120V Wiring: 2x 12-2 runs or 1x 12-3 run with shared neutral?

[Jay Maiers]

In planning my soon-to-be workshop space, I've laid out a room with 10 wall mounted quad boxes. I'd like to run these on two circuits, every other box on the same circuit. The inspector said it's acceptable to use 12-3 wire to create 2 20A circuits for these outlets. These are non-dedicated outlets.
Is this a good or even reasonable idea, or would it be better to just run 2 12-2 lines along the same path? The room is in an unfinished basement. Pulling 1 wire vs. 2 is not a problem.

I guess this is really a "Best Practices" kind of question. The only real issue I see here is the possibility of overloading the neutral line. I can't imagine doing so (having come from a work area with 1 15A circuit), but I thought I should ask...

Thanks so much!
Jay

[Rod Sheridan]

Hi Jay, if you use the multi-wire branch circuit (3 conductor) you need a double pole breaker.

My entire shop is wired this way with one double pole breaker supplying all the 240 and 120V outlets except for the cyclone.

You cannot overload the neutral in this scenario as the lines are 180 degrees out of phase so the neutral only carries the unbalanced current.

The multi wire branch circuit requires pigtails for the neutral, check with your inspector on the details.

It's the most cost effective circuit configuration.

Edison may have picked the wrong system (DC vs AC) however his multi-wire circuit was a keeper.

Regards, Rod.

[Mike Henderson]

As long as everything is done correctly, a three wire circuit is okay. But you have to think down the road. If someone later comes in and puts both hots on the same side of the transformer they're going to cause problems. Also, if someone changes the double pole breaker to two single pole breakers someone working on the circuit can get shocked.

According to an electrician friend of mine, those circuits are not permitted in the LA area any more for the above reasons.

Personally, I'd put in the additional wire.

Mike

[Ken Fitzgerald]

Be aware that if you do the "EDISON" that GFCIs won't work properly.

As our own electrical engineer Moderator Chris Padilla about his experience.

He ended up repulling his circuits after he had covered the wall IIRC.

[Jay Maiers]

Be aware that if you do the "EDISON" that GFCIs won't work properly.

As our own electrical engineer Moderator Chris Padilla about his experience.

He ended up repulling his circuits after he had covered the wall IIRC.

Well, that's a problem. Given that this is all in an unfinished basement, and it will remain unfinished for quite a while, all non-dedicated outlets require GFCI (breaker or outlet). Sigh.

Oh well, it was a good thought...

[David G Baker]

I may have wired a few circuits using the #12-3 and always wondered if it was wrong. I searched the net and didn't find an answer. The answers given here have convinced me that I should not have used the #12-3 wiring method. Thanks guys. I usually find out that if I wonder about how to wire something it is usually not a good idea to use the method in question without consulting a pro.

[Jay Maiers]

I may have wired a few circuits using the #12-3 and always wondered if it was wrong. I searched the net and didn't find an answer. The answers given here have convinced me that I should not have used the #12-3 wiring method. Thanks guys. I usually find out that if I wonder about how to wire something it is usually not a good idea to use the method in question without consulting a pro.

I'm pretty good about researching and finding my own answers even if I do ask the question to verify my findings. This one, however, was something I'd never considered; it just saved me the hassle of correcting my permit application drawings :)

That advice is worth way more than I paid, so to balance things out I just donated to Sawmill Creek :)

[Rod Sheridan]

Well, that's a problem. Given that this is all in an unfinished basement, and it will remain unfinished for quite a while, all non-dedicated outlets require GFCI (breaker or outlet). Sigh.

Oh well, it was a good thought...

The Edison circuit works fine with GFCI receptacles if they're installed properly.

You're distribution panel is an Edison circuit in itself.

For example you can take the red, black and white conductors to a convenient box, then feed the red/Black off to your 240 volt receptacles.

The 120 V circuits will be black/white and red/white which can each feed a GFCI, then those 2 GFCI's can subfeed all the other 120V receptacles that are required to be GFCI.

Other 120V circuits are red/white and black/white, fed from before the GFCI's.

Regards, Rod.

[John Coloccia]

The only time I've done it is when I was one wire short on box/conduit fill. Other than that, I would never do it just because it adds a complication that needn't be there for no obvious advantage.

[Curt Harms]

The only time I've done it is when I was one wire short on box/conduit fill. Other than that, I would never do it just because it adds a complication that needn't be there for no obvious advantage.

That would be my thought as well. Wire just isn't that expensive or hard to pull in an unfinished basement.

[Bruce Wrenn]

Instead of Edison, look up "multiwire branch circuits." Very doable as long as you don't mind putting a GFCI recpt at each location. Down stream from GFCI devices, they can't share a single neutral. Each circuit has to have it's own neutral. Otherwise there is no way for the GFCI device to figure out if the circuit is bad, or if it is sensing current from other leg of multiwire. Because my shop is in a detached building, and when it was built, I only have to have GFCI protection on recpts nearest the doors.

[Van Huskey]

I like MWBC however you have to understand them to install or modify them!

Rod mentioned pigtails, which are required. NEC "The removal of a wiring device, such as a receptacle, must not cause an interruption of continuity for the grounded (neutral) conductor in a multiwire branch circuit [300.13(B)]."

You also have to make sure hots are wired out of phase (also mentioned by Rod) not doing this can overload the neutral and cause wire fires. The hots will be wired to DIFFERENT busses in the service panel. The double pole breaker with a trip tie (also Rod mentioned) ensures it is wire correctly (every other space on a common service panel is on a different bus, thus out of phase with the one beside it). Going off memory I am not SURE NEC requires it but it is THE way to do it. Upon checking it looks like NEC now requires it as of 2008 which may or may not be what is required in your area, but again do it whether required or not.

If you have MWBC coming out of the service equipment you have to make SURE you NEVER remove a nuteral from the ground bar if the phase conductors are energized on a MWBC, if you do this the circuit goes from being a 120V parallel circuit to a 240V series circuit and can wreak havoc on items connected to it.

Also be aware that non-touch induction type testers may NOT show the leads as being hot even when they are due to the long distance and close proximity of the hot legs.

There are several problems with GFCI receptacles but the way around all the issues is a a three conductor (ie 12-3) home run, and 2 two conductor (ie 12-2) runs on the downstream (load side) of the GFCIs. This is useful for cost/labor if you have long home runs or a difficult run in retrofit but not usually an issue in a shop which is often serviced by a sub panel and not the main service panel. This is used often to accomodate the two required kitchen circuits. There are ways to make this work BUT the simple thing to do in a shop is wire seperate circuits. The extra costs will not be much and will simplify the wiring and prevent issues down the line if you or someone else doesn't notice that there is a MWBC in place.

Personally, I wired my shop with no MWBC and used only GFCI breakers not receptacles, it was a little more expensive (maybe $30 more for each circuit) but I prefer this approach. Remember they only exist to save money/labor, this is a big issue with a professional but is modest for a 1 time wiring job and is one less hassle for a non-pro to have to figure out.


To answer the original questions:

1. no best practice issue both are acceptable one just happens to be cheaper BUT has more potential issues

2. as long as it is wired correctly there can not be a neutral overload since they are out of phase