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

[amateur_russ]

I recently added on a 3rd garage stall which I will use the majority of it as a woodworking shop. To save some money I did the wiring myself. Here is my situation.

I wanted two 20 amp circuits alternating from outlet to outlet as you go around the shop. I ran 12/3 wire from my breaker box (new Cutler-Hammer with Type CH breakers) to the garage and alternated red and black wires to alternate circuits. I intended to install a GFCI outlet at the beginning of each circuit. The inspector pointed out that sharing one neutral between the two circuits would cause both GFCI to trip at the same time. I asked if I should re-wire and he said I could get a 2 pole GFCI breaker to protect both circuits.

My questions...

1. Can I still go with 2 GFCI outlets and deal with the inconvenience of both tripping at the same time? Is there any risk in that setup?
2. Wouldn't a 2 pole GFCI breaker be producing 40 amps or am I missing something?

Help! Thanks in advance Russ

[SQLit]

Shared neutrals will be a problem, usually when there is a load pulled from both circuits at the same time. If you only use one circuit at a time you might have infrequent tripping.

The only way a 2 pole breaker is giving out 40 amps is if it is a 40 amp breaker.

I suggest that you get 20 amp GFCI's my little buzz box wire feed welder, is hard on the 15 amp varity.

[John Grabowski]

Do you really want to install something half-ass that will continue to give you problems? The alternative to a two pole 20 amp GFCI circuit breaker would be to install a pigtailed GFCI receptacle at every location. That would alleviate the unbalance problem.

You're missing something.

[amateur_russ]

Are you thinking two 20amp GFCI breakers? Don't I have the same problem with neturals?

Thanks, Russ

[zxcvbob]

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

[RBM]

No, One double pole 20 amp GFCI circuit breaker and you won't have neutral problems

[Chris Lewis]

According to amateur_russ :

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.

[John Grabowski]

I'm wondering if that is a viable solution. If he has a 5 amp drill connected to phase A and a 1 amp shop light on phase B, won't the imbalance on the hot legs cause the two pole GFCI breaker to trip?

[RBM]

I wouldn't think so, as some hot tubs have "main" gfci protection feeding assorted 240 and 120 volt pumps, blowers, and heaters

[Mark Lloyd]

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.

[Mark Lloyd]

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).

[Mark Lloyd]

If sharing neutral before the GFCIs (line side) is causing a problem, you won't have eliminated it. The design of breaker boxes (and connections to the street) has a single neutral.

[Mark Lloyd]

It would if the GFCI is miswired.

[Mark Lloyd]

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).

[kevin]

Imbalance won't be a problem with a twopole GFCI breaker. Confusion comes from this statement:

This is true, but not the way the GFCI works. The GFCI has a special ring through which both the hot and neutral wires run. Like a circus lion jumping through a hoop. The ring senses the sum of the current running through it, and the GFCI trips if this sum is ever more than

5ma (in either direction: up or downstream). The only way to keep the sum of the currents zero is obviously to have both hot and neutral exactly the same current, but in opposite directions.

The double-pole GFCI breaker just runs both hots and the single neutral through the same single ring. In your example, you get 5amp phase A on one hot, 1amp phase B on the second hot, and negative (5amp A + 1amp B) on the neutral. Course, since phase A is exactly the negative of phase B, that's really 5ampA on one hot, -1ampA on other hot, and -4ampA on the neutral. And wonder of wonders if it doesn't sum up to zero so the breaker doesn't trip.

-Kevin

[John Grabowski]

Thanks for the explanation Kevin. I never installed a two pole GFCI breaker like the original poster intends to so I had no personal experience as to whether it would work or not. Your example makes perfect sense.

[Chris Lewis]

According to kevin :

Actually it is. The whole point of the toroidal coil is to detect when they imbalance. With a two pole GFCI, the coil's just wired differently so that they detect when the neutral current isn't the difference between the two hot currents.

If you tried to do a multi-wire branch circuit with two single pole GFCIs, the toroids (plural) aren't wired right to detect imbalance properly. So they be tripped all the time.

[Chris Lewis]

According to Mark Lloyd :

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.

[Chris Lewis]

According to Mark Lloyd :

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 ;-)

[Mark Lloyd]

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