Do I use 12/3 Romex?

Old 09-12-04, 09:13 AM
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Do I use 12/3 Romex?

I'm trying to wire a new 20AMP dedicated circuit to the garage and another 20AMP dedicated circuit to a room right next to it. I was told I could use a 3 conductor Romex cable(12/3) and use the red and black wires for the separate circuits, each going back to the breaker box to two separate 20AMP breakers. The neutrals would all be joined near the two rooms, would this work?
Old 09-12-04, 10:31 AM
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It would work if done properly. It is called a multiwire circuit. It is generally not advisable for a DIY to use a multiwire circuit due to some of the anomalies that may happen. Just install two 12-2 cables.
Old 09-12-04, 11:08 AM
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There are at least a dozen special rules for a multiwire circuit, and it limits current and future options, so unless you have a special reason to want one, I strongly agree with Ron that you should probably not do it. The most compelling reasons you might want to use a multiwire circuit are: (1) very long run (at least a 100 feet) and high power needs including motors, such that voltage drop might become significant. (2) very difficult conditions to pull wire, such that pulling one 12/3 is significantly easier than pulling two 12/2. (3) wiring a detached structure without a subpanel.
Old 09-14-04, 07:37 PM
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I agree with all that is said about multi-wire. If you follow the rules (NEC article 210 is a good place to start), then you are according to code, but doesn't mean that someone (even you) can't come later and mess it up (boom).

Tag that circuit at every location as "Multiwire". Wrap a sticker/piece of tape around the wires and mark as "Multiwire". A small hangtag is also nice.

Do your best to communicate that those two hot wires have a relationship that must be respected. Tag at every opportunity, ESPECIALLY inside the breaker panel. I also ziptie the two hots together as they exit the two breakers, to reinforce the relationship.

I don't feel comfortable using multiwire for computers or high dollar electronics, they might be at an increased chance for surges/transients.

Multiwire for three-phase circuits is downright confusing.

You may find that the cost-saving isn't there, if you use only a half roll of 12-3, it would be cheaper to use a full roll of 12-2.

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