How many circuits can one neutral share?
#12 stranded wire.
Code wording is unclear to me, it reads -
TWO or THREE sets of 3-wire feeders or TWO
sets of 4-wire or 5-wire feeders.

(Can I share a common neutral with three 20 amp single phase circuits?)Thanks in advance.

on single phase you can share a common neutral with 2 circuits of the same ampacity as the neutral. for example, two 20 amp circuits can share one 12 gauge neutral. The catch is that the 2 circuits must be on OPPOSITE legs of the 240 volt breaker panel. The neutral will only carry the differnece in current between the two circuits. for exampl, if one circuit has a 12 amp toaster on it and the other has a 9 amp coffee maker, the neutral will be carrying 3 amps

Don't forget the requirement of D/C'ing BOTH current carrying conductors with one motion (tied OCPD's) for the circuit in questions.

Doug, that only applies if the two circuits supply device(s) on one yoke such as split wired duplex receptacles. (210.4) If this is simply a 3 wire home run then a two pole breaker is not required.

You are correct. Thought that the interp. included shared neutrals in general.

Its amazing how mis-interpreted that provision is.

Rob, let's look at your question closely.

First of all 215 is for feeders so we will leave this section and go on to 225.7(B) which will allow you to put as many circuits as you want on a properly sized neutral. Yes this section is specific to "Outside Branch Circuits and Feeders" but there is nothing in any other section prohibiting it for other applications.

If you are using a neutral conductor the same size as the circuit conductors, then you could have a neutral common to both ends of a single winding, (transformer) or common to three windings connected in a wye or star configuration. This goes with Cpalm1's post

I know, I'm causing trouble.

Roger

[This message has been edited by Roger (edited 01-13-2004).]

Say that there were 4 15 amp circuits sharing one neutral. If 2 circuits come from one side of the supply, and the other two are fed by the other side, then can I use one neutral that is sized to take 30 amps (AWG 10 IIRC)? This would have to be in one conduit I would think.

Wa2ise, yes that would be allowed per 225.7(B)specificaly. Seems strange doesn't it.

Roger

Roger;


Not even if you tried to!

Addressed things which are indeed very related to this scope.

I like the descriptions of Transformers, which basically caught my eye - and led to this reply!

To the original poster:

Although the idea of using an oversized Common Grounded Conductor for several multiwire circuits is compliant to the NEC, it still just sounds odd to me!
It works, and should be safe - if installed properly; it's only a "different" method, very much "unthought of" by many of us here - including myself!

Regardless of which option you choose - be sure to have a "Balance" of Ungrounded Conductors which are derrived from Breakers on "Opposite Polarity Points" in the Panel.
Simply, if you can measure 220-240 VAC across two Breakers, use these two circuits for a single multiwire circuit. Step down 1 space from that multiwire for the next one.

BTW: Per the term "Opposite Polarity Points", I use that term because "Opposite Phases" really does not apply to a 1Ø 3W system.

Talk about starting trouble

Scott35

Hello Scott.




Roger

I see that the definition of multiwire branch circuits requires the non-grounded conductors to have voltage between them. Having two single-phase 'line to neutral to line circuits' sharing an oversized neutral would NOT be considered a multiwire branch circuit.
110.8 says that only wiring methods recognized as suitable are included in the Code. By inference, other wiring methods not recognized by the Code must then be questionable, and therefore the inspector can freely reject them.

Anyway, back to curiosity prompted by Rob's original question, I too would like to know why the Code allows such a freak multi-multiwire feeder circuit. What would be a typical installation for such a scheme?