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I want to run 12/3 wire to gfci-outlet-outlet-outlet-gfci-outlet-outlet-outlet. How can I accomplish that? There will also be 2 separate breakers

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    So a MWBC. Only with a double pole GFCI breaker. Either use that or make it not a MWBC. – Dan D. Aug 5 '18 at 18:52
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    What make and model is your breaker panel? Can you get 12/2/2 NM? – ThreePhaseEel Aug 5 '18 at 19:46
  • Why are you wasting money on 12/3? 12/2 is the correct stuff for a string of outlets. – Harper Aug 5 '18 at 20:18
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    RE: "12/3" and "...2 separate breakers", are you planning on using a common neutral? – BillDOe Aug 5 '18 at 20:21
  • I must confess that I don't understand the circuit the you propose. Do you want a total of 8 (duplex?) receptacles on two breakers in which each breaker feeds one GFCI and three standard duplex receptacles? Presumably each breaker would feed one GFCI and three std duplex receptacles? This must not be the case because you would just have two separate breakers and not be thinking of 12/3! – Jim Stewart Aug 6 '18 at 1:01
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You aren't allowed to do it that way anymore

That trick you learned a long time ago has been hit by a bunch of code changes, which almost moot what you plan. It also has a name: Multi-Wire Branch Circuit.

  • Neutrals must be pigtailed at every splice point or receptacle. You cannot land the two white wires on a device. If you ever removed the receptacle, it would sever the neutral for the other half of the circuit, which is not allowed.
  • You must use a 2-pole breaker with common trip, or you must use two adjacent full-size breakers with handle ties so you must turn them off together.
  • Using a double-stuff is NO; that will set your wiring on fire. A quadplex can be used if you really know what you're doing and put it on the 2-pole middle.

If you do all those things, and stop to make sure you follow the other MWBC rules, then it is OK. But...

This does not play with GFCI

The problem with GFCI and MWBC is the shared neutral. A neutral cannot be both the MWBC shared neutral and also the protected-zone downline neutral coming off the load terminals of a GFCI+receptacle.

Every GFCI+receptacle device comes with a warning sticker across the LOAD terminals. It says "Do Not Use. for Wizards Only." Or words to that effect.

Option 1 is to heed that advice and fit GFCI+receptacles where you need them, not using the LOAD lines at all. That means up to eight GFCI+receptacles.

Option 2 is fit a 2-pole GFCI breaker, and wire the circuit in the normal way you intended, but with normal outlets. DONE. This works because the entire MWBC complex is inside the GFCI protected zone, so you can do all the normal MWBC things therein. A 2-pole GFCI breaker is typically $80.

Option 3 is to fork the MWBC at the first receptacle, and carry them as 2 separate circuits each with their own hot-neutral. You'd need /2/2 cable (black red white white-striped-red): /4 would not do since it only has one neutral. You could also use two separate /2 cables in the obvious way.

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    Option 4 is to run 12/2/2 for the homerun as well, simply making it two branch circuits all the way thru, btw. – ThreePhaseEel Aug 5 '18 at 22:59
  • What about pig tailing the supply line neutral to the two GFCI receptacles, black in the first and red in the second. Then use the load terminal from each GFCI down their separated chain. Wait this is option 3, isn't it? – J. M. Becker Sep 11 at 21:25
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Why do you want to run 12/3? What function or operational capability do you expect from this?

This would use 12/3: std 2-pole common-trip breaker with 12/3 to a std duplex receptacle with the hot tab broken. Then split with 12/2 to a pair of GFCI receptacles with following receptacles fed from the loads.

Or 12/3 from the 2-pole breaker to a 2-gang box with two GFCI receptacles. From the load contacts go to separate branches with std receptacles.

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