Multiwire circuit on 208v?

[jack]

I want to add a circuit to a 208 panel, but there are no holes and I really don't want to go at it with a hole saw.

There is a circuit consisting of one outlet connected to the box by 1/2" conduit. If multiwire circuits are allowed on 208v, I could put the outlet on one leg of the multiwire circuit and put my new circuit on the other. That way I can just put my new MC cable to the bottom of the outlet box, which does have a hole.

Seems to me that it should be pretty much the same as 240v, except that some current will always be returning over the neutral; but there is no harm to that. But maybe I am overlooking something (which, given my knowledge of

208v, is not unlikely)

If I can't use a multiwire circuit, can I simply attach the MC cable to the bottom of the outlet box and run it up through the conduit to the panel box? I think 1/2" will take 6 #12 wires.

Thanks.

[Mikepier]

Do not split a 240V circuit. If you need an extra knockout, why don't you use a duplex BX connector like this:

[RBM]

What you're proposing is fine. Bring your cable into the outlet box, splice the two neutral wires together with a pigtail to the outlet, as the neutrals of a multiwire circuit cannot be dependent upon a device, and run your new hot leg for the new circuit to a circuit breaker adjacent to the one feeding the outlet circuit

[Wade Lippman]

Do not split a 240V circuit. If you need an extra knockout, why don't you use a duplex BX connector like this:

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If I had known that existed I would have used it. But now the multiwire idea just seems more elegant. Do you know of any reason not to use a multiwire circuit?

[Wade Lippman]

I was planning on taking the neutral off the outlet, so I appreciate you pointing out that it is improper. Pigtailing is almost as easy.

I guess what you don't want is the neutral connection to the box breaking. With a pigtail you lose everything or nothing, so you can't get a floating neutral; is that the idea?

[bud--]

That works. Normally you would add 2 wires to the conduit and splice to MC at the outlet box. In both cases the outlet box needs to be big enough.

You have 4 conductors in the conduit. The wire ampacity has to be derated to 80% (4-6 wires). If you are using #12 THHN the ampacity is

30A which derates to 24A (and can only be used at 20A).

I agree except the 2008 NEC wants a common disconnect on multiwire branch circuits (210.4). Could be a breaker handle tie. (The change should make multiwires much less attractive.)

[gfretwell]

Harmonics ... what kind of load is it

[Wade Lippman]

One circuit is a small water heater, the other a convenience outlet next to the panel box. I suppose a light or a radio occasionally.

[metspitzer]

Unless he answers........a bank of computers. What have we learned here?

I have never heard a situation of harmonics being a problem in residential wiring. Maybe someone can give us an example.

[gfretwell]

He said 208, that implies 3 phase, where triplin harmonics can be a problem ... and it is any reactive load.

The water heater and a light C&P load should not be a problem.

[RBM]

That is the idea, exactly, and as Bud correctly points out, the 08' NEC requires the breakers of the two circuits to be tied. This may or may not apply, as I know in my area we're still using the 05' code, but it's a good practice to get into, to prevent Edison circuits from accidentally getting screwed up

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