Garage Wiring Advice

[elav]

I just had a sub-panel added to my garage and would now like to wire it. I'm thinking that I would like to run 2-gang boxes where one of the outlets would be from one circuit and the other outlet would be from a 2nd circuit. Interested in wiring this and meeting code. Is getting a double pole GFCI circuit breaker and then running 12-3 wire to the outlets to code? Also for my education, is this possible because the double pole breaker is pulling from "hot" wires that are out of phase and therefore the neutral is only seeing power from one leg or the other and therefore not running double the current through the common neutral?

If there is any questions as to whether this is to code or not I'll just run two separate 12-2 lines. Thanks in advance! There is a lot of information on this forum and sometimes because of that it is diffficult to figure out what is correct...

 

[Engineer61]

If you are considering using a common neutral, then what you are asking won't work. GFCI breakers go off when the current in the hot wire isn't equal to the current in the neutral wire (i.e. some current is going to ground through some other path other than the neutral line). So every time you have a draw on both hot lines both GFCI breakers will blow because the amount of current in the common neutral line is the sum of the currents in the two hot lines.
You need to run at least five wires to each box, two pair of neutral & hot lines, and you can use a common ground if it's big enough to handle the max current from both circuits.

 

[Sureshot]

What about running two runs of 12/3 and splitting the plugs and having 4 circuits?

 

[Engineer61]

Again, you have 2 neutral wires handling the current of 4 hot wires; it won't work with GFCI's and is a bad idea with normal breakers unless you use a larger neutral wire that can handle the doubled current.
Remember that electricity goes round the circuit, all the current that comes from the panel on the hot wire needs to go back to the panel on the neutral wire. Sharing 1 neutral wire between 2 hot wires means the neutral wire has to handle double the current, so it needs to be larger so it doesn't heat up and cause a fire inside the wall somewhere.

 

[MrMark]

Not really if set up correctly. If on opposite phases the current actually cancels or subtracts. You could actually go smaller on a neutral in some shared neutral scenarios, like the service entrance itself.

To the op, you can't do what you want (an excellent idea by the way) with a gfci breaker , but you can do it gfci receptacles with a mwbc (shared neutral (12/3))

 

[lakee911]

If you use a 2P GFCI breaker you can wire up a MWBC just fine. You will have common trip (not needed) and common disconnect (needed) on both circuits. Unfortunately a 1P 20A GFCI breaker costs about $70 and a 2P 20A GFCI breaker costs about $120. The savings to use a 2P GFCI breaker would require hundreds of feet of wire to come out ahead.

Alternatively, you can wire up a MWBC to feed one or two GFCIs and use two wires on the load side. Probably not worth it.

 

[wyliesdiesels]

If you are considering using a common neutral, then what you are asking won't work. GFCI breakers go off when the current in the hot wire isn't equal to the current in the neutral wire (i.e. some current is going to ground through some other path other than the neutral line).

Unless the circuit is wired wrong like neutral and ground bonded together in the outlet, how would this happen?

Yes, a GFCI breaker won't work if he goes with a MWBC. Instead, he should use GFCI outlets.

So every time you have a draw on both hot lines both GFCI breakers will blow because the amount of current in the common neutral line is the sum of the currents in the two hot lines.

Incorrect! If he wires the circuit as a MWBC(Multi-wire branch circuit) where the 2 hots come from opposite busses(240v potential between 2 hots) then the current flowing on each hot back through the neutral cancels itself out. For example, if the current flowing over both hot legs is 5a each, then the neutral carries nothing. If instead, one leg carries 10a and the other 5a, then the neutral will carry the difference- 5a!

MWBC with a shared neutral are used all the time. One example being the circuit feeding the dishwasher and garbage disposal using 12/3.

You need to run at least five wires to each box, two pair of neutral & hot lines, and you can use a common ground if it's big enough to handle the max current from both circuits.

Incorrect. The EGC only has to be as large as the largest ungrounded conductor in the circuit. In the OP's situation, this would be a #12. However, he can't share an EGC of one NM-B cable with another because EGCs are supposed to be run with the other conductors of the circuit.

Again, you have 2 neutral wires handling the current of 4 hot wires; it won't work with GFCI's and is a bad idea with normal breakers unless you use a larger neutral wire that can handle the doubled current.
Remember that electricity goes round the circuit, all the current that comes from the panel on the hot wire needs to go back to the panel on the neutral wire. Sharing 1 neutral wire between 2 hot wires means the neutral wire has to handle double the current, so it needs to be larger so it doesn't heat up and cause a fire inside the wall somewhere.

Once again, this is incorrect as long as the 2 hots legs are from opposite busses. Yes, the neutral would see the sum of the current flowing on the 2 hot legs IF the 2 hots were from the same buss in the panel. But, it would be the SUM OF, NOT double of! Where do u come up with this doubling theory? The only time a neutral could POSSIBLY see double the current is IF the 2 hots are from the same buss AND the 2 hots both had the same amount of current flowing on them.

BUT, if this scenario is happening in someone's electrical system, then they need to get a QUALIFIED electrician to fix it and do their other electrical work, as it shows that someone really didn't know what they were doing if they dangerously wired a MWBC in this manner!

 

[snorky18]

To the OP, why not just use a regular 2 pole breaker w/ 12/3 (MWBC), then install GFCI outlets for each circuit after you split it at the first junction box? Seems like it would be considerably cheaper than some of the GFCI breakers I have seen.

 

[elav]

Thanks everyone for the great input. This is for my 2-car garage and my house doesn't have an attic or a basement so my garage does double duty providing long-term storage of some items. So, there is a fairly good chance that some of the outlets will be covered/blocked so I've decided that GFCI circuit breakers will be easier to find than GFCI outlets if one trips. Price wise, the outlets are like $18 and the GFCI circuit breakers are around $45 so not a huge added expense. Based on all the feedback in this thread so far I think I'll run 2x 12/2 to each box.

 

[wyliesdiesels]

Thanks everyone for the great input. This is for my 2-car garage and my house doesn't have an attic or a basement so my garage does double duty providing long-term storage of some items. So, there is a fairly good chance that some of the outlets will be covered/blocked so I've decided that GFCI circuit breakers will be easier to find than GFCI outlets if one trips. Price wise, the outlets are like $18 and the GFCI circuit breakers are around $45 so not a huge added expense. Based on all the feedback in this thread so far I think I'll run 2x 12/2 to each box.

You only need 1 GFCI outlet per circuit. You feed the downstream receps. from the load terminals on the back of the GFCI. So just put the GFCI in a location where it won't be covered! That way u can save a bit of $!

 

[VHF]

Based on all the feedback in this thread so far I think I'll run 2x 12/2 to each box.

Unfortunatly, that ends up being the simplest solution. I've gone in circles trying to figure how to do MWBC while maintaining GFCI, but never came up with an elegant or cost-effective solution.

12/4 NM does exist, but you won't find it at the big box store. Probably not in stock at most electrical suppliers, either. It would save 1 conductor over two 12/2 cables as you would only have a single ground wire for both circuits. But even with one less conductor I doubt it would save any money over running dual 12/2 NM.

(Addendum: 12/4 is also called 12/2/2, meaning it has 2 hots, 2 neutrals, and ground (as opposed to a different "flavor" of 12/4 that contains 3 hots, 1 neutral, and ground.) It may be becoming more readily available as electricians start using it for residential work due to AFCI requirements.)

 

[MrMark]

If you use a 2P GFCI breaker you can wire up a MWBC just fine. You will have common trip (not needed) and common disconnect (needed) on both circuits. Unfortunately a 1P 20A GFCI breaker costs about $70 and a 2P 20A GFCI breaker costs about $120. The savings to use a 2P GFCI breaker would require hundreds of feet of wire to come out ahead.

Alternatively, you can wire up a MWBC to feed one or two GFCIs and use two wires on the load side. Probably not worth it.

Please explain how this would work with a 2P GFCI breaker. The breaker detects load imbalance on the 240V load vs. line and I assume it does the same for any 120V load. But, only one phase is connected to the 120V side of the GFCI breaker. This scenario has 2 120V loads of opposite phase with subtracting current. Can you confirm that you have done this yourself?

 

[Charles (in GA)]

If you use a 2P GFCI breaker you can wire up a MWBC just fine. You will have common trip (not needed) and common disconnect (needed) on both circuits. Unfortunately a 1P 20A GFCI breaker costs about $70 and a 2P 20A GFCI breaker costs about $120. The savings to use a 2P GFCI breaker would require hundreds of feet of wire to come out ahead.

Alternatively, you can wire up a MWBC to feed one or two GFCIs and use two wires on the load side. Probably not worth it.

Won't work. GFCI will trip every time you try to use a circuit.

Charles

 

[MrMark]

Won't work. GFCI will trip every time you try to use a circuit.

Charles

That's what I thought too. But, I have never used one of these "pool" gfci's, which have a neutral pig that I suppose the internals need for some reason, assuming that the pool is just 240 (if 240/120 it obviously needs it). This stuff is fairly complicated and people should be sure they know what they are talking about before . . .

 

[Charles (in GA)]

Found this via Google on the askmehelpdesk.com web site and it makes sense. Don't know if its right, just makes sense.

There are two types of two pole GFI breakers, straight 240 volt, with no neutral lug, and 120/240 volt with a neutral lug. Not all manufacturers make both, so this is something to consider before installing a panelboard. The 120/240 volt works for both applications.

A 240 volt load, such as a heater element, needs no neutral, but must have a n equipment ground. A straight 240 volt works simply. It will monitor the current flow in both hot legs. If the current flow differs between both by 5 milliamps or more, the difference is assumed to be leaking to ground, and the CB will trip.

A unit that needs 120 and 240 volt needs a neutral. For example, the heater needs 240 volts, no neutral, and the current in both hot legs will be the same, let's say 10 amps. If only the heater is running, there is no current in the neutral, only 10 amps on each hot leg.

Now add a 120 volt pump that draws 5 amps. It only connects to one hot leg, so it impose 5 amps on one hot leg and you can measure 5 amps on the neutral.

The one hot leg the pump is connect to sees the 10 amps of the heater plus the 5 amps for the pump, while the remaining hot leg stills only sees the 10 amps of the heater.

I believe it is this type of scenario that is concerning you.

They contain intricate and intuitive electronic measuring circuits that must monitor both hot legs and the neutral, and do the all the comparisons, by knowing that one hot leg, while has more load on one due to the pump, adds it, compares to the neutral, and as long as it sees the proper current , no tripping occurs.

Once the current flow differs 5 ma or more in any hot leg, and does not see the current flow in the return, whether the other hot leg or the neutral, it then assumes a leak to ground and trips.

This is why these breakers are so costly, in the range of $130.00 each.

Contacting the manufacturer is a great idea. I know, as I have done this many times before for various reasons, due to proprietary information and industrial secrets, you probably will not get a much better answer.

Plus, on your initial call ,I guarantee you will get some layperson in a call center reading a generic script for answers, and you will hang up not knowing any more than your when you first called.

Be patient and persistent to speak with a product engineer. He/she will do their best to explain the operation without giving away trade secrets.

I find that GE and Square D are fairly useless getting tech info. Siemens and Eaton ( Westinghouse and Cutler Hammer) are much better.

Charles

 

[MrMark]

Makes perfect sense. The person who wrote that knows the basics. Still don't know why the device needs a neutral connection for a straight 240V load unless the internal electronics of the gfci need 120v, which is probably the answer.

 

[Charles (in GA)]

Here is another link that is written by someone with quite a bit of knowledge on the subject. Better, it has drawings.... yes, I like "cartoons" (slang for pics, drawings, etc) easy to understand.

http://www.iaei.org/magazine/2012/06/gfci-and-afci-basics/

Charles

 

[wyliesdiesels]

I agree with MrMark and Charles. A DP GFCI breaker will not work in this scenario. The OP should just go with GFCI outlets and be done with it.