two of the three circuits will feed out to my kitchen countertop required small appliance branch circuits (GFI protected). The other circuit will be a dedicated receptacle for a plug and cord garbage disposal. At what point do I know that I am overloading the neutral? What are the rules about how many circuits you can run off of one neutral? Any help would be appreciated.


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#98067 - 05/03/06 01:14 PM Re: Multiwire circuit neutrals  
macmikeman  Offline
Member
Joined: Jul 2002
Posts: 717
Honolulu, Hawaii
What is the system voltage in this kitchen you are refering to?


#98068 - 05/03/06 02:16 PM Re: Multiwire circuit neutrals  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,124
Estero,Fl,usa
If this is not 3 legs of a 3p wye it is a violation and you will be overloading the neutral.

Art 100 definition of multiwire;
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.


Greg Fretwell

#98069 - 05/03/06 02:49 PM Re: Multiwire circuit neutrals  
dlhoule  Offline
Member
Joined: Apr 2005
Posts: 73
Saline, MI 48197
If this is not 3 legs of a 3p wye it is a violation and you will be overloading the neutral.
Art 100 definition of multiwire;
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Perhaps I'm just dense, but how does that definition make it illegal if it is not 3 legs of a wye. If you used a #10 gc and 3 #14s from any 240 V source; IMO it would be legal and satisfy the NEC.


#98070 - 05/03/06 04:03 PM Re: Multiwire circuit neutrals  
Roger  Offline
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Joined: May 2002
Posts: 1,716
N.C.
Dlhoule, in your example it would be legal. The #10 would carry the maximum amperage of the two # 14 ungrounded conductors on the same leg.

Roger


#98071 - 05/03/06 05:30 PM Re: Multiwire circuit neutrals  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
I don't agree with the example of using a #10 and 3 #14's for a few reasons:

1. Each ungrounded conductor must be a different color- which is not clearified in the example.
2. If it is a multi-wire branch circuit by virture of the fact that it is using 2 ungrounded conductiors that have a voltage between them per the definition, then I would not approve the additional ungrounded conductor tagging along on the grounded conductor of the multiwire branch circuit.
3. I am assuming that "#10 gc" means grounded conductor.
4. 210.4 (C) makes it to debateable and on top of that ir's wierd.


George Little

#98072 - 05/03/06 05:43 PM Re: Multiwire circuit neutrals  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
George than I have to ask if you think this is a case where the handbook is wrong?

[Linked Image]
Exhibit 225.3 A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).

Bob

[This message has been edited by iwire (edited 05-03-2006).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#98073 - 05/03/06 06:26 PM Re: Multiwire circuit neutrals  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
The diagram posted by Iwire comes from Article 225- Outside Branch Circuits and Feeders and is permitted outdoors (225.7(B)), but would not be permitted indoors as far as I can tell since there is no similar language in Article 210 Branch Circuits.


Few things are harder to put up with than the annoyance of a good example.

#98074 - 05/03/06 06:47 PM Re: Multiwire circuit neutrals  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
True Tom, but where is the language that prevents it?

What code section can be cited to fail it and why should there be one.

It is as safe as a feeder.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#98075 - 05/03/06 06:48 PM Re: Multiwire circuit neutrals  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
You are correct Bob. That is a good example. When I made that post I somehow knew you'd bring that up. I still say it's a poor design and an installer who installs this way is just asking for trouble. I can't give him a violation but I can first of all try and talk him out of it due to the potential for confusinon down the road and if that don't work you can bet your beppi he will be submitting an asbuilt showing all the intimate details of the cicuiting, wire numbering, loading and not just a skimpy sketch on piece of scratch paper either.

I have never seen anyone install anything even remotely close to that in real life. I've been in the trad for 40 years and an inspector for over 20.

I take that back- I did have a guy one time install some festoon lighting where he had a common neutral.


George Little

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