Common Neutrals

[Jimmy4645]

I have a question regarding multi-branch circuits utilizing a common neutral. For example, you have a network: Phase A, Phase B, Phase C & Neutral. According to my co-workers, in the NEC 2008 each phase needs a separate neutral home-run back to the panel and no sharing of neutral 'common neutral? is allowed. I?m wondering if this statement is valid, because I cannot find it in the NEC 2008 code book. I would appreciate any help?

Thanks

 

[infinity]

It is not valid. The new requirement for the 2008 NEC is that MWBC's have a handle tie or multi-pole CB serving the circuits sharing the neutral.

 

[iwire]

The NEC allows it as Rob mentioned, many job specifications do not allow it.

Perhaps your coworker was confusing job requirements with NEC requirements.

 

[marti smith]

They may be considering 210.4 (D), although the rest doesn't make much sense, unless your jurisdiction means to specify their own ruling.

Electrical theory shows us each phase is 120 degrees apart: if the return current is from 3 phases on one neutral, there will be little heat increase that is any different from the outgoing current, on the neutral. If more than one conductor of the same phase is sharing a neutral, the return current is no longer out of phase (because the same phase current exists) and heat is created, usually showing up in fried insulation at the point of greatest resistance. I'm sure I missed something but it's something to chew on.

 

[SG-1]

One reason for doing this would be with a load that could be found in a Data Center where the third harmonic could, I said could add up on the neutral to a point that a 100% neutral would be overloaded.

I think I remember something in the notes of the 2008 code that referenced this situation. It was before section 250. I am separated from my Code Book until tomorrow though.

Steve

 

[daleuger]

It is not valid. The new requirement for the 2008 NEC is that MWBC's have a handle tie or multi-pole CB serving the circuits sharing the neutral.

That's gonna cost some extra on jobs. Anybody still say there aren't a lot of money rackets in code?

 

[infinity]

That's gonna cost some extra on jobs. Anybody still say there aren't a lot of money rackets in code?

Do you mean because of the cost of handle ties? IMO they would be cheaper than using a separate neutral for each circuit.

 

[hunt4679]

I think it was a safety issue. Even if you use a breaker tie I think you still have to bundle all wires sharing a neutral or id the cir numbers on that neutral.

 

[infinity]

I think it was a safety issue. Even if you use a breaker tie I think you still have to bundle all wires sharing a neutral or id the cir numbers on that neutral.

You're correct about bundling if there is more than one MWBC in a raceway. Where do you find the "id the cir numbers on that neutral" permission in the NEC?

 

[daleuger]

Do you mean because of the cost of handle ties? IMO they would be cheaper than using a separate neutral for each circuit.

No no....I mean the cost of pulling all those extra neutrals possibly upsizing conduit. Ever had a set of prints where it calls for say 37 (black) and I dunno 41 (blue) in the same vicinity but 39 (red) may run the other way from the panel? Oh no 37 and 41 can't share because a tie would throw in 39 too.

I think it was a safety issue. Even if you use a breaker tie I think you still have to bundle all wires sharing a neutral or id the cir numbers on that neutral.

Nothing wrong with that. I'm probably wasting my breath....or in this case keystrokes, but in a perfect world if people would simply label and bundle a little bit no reason for the next guy to get bitten by a loaded neutral. OOOOOH or I know.....always treat it like it's hot. That ring a bell anywhere? Safety is one thing but at what point does it become overkill? IMO eventually they're going to make our trade so that any untrained monkey could do it. Then see how much you complain about the money you don't make. :grin:

 

[infinity]

IMO individually numbering each neutral of a MWBC is better than a piece of tape around those conductors. Only problem is that the NEC hasn't figured that out yet. :roll:

 

[e57]

IMO individually numbering each neutral of a MWBC is better than a piece of tape around those conductors. Only problem is that the NEC hasn't figured that out yet. :roll:

I fail to see the difference - other that with a bundle you don't actually have to see the number - just get all the conductors together - knowing what circuit they are on is a different set of problems. But I agree it is nice to know, but don't see it necessary to make it a code requirement.

 

[infinity]

I fail to see the difference - other that with a bundle you don't actually have to see the number - just get all the conductors together - knowing what circuit they are on is a different set of problems. But I agree it is nice to know, but don't see it necessary to make it a code requirement.

I'm not advocating a code requirement but a code option. IMO a piece of tape is inferior to individually number neutral conductors, for example, those in a panel. The NEC does not provide us with that option. Even fully numbered neutrals would still require that piece of tape.