Wiring a 3rd garage stall. Do I need to start over?

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Sharing the neutral _after_ the GFCIs (load side) is what causes the problems.
It's perfectly reasonable to put two gfci's on the end of a 4 wire 240V circuit, but the neutrals on the load side of the GFCIs MUST NOT connect to each other in any way.
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On Fri, 03 Feb 2006 21:07:48 -0000, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

Something I posted a few times recently. Those posts were ignored.
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wrote:

Looks like you will if you do, you won't if you don't.
The neutrals will always be shared before the GFCIs (line side). I expect that the previously mentioned problem had to do with sharing neutrals after the GFCIs (load side).

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Some people would add the 20 on one pole and the 20 on the other to get 40, without realizing that this doesn't correspond to reality (considering different phases). Like those 3 12-year-old boys who thought they were 36 together.

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amateur_russ wrote:

1) Use a regular 2-pole 20A breaker instead of a GFCI unless there you have some other code requirement for GFCI breakers.
2) Start over from the 1st outlet in the garage. Use a deep double-gang box and install *two* GFCI's. Put one on the black wire and one on the red white and the white goes to the LINE side of both.
3) Connect a 12/2 cable to the LOAD side of each GFCI, and run these 12/2 cables to the rest of the outlets (alternating as you go).
4) It's a lot simpler than it sounds! ;-)
Bob
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You misunderstood him. You _cannot_ use two independent GFCIs with a shared neutral circuit. It just _won't_ work at all, period.
He didn't mean "trips at the same time", he meant "they _both_ will trip" continuously.
The reason for this is simple: the hot and neutral current though _each_ GFCI must exactly balance. But in a shared neutral circuit, the current through the neutral is the _difference_ between the two hot legs. So as soon as you plug _anything_ into any one of these outlets, both GFCIs will trip.

Yes.
You don't understand 240V multiwire circuits.
You simply need a 2-pole 20A GFCI breaker. It's the same thing as two independent 20A breakers, except that the poles are tied together so they both trip at the same time. And it has a GFCI function too.
MAKE SURE that the poles are on opposite legs of the main panel. If the current breakers aren't on opposite legs (best: adjacent to each other), you're going to overcurrent the neutral.
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On Tue, 31 Jan 2006 22:54:21 -0000, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

This would apply to sharing a neutral on the LOAD side of the GFCIs (you have to have a separate neutral for each here). Sharing a neutral on the LINE side shouldn't cause anything like this. I think people are having trouble because they're getting these mixed up.

This limitation does not apply to the wiring supplying power TO the GFCIs.

A double-pole breaker wouldn't let you get it wrong (at least not without some extra work).
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Right. I should have made that clear. But the application at hand does share neutral on the load side fo the GFCIs.


Heh. Depends on the panel. It is quite possible to install a FPE double-pole breaker into a FPE panel such that _both_ poles are on the same leg.
Or at least it was about 15 years ago ;-)
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