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How do I install a GFCI receptacle with two hot wires and common neutral?

I am planning on changing the old large gauge/3 wire that fed my electric dryer into a multi wire branch circuit with shared neutral for the washer and new gas dryer to operate from a new receptacle, with the hot tab broken off, but a shared neutral. Since this is going to be in the laundry area, I want it to be GFI, but I think the shared neutral will cause it to have issues. What's the best way to do this?

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@Tester101 Since these outlets aren't daisy chained, I think this question is unique enough. –  BMitch Dec 29 '11 at 22:49
    
What amperage is required for your dryer? –  shirlock homes Dec 30 '11 at 12:01
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marked as duplicate by Tester101, ChrisF Jan 2 '12 at 11:49

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2 Answers

I believe I've seen that GFCI isn't recommended for high load appliances, particularly those with a motor (e.g. sump pump) and that it's not even required if the circuit is dedicated with a single outlet.

If you insist, then consider separate GFCI outlets for each device, wired in a double gang box. The GFCI will function perfectly normally since each is only protecting the outlet and not any downstream devices. That said, I would label this box as containing two separate circuits, so that anyone working on it in the future knows to turn both breakers off.

If you find and were to use a double pole GFCI breaker, I'd worry that this would confuse any electrician that works on this in the future, since that would usually indicate that a 220v device is installed on that circuit.

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You don't have to label it as having 2 circuits, if you're using a double pole breaker (which is required in multi wire branch circuits). There is no way to shut off one breaker, without shutting off the other. –  Tester101 Dec 30 '11 at 3:02
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If you want the GFI to be part of the breaker in the panel (assumed because you mentioned breaking the tab on the receptacle), it will work if you use a 2-pole GFI breaker (20 amps, using NEMA 5-20R outlets). NEC also requires a common device (e.g. untabbed duplex receptacle) be on a single breaker handle anyway, so you're stuck with the disadvantage of an overload or fault on one causing both to be shut off. A two-pole GFI breaker senses both hot wires and the neutral concurrently (e.g. it has just one current transformer inside).

But there is another option. You can use a pair of GFI receptacles. That will also still work since the GFI current sensor in each receptacle will be comparing the hot and neutral from just its own receptacle connections. The old dryer branch circuit is electrically equivalent to the split-phase service drop coming into your panel. So this means you need a 4x4 two-gang box. But that's what the dryer outlet usually already has. If you have an older one-gang box, you need to expand it to two gang to do this. You can use ordinary duplex GFI receptacles for both phases, but simplex GFI receptacles do exist if you really want just one on each.

If this is an older dryer circuit with no separate ground wire (and had the "sad face" NEMA 10-30R outlet instead of the newer 4-slot one), you will need to disable use of the ground hole. The GFI sensor and test button will still work, since the test button uses the feed-side neutral instead of actual ground.

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The dryer was actually hardwired into a weird looking outlet box with some plug in fuses for the black and red wires. There are 4 wires total in the box, black, red, white, and ground. We're switching to a gas dryer, so this is the reason for the change. –  Aaron Dec 29 '11 at 23:05
    
Ok, now your question make more sense. Treat it like any other 3 wire (120/120) feed. Just be aware that motor loads often will trip GFICs on start up or rapid shut down. –  shirlock homes Dec 30 '11 at 12:06
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