Protecting multiwire circuit with a GFCI breaker


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Old 03-15-15, 05:30 AM
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Protecting multiwire circuit with a GFCI breaker

Just trying to confirm that my logic is correct in this application. My friend is in the process of selling his house. Old cloth wire no ground wire in some of the house. Meantime, buyer's inspector found some receptacles not grounded so I suggested is cheaper to go with a gfci breaker to protect them all since they are on one circuit. An individual gfci receptcle not good because the existing boxes too shallow and would require a wire mold box or cutting out old and putting in new etc. we decided on a breaker for ease.

So we buy a single 20amp gfci breaker and I pull the panel cover off to install it. Of course I have to trace back the hot to find the neutral. Upon finding where the romex comes into the panel I noticed it is a 12/3. Oups!! Who ever hooked up this multiwire did not use a two pole breaker so when i originally looked at the breaker with the cover on there was no way for me to know this. I told him it won't work. He insisted i try it anyway. Knowing, no harm and it would just trip it did. So I told him why it would not work. Well he had a few thoughts thinking maybe i was not sure but i knew. I told him we have to get a 2 pole 20amp gfci breaker in order to accomplish what we wanted to do. More than likely the home depot would not have this in the store and would have to be ordered from their site. (using home depot because he wants to use his credit card from them). So he wanted to go to the store anyway to see if they have one. He decided he was going to run this past the guy in the electrical isle. Well that was a mistake. The guy was insisting that if we used a two pole breaker we would be sending 240v to the receptacles. I kept explaining to the guy that it is a multiwire circuit but it seemed to go over his head.

The other thing is too that even though there was no tie between the two breakers for the multiwire circuit one breaker was a 15amp and the other a 20amp. So my friend now is kind of in question as to why I would know more about this than the guy in the electrical isle (who is probably from the paint isle covering for the electrical guy that is on lunch).

Granted the 20amp 2 pole GFCI is $98 it is still better than trying to use a gfci receptacle because of the appearance of a wire mold box and all the other stuff that needs to be done with the expensive molding, painting etc in order to protect the circuit from this first receptacle on the run.

Is my thinking correct here? You need the two pole because using a single pole it senses an imbalance on the neutral immeiaately because it is only monitoring one hot going out.
 
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Old 03-15-15, 05:43 AM
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Using a ground fault circuit interrupter breaker on a multiwire branch circuit (two hots sharing a neutral) you need a GFCI breaker unit intended for that purpose namely a double wide double pole GFCI breaker.

The double GFCI breaker, like any breaker set serving an MWBC, must be positioned in the panel to provide 240 votls across the two hot lines. No, the individual 120 volt receptacles will not receive 240 volts when everything is wired up correctly.

Yes, your thinking is correct. You need the two pole GFCI breaker because using a single pole it senses an imbalance on the shared neutral immediately because it is only monitoring one hot going out.
 

Last edited by AllanJ; 03-15-15 at 06:30 AM.
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Old 03-15-15, 06:25 AM
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Yes, a 2 pole GFCI breaker is appropriate for a MWBC. Though they are not available for all panels.

1 - It may need to be 15 amp on both poles - check if the wires are 14 gauge or 12 all the way to the receptacles.
2 - Easier option is to buy 2 prong receptacles. Leviton 15 Amp 2-Wire Duplex Outlet - White-R52-00223-00W - The Home Depot
 
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Old 03-15-15, 06:34 AM
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Yes, the 15 amp breaker for one half of the multiwire branch circuit should arouse suspicion. If there is any 14 gauge wiring within that branch circuit (including to a light fixture but not including the tiny wires going from the outlet box to the lamp socket or ballast). then the maximum breaker for that circuit is 15 amps. If you isolate the 14 gauge wiring to something that was hung off the circuit at a later date, that 14 gauge subcircuit might be disconnected and decommissioned pending the running of a new feed for it and the plan for the 20 amp breaker carried through.

For most older homes with metal outlet boxes, the boxes are too small to replace existing duplex receptacles with ground fault circuit interrupter receptacles.
 
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Old 03-15-15, 07:32 AM
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The guy was insisting that if we used a two pole breaker we would be sending 240v to the receptacles
The guy was probably not very knowledgeable. But, you will have 240V inside the first receptacle box. The receptacle will not be 240, but you do have 240 available there.
It's important to pigtail the neutral at this first wiring box/receptacle. Don't use the side terminals on first receptacle to connect neutrals downstream to additional receptacles.
 
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Old 03-15-15, 07:46 AM
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If this is wired using one cable there should be no need to have the 15 amp breaker and a two pole 20 could be used. Hopefully no one installed any #14 down the line.
 
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Old 03-15-15, 06:24 PM
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Who ever hooked up this multiwire did not use a two pole breaker so when i originally looked at the breaker with the cover on there was no way for me to know this.
Having a common disconnect by using a 2 pole breaker or breaker handle ties wasn't required till the 2008 NEC.

He decided he was going to run this past the guy in the electrical isle. Well that was a mistake. The guy was insisting that if we used a two pole breaker we would be sending 240v to the receptacles.
That doesn't surprise me at all.
 
 

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