12/3 split to two outlets with shared neutral?

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Old 11-26-16, 07:02 AM
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12/3 split to two outlets with shared neutral?

So I've got this 250' roll of 12/3 that I picked up on sale somewhere along the line. I've also got this 24' x 24' garage that I'm going to wire up as my shop. I remember something about being about to use a shared neutral on a split outlet but I don't remember if anything special is required or not. What I'm thinking of doing is string it around the shop to all the outlets and either split the outlet with one circuit on top and another on the bottom or more likely just use dual gang boxes every where with the left hand outlet for one circuit and the right hand for another, sharing the neutral.

If that's not cool then I'll just do it normal with 12/2. I was just trying to figure out a way to use this 12/3. The goal if it's not obvious is to make sure I can run two fairly high draw devices from the same location without popping breakers. Say a 15A table saw and a 10A shop vac or something.

Thanks in advance!
 
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Old 11-26-16, 07:29 AM
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12/3 can share a neutral, if you use a 2 pole breaker (2 pole 20A).
12/3 will not work on two GFCI circuits. Each GFCI needs it's own neutral and cannot share one. If you tried that, the GFCI would trip ever time.
Your garage outlets require GFCI protection.
 
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Old 11-26-16, 07:36 AM
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You could use a 2 pole 20 amp GFCI breaker and share the neutral. Then you can split the 2 circuits anyway you want.
 
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Old 11-26-16, 08:56 AM
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Also note that the neutral needs pigtailed in boxes when shared - it can not use the receptacle to pass-through. Idea is if receptacle breaks or gets pulled the voltage stays the same for devices down the line.

Multi-wire branch circuits (MWBC) work well. Just require some thought in laying them out.
 
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Old 11-26-16, 09:09 AM
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All good stuff, thank you. Pig tails check, pricey breaker check. But it would physically work.

Any other pros/cons to this? The main con I see is that a nuisance trip on the GFCI, not uncommon with cheap tables saws and other corded motors, will kill both circuits.

I can go look this up in my NEC books but what are the circumstances GFCI not required? What I remember from when I wired the house was GFCI was not required for certain dedicated outlets with just a single outlet. Like a washing machine with one outlet and marked washer only. So nothing else could be plugged in. But it's been a few years since I took those resi electric classes and wired the house, things are fuzzy.

I'm specifically thinking of the garage doors and the air compressor. I didn't get a very large panel, which I will regret. I was thinking a 20a circuit for the two doors and ending at the small compressor. Reading above makes me realize that probably has to be GFCI as well.

Thanks.
 
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Old 11-26-16, 11:15 AM
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GFI's are required just about everywhere where there is a damp, wet location or any place where leakage to ground is possible.

The only place a single receptacle may squeak by is on a refrigerator or freezer and that's up to your AHJ.... otherwise those appliances are also required to be on GFI's.
 
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Old 11-26-16, 04:21 PM
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You can put ground fault circuit interrupter receptacles on a multiwire branch circuit provided that you do not continue the MWBC using the load terminals of a GFCI receptacle unit.

Non-MWBC subcircuits using 12-2 wire can branch off of the load terminals of any of the GFCI receptacles.

Given that the incoming and continuing neutrals of the MWBC itself must be connected directly to each other (with a pigtail to the receptacle itself) there is no compelling need to use the load terminals of the GFCI receptacle except that each outlet box utilizing the MWBC main line without a GFCI breaker will need its own GFCI receptacle.
 

Last edited by AllanJ; 11-26-16 at 04:38 PM.
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Old 11-27-16, 06:02 AM
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I've also got this 24' x 24' garage that I'm going to wire up as my shop
That pretty much is the key right there. In a residential garage, all 120 volt, 15 and 20 amp receptacles are required to be GFCI protected. General purpose receptacles are also limited to 20 amps max. The rule for equipment that is fixed in place not required to be GFCI protected went out in 2005 or 2008 code cycles.
 
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Old 11-27-16, 09:00 AM
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Well, I'm a 1970s kind of guy

My resi program that I took before rewiring the house would have been 2004 so that makes sense and I'm glad you all pointed it out.
 
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Old 12-01-16, 10:48 AM
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120v / 240v

One other advantage, and I'm using this in my new wood shop. With the 3-wire cable to each box, you have the option of wiring a 240V receptacle by using the two hot wires (red & black, I presume). Makes for some flexibility down the road.
 
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Old 12-01-16, 11:23 AM
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No.... that is not correct. You would need to run a dedicated red/black wire for a 240v circuit.
 
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Old 06-07-17, 01:38 PM
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Bringing my own thread back. I finally wired this circuit, but it's been fired up so far with a regular two pole 20a breaker that I already had. Need to finish this out right before I forget about it and never do it.

I see now I only needed to pig tail the neutral. I pig tailed the hots as well. I suspect I may not meet box fill requirements with all that 12/3 coming and going, but it's wired cleanly and working out well. Finished it up a couple weeks ago and have already used it a ton. Lots of uses with the 12a shop vac running at the same time as another high draw device and it's easy to make sure I'm on different legs and not tripping breakers. Not sure I would have done it this way if I didn't have that 12/3 sitting there, but it's working great.

Is this the correct breaker for my Siemens panel? QF220a?

https://goo.gl/lpq7FN

Thanks!
 
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Old 06-07-17, 06:35 PM
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Yes, that breaker will give you GFCI protection on both 120 volt legs.
 
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Old 06-07-17, 08:18 PM
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Just be aware that those 2 pole Siemens GFCI breakers are longer than standard ones. They intrude into the gutter space so can make things tight.
 
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