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Insane MWBC

Posted by pharkus (My Page) on
Thu, Nov 11, 10 at 10:24

Anyone who knows me knows I love these things.

Now, I have a question. This will work, electrically, and is safe, but I can't figure out if there's a rule against it.

You already know I can take two 14awg hot conductors, each on a 15A breaker but on opposite legs, and share one 14awg neutral between them. The neutral currents from the two circuits cancel each other out, and the maximum current through the neutral wire is 15A.

I can't add a third circuit, because two of them will be on the same leg, and their neutral currents would add up instead of cancelling. This means the maximum current through the neutral wire would be 30A...

... so why the heck can't I just upsize the neutral to accomodate? ie, shove three 14AWG hots and one 10AWG neutral down a conduit?


Follow-Up Postings:

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RE: Insane MWBC

"This will work, electrically, and is safe, but I can't figure out if there's a rule against it."

And you'll be connecting these circuits (and the 'super neutral') only to devices and fixtures rated for 30 amps? If not, I'm not altogether convinced that it's as safe as you think it is.

Consider, for example, a daisy chain of receptacles used to carry pass-through current down the line. Regardless of the current drawn by a single receptacle, it'd potentially be subjected to 30 amps of current.

***

You prolly already know this, but there is such a thing as a "super neutral". I believe they're mostly used in office and other commercial settings with scads of computers, electronics and/or lighting that tend to generate additive harmonic currents from non-linear loads.

Never heard of one being used in residential single-phase wiring, however. :-)


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RE: Insane MWBC

And you'll be connecting these circuits (and the 'super neutral') only to devices and fixtures rated for 30 amps? If not, I'm not altogether convinced that it's as safe as you think it is.

Consider, for example, a daisy chain of receptacles used to carry pass-through current down the line.

I believe you're not supposed to do this anyway. Pigtails, because, otherwise, failure of one receptacle can result in major problems downstream...


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RE: Insane MWBC

"I believe you're not supposed to do this anyway. Pigtails, because, otherwise, failure of one receptacle can result in major problems downstream..."

Not buying.

1. Even if you pigtail, the device is still potentially subjected to 30 amps.

2. Whether to pigtail or to use the second pair of receptacle terminals to continue a circuit is purely a matter of tradesman's preference. IMO, there are credible arguments for either option.

3. When they start offering only receptacles with two terminal screws -- one brass, one shiny -- maybe we can assume that everyone will pigtail.

4. And besides, this was only one of many possible examples of how a device rated for 15 or 20 amps could be subjected to 30A current.


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RE: Insane MWBC

How? If ...

okay I'm making all sorts of assumptions that I didn't specify, and you seem to be psychicly finding them.


I'm not planning to split-wire the outlets. I'm planning on there being three circuits that leave the panel through one piece of conduit using only one neutral.

The receptacle is not split-wired. The neutral wire to the receptacle is pigtailed. Unless the end user is doing something *REALLY* stupid, the only neutral current that can flow through the pigtail (and thus the device itself) is the current that originally went through hot on the same device, which is only on one of the circuits, which is limited to 15A.

If I DID want to split-wire the receptacles, which I don't, then I could achieve this by either A- keeping the 'two circuits' involved with the split receptacle as being the two that are on opposite legs or B- splitting the neutral jumper also and using two pigtails - or a 10AWG pigtail cleverly woven between both screws.


I can't wait to hear your other examples. :)


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RE: Insane MWBC

Assuming you pigtailed the neutrals as required, the receptacle is NOT subjected to 30 amps unless you split wired it from the two hots on the same side. Each hot is protected by a 15 amp breaker. That is the current limit the device will see. The neutral will not see more current than the hot.


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RE: Insane MWBC

"Whether to pigtail or to use the second pair of receptacle terminals to continue a circuit is purely a matter of tradesman's preference."

The option is NOT "tradesman's preference" for the neutral on a MWBC.

They are required to be pigtailed.


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RE: Insane MWBC

Ya know, pharkus (et al), I'm seriously beginning to think my original opinion is wrong about this, so long as things are otherwise correctly wired.

I'd be interested in brick's opinion as to whether or not the approach violates code in residential wiring. (IIRC, that was your original question before I helped derail it.) :-)


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RE: Insane MWBC

first of all the nonsense that you are describing is in no way, shape or form a mwbc. you should probably review the definitions in the front of the code book as calling things by the wrong name could cause confusion.

second, the "super neutrals" referred to in an earlier reply are for a completely different purpose. they are up sized to allow for the extra current generated by non linear loads, but are used on normal mwbc's. as i already stated, the strange conglomeration you are talking about doesn't qualify as a mwbc.

now, with that said, what you are talking about doing IS allowed by the code, but only for outdoor lighting i believe. i want to say to check 225.7b (no code book handy, been out of work for a while now). if you have the handbook i believe there is even extra explanatory material on this topic. since this particular section of the code is the only section i can recall that specifically allows this wiring method, i ASSUME that means that it is the only situation where this is permitted. of course, i could be mistaken about that.

what i don't understand though, is if you are such a big fan of mwbc's, why would you even want to do this? you would be completely negating any of the benefits associated with running mwbc's. you would eliminate the possibility of balancing neutral current (unless you added yet another hot leg off the other phase), and without the neutral being balanced it also would not help with voltage drop at all.

long story short, even though it's feasible, you'd be much better off just running multiple mwbc's.


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RE: Insane MWBC

It doesn't really matter, jmorrow, what you call it. The electrons don't care what the installer named it.

All I want to know is if it's legal.

And it is not your position or right to judge my decisions or applications.


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