My situation is this... I have a pair of outdoor GFCI outlets which are on separate 15A breakers and are in phase with each other so I want to combine them using a special wiring harness such that I can draw up to 30A total at 120VAC (60 Hz). So I carefully tested that the 2 outlets are indeed in phase by checking them individually first for power, then checking the voltage between the 2 hots. Unfortunately, there is a fraction of a volt difference between them and that is enough to "pop" them as soon as I interconnect them (parallel them). Both GFCI breakers trip immediately even without any load. My application is so I can run a 120V device that is MORE than 15A draw without overloading either circuit. Also because there are other appliances on one of those same circuits inside the house so I would rather split the load (let's say 17A total) so that it is about 8.5A per outlet. The split doesn't have to be exactly 50/50 but something close to that would be nice.

So my question is how do I do this with GFCI or will those prevent me from doing this?

It appears there are "random" loads on the one outlet (lights, fans...) but the 2nd outlet is dedicated (with no other loads). I think that might be part of the problem of the voltage difference. Should I perhaps load up one of the outlets BEFORE combining it with the other one such that the voltages match exactly, thus no current should flow between them? That might work if the load is constant on the one outlet but then what might happen when I put the heavy load in the paralleled outlets? This seems like a challenge to get to work.

I think with non GFCI outlets this would be "cake" (easy) but my application is to rapid charge a golf cart battery bank and that has to be done outside and GFCI outlets are required (by law) on outside receptacles.

  • 4
    If one of the 15A breakers opens, then the remaining circuit is carrying 30A. Hopefully the second breaker would then open, but I don't see "hopefully" anywhere in the Electrical Code.
    – gbarry
    Oct 16, 2016 at 17:29
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    What is the nameplate voltage and amperage on this golf cart charger? Oct 16, 2016 at 23:05
  • @ThreePhaseEel - 36V 40A. While charging at 40A, it will likely be around 40V so that is 1600 watts of charge power. The limit here in USA for a standard 15A outlet (receptacle) is 1875 watts (125V * 15A). I could get lucky and it will only draw the 17A of AC for a short amount of time, not long enough to trip the circuit.
    – David
    Oct 17, 2016 at 3:26
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    No, the input nameplate voltage and amperage.... Oct 17, 2016 at 3:27
  • 120VAC 17A (max) 60 Hz input I think.
    – David
    Oct 17, 2016 at 3:27

8 Answers 8


If you're in the US, what you're doing is dangerous, a code violation, and will not work. If your equipment requires a 30 ampere circuit, you should install a proper 30 ampere circuit.

The National Electrical Code only allows circuit breakers to be connected in parallel, where they are factory assembled in parallel. Simply connecting two breakers in your panel in parallel, is a code violation. There's also a section about not being allowed to connect conductors in parallel, unless the conductors are 1/0 AWG or larger.

National Electrical Code 2017

Chapter 2 Wiring and Protection

Article 240 Overcurrent Protection

240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not be connected in parallel.

Chapter 3 Wiring Methods and Materials

Article 310 Conductors for General Wiring

310.10 Uses Permitted.

(H) Conductors in Parallel.

(1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).

  • 1
    My equipment does NOT require a 30A circuit. If the battery charger maxes out at 17A 120VAC draw and slowly tapers (over hours) to perhaps 3 or 4A draw, it is possible it can work on a 15A GFCI outlet or maybe it will "pop". The simplest solution would be to replace that outlet with one that can handle 20A and encroach on some of the "headroom" in the wall wiring since it will only briefly be over 17A draw (perhaps 15 minutes max), then it should be 15 amp draw or less. Who said you are not allowed to load 2 different outlets at once with the same load? Where is that in the code?
    – David
    Oct 17, 2016 at 15:13
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    Can you provide the make and model number of the charger, and/or a clear, readable photo of the nameplate on the charger? If the equipment requires a 20 ampere circuit, then you should install a proper 20 ampere circuit.
    – Tester101
    Oct 17, 2016 at 15:25
  • Yes I could but why would it matter? The specs are 17A max and 120VAC single phase input. That is all you need to know. Telling you the exact make and model of the charger is irrelevant to this discussion. I would suspect the PF (Power Factor) at full charge output power is about 0.8. This is based on observations from other similar but less powerful chargers. @Tester101
    – David
    Oct 17, 2016 at 17:22
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    @David Telling us the make and model of the charger, would allow us to look up documentation on the unit. From that we can find any special installation requirements, or other information provided by the manufacturer. Equipment that has a nameplate rating of 17A @ 125V, would require a 20 ampere circuit. You can't put it on a 15 ampere circuit, and you can't parallel two circuits together.
    – Tester101
    Oct 17, 2016 at 17:43
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    @David But if one of the people trip, the other gets crushed. Doing what you want may (an likely will) work, however, it is a dangerous code violation. Install a proper 20 ampere circuit (not just a 20 ampere receptacle), and use that to power the charger.
    – Tester101
    Oct 17, 2016 at 17:56

This is not possible, not with regular breakers and especially not with GFCIs.

You can't just combine 2 circuits and get twice as much juice. In an ideal world on you theoretically could, but in real world no 2 paths are perfectly same. So in case of regular breakers, one will get loaded more than the other (20/80 distribution would be a good outcome), which means it'll trip and then you're left with the second one already overloaded which will trip it as well.

With GFCIs it's even less possible. They are designed to detect and trip on imbalanced tiny stray currents, and as you connect them together you create exactly such imbalanced current which trips them as you observed. (You correctly concluded that the minute voltage difference is created by circuits being not loaded precisely the same. A zero loading could have worked for plugging the interconnect part, but the very millisecond you start charging you'll get similar situation to previous point - the current won't split evenly across GFCIs and they'll trip simultaneously.)

Another thing is does your power quota really go up to 30A? The fact that you have 2x15A circuits doesn't mean you can load both to the max, because there can be eg. 25A main breaker for the entire house.

Sadly, there is no way for you to go around it. You need to call a licensed electrician to design and install another 30A circuit, with 30A GFCI and all bells and whistles as the code requires.

Connecting power sources in parallel is unpredictable and risky. It's an unpleasant thing to say, but I can tell that you need more background just because you even thought about it.

  • Actually if you read the comments carefully, I stated the charger is 17A draw max rated at 120VAC so even if I do get a 80 /20 split with 15A breakers, that will put both draws BELOW the 15A limits and no the house is not limited to a 25A breaker that is ludicrous you wouldn't be able to run anything except a few electric toothbrushes. Common here is 150 or 200A main breakers. My test (soon) will be putting a large load (over 15A) on parallel 120VAC circuits but first making sure they are on different breakers. That way (in theory) I can get UP TO 30A total which 17A falls within. @Agent_L
    – David
    Oct 17, 2016 at 17:05
  • In theory this should work because most of the resistance is in the load, not in the wiring to the load. I suspect the load distribution will be "tighter" than 60/40 meaning no worse than that. If I got really lucky (which is not required for this to work), it will be close to 50/50. I can easily test this using a Kill-A-Watt "wattmeter" with a purely resistive load (900 watts of incandescent lamps for example). Those have a "perfect" P.F. (Power Factor) of 1.00. I might see 450 watts at each feed or I might see 500+400 but whatever it is, I doubt it will be something like 720+180. @Agent_L
    – David
    Oct 17, 2016 at 17:10
  • If the 900 watt load splits fairly evenly (let's say 60% / 40%), then there seems like a heavier load should do something similar. Even if I only get 2A from one of the paralleled outlets, that will force the other 15A to come from the 2nd outlet (on a different 15A breaker). There is virtually NO WAY for the split to be that bad (away from 50/50). You would have to put one of the feeds on about 1000 feet of extension cord to make the resistance so far out of whack for that to happen. My test should confirm my idea works. For short term it should be ok and fun to test too. @Agent_L
    – David
    Oct 17, 2016 at 17:17
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    Rather than getting a 240V charger or a charger than requires more than 15A of input, I decided to keep it simple and just use a slightly weaker charger that is 15A max rated. Since it will not sustain 15A of draw and will taper to much less, it seems it will work. I doubt there is anything else on that same 15A circuit so using all of the 15A in the charger seems like it will work. Inside the house when I threw all the breakers off (including the 150A main breaker), the charge outlet stayed energized which tells me it may be dedicated (not shared). That seems good to me. @Agent_L
    – David
    Oct 19, 2016 at 12:31
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    @David A TV with a plain old transformer would not work, but most modern electronics have switching mode power supplies with stabilization so they're rated 100-250V. And the higher voltage, the better efficiency you get so technically a modern American should run most devices from the dryer socket : )
    – Agent_L
    Oct 19, 2016 at 12:54

As Tester101 stated, this is illegal and against code.

I also wanted to throw in that there is more to electrical safety than wires being sized appropriately to carry current to prevent excessive heat/fires.

Presuming your charger uses a standard 20A plug, there are no NEMA outlets which match this plug configuration which are rated for 30A. You are now potentially exposing someone who might unplug your charger to a fault current of 30A--increasing the danger of using these outlets.

Also, your proposal goes against convention and common conventions are defined to help promote easy understanding and to ensure safety.

Your proposal to use two separate 15A breakers (if it works) is confusing and therefore dangerous for anyone other than you who might do some service to this electrical equipment. To do what you are proposing to have two independent in-phase breakers controlling power to a single outlet, based on the ubiquitous design of interleaved breaker panels, when someone wants to service this outlet, he/she would then have to remember to:

  1. Turn off two breakers on opposite sides of the panel, or
  2. Turn off two breakers with another breaker space in between

This setup is:

  1. At the very least baffling to someone else in the future who works on the outlet (e.g. an electrician or a future homeowner). "Which combination of breakers turns off this outlet?" There is nothing like it in the national electrical code.
  2. Dangerous because someone may think they have disabled power to the outlet according to common conventions. I.e. Someone turns off one of the breakers according to common convention but forgets or doesn't realize they need to turn off the second randomly placed non-tandem breaker.

To add: Since the work is illegal, if someone is injured by the result of your work, you can be held accountable for damages (i.e. lawsuit). Also, it can devalue your home if discovered by an inspector when you go to sell your home. I know I would personally question the integrity of the rest of the wiring in the home if I found out about this setup before making a purchase.

  • All of these points are "defendable". I will be the only one using it and only on a temporary basis using a special wiring harness I make myself. Yes the breakers would be "gapped" normally but you didn't follow mt description earlier that one of the outlets is nonresponsive to the breaker panel, meaning I flipped ALL the switches off there (including the main) and one of the 2 outlets stayed energized. If this dual 15A breaker design was for someone on a more permanent basis, they could simply mark the breaker panel index (directory) appropriately so there is no confusion. @statueuphemism
    – David
    Oct 18, 2016 at 2:21
  • How do you know it is illegal in the USA? Where is the code that says you cannot put a single load on a pair of 15A branches? This is VERY similar to wiring up 240V 15A by using split phase 120V 15A except here we have same phase, not opposite phase. Someone could just as easily plug a custom made wiring hardness into 2 split phase 120V 15A outlets (on different breakers) and get 240V 15A like for a small welder. Is it legal to do so? Well I am not sure but it is reasonable because you are using power coming into your house which was setup by a real electrician, just not in a common way.
    – David
    Oct 18, 2016 at 2:26
  • This is exactly why they have breaker box directories. Someone could easily write in something like bays 5 and 7 are paralleled together to form a 30A capable circuit and must be turned on and off in pairs (although the turning off in pairs is debatable). This idea is not so far fetched that it hasn't been thought up or tried. I am confident that many tinkerers have tried this and even way more radical things. I have never seen or read anywhere where you are not allowed to put a single load across multiple breakers using more than 1 outlet here in USA so someone show me.
    – David
    Oct 18, 2016 at 2:53
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    @David Please pick up a copy of the NEC (NFPA-70) applicable in your locality and give it a read before messing with your home's wiring--Tester beat me to the punch on code references, so please take a look at his answer. Also, make sure you apply for any necessary permits and inspections of your electrical work with your local building department if you still decide you wish to pursue your own home wiring projects. Oct 18, 2016 at 10:16
  • "meaning I flipped ALL the switches off there (including the main) and one of the 2 outlets stayed energized." - wonderful, so they're fed off different panels, and maybe even transformers? Also, new MWBC circuits are required to be in one cable, and have all poles trip simultaneously. Mar 13, 2017 at 3:23

Remember that for the same power (in watts), voltage and current are inversely proportional. If the input is 120V 17A, then dividing voltage by three and multiplying current by three yields 40V 51A. In fact no transformer or power supply is 100% efficient so some of that amperage is lost as heat, but that's still plenty of headroom to provide the desired output. You do not need 30A in if you are stepping down the voltage.

For 120V 17A, a 20A circuit should be fine, and is probably what this beast was designed for. There is one in many garages. If not, run one. Problem solved.


Exactly matching voltage isn't the problem. If even 20mA from one Hot returns via the other Neutral, both GFCIs detect the imbalance and trip (because the imbalance could be through you, to Earth).

Will your code let you run both 15A circuits from the same GFCI? Each circuit should still have its own 15A overcurrent breaker but they share a 30A GFCI, so both will trip if a real fault occurs.

That's roughly how my garage is wired, though I'm not in the US and you'll need local advice before following this path.

The alternative is a single 30A outlet for this purpose.

  • But the wiring on the dedicated 15A outlet is probably only code compliant for 15A draw (perhaps 20A for some "headroom"), so if I put in a 20A GFCI, I would not be certain that the wiring could handle more than 15A safety. How could I test this? Should I try putting a 17A load on a 15A GFCI and see if it trips and if so at what amperage? What AWG wire do I need to safely carry 17A at 120VAC at 60 Hz. Luckily the device is a battery charger so it may only be at 17A at the beginning of the charge then taper to 15A or less after a few minutes but it is still risky on a 15A breaker circuit.
    – David
    Oct 16, 2016 at 16:57
  • It sounds to me like this is more trouble than it is worth on GFCI outlets (or even if 1 of the 2 is GFCI). For non GFCI outlets it seems like it would easily work but what would happen to the current that flows due to the imbalance between the 2? I don't even know why the 2 outlets are different voltage anyway other than maybe one branch had a load (or loads) on it and the other did not. I think with GFCI it would be very difficult to not get the breakers to trip cuz it seems there will always be some imbalance unless maybe they both start out with no load but even then I am not certain.
    – David
    Oct 16, 2016 at 17:09
  • 1
    No, you'd have to use correctly rated wiring up to the GFCI (it was 30A in the question, 20A now?). Best answer is a single correctly rated circuit for the charger. Or find a way to restrict the charger to 15A.
    – Brian Drummond
    Oct 16, 2016 at 17:11
  • What I am getting at is I doubt they use wire that will heat up greatly at the max rated output of a receptacle so what I am saying is 15A wire might be ok for a brief 17A draw (maybe 10 to 15 minutes tops) which likely the charger will be at. The problem is the 15A breaker in the GFCI itself may pop so I would have to replace that GFCI receptacle with a 20A one but to make things more complicated, that receptacle is NOT on a panel breaker inside the house. They have it somehow "hardwired" so that it seems to me the GFCI itself is the main protection for anything plugged into that.
    – David
    Oct 16, 2016 at 17:21
  • The GFCI must be on a breaker inside the house. An ordinary GFCI breaker as commonly sold in the USA contains no 15A trip mechanism it has only the GFCI function.
    – Charles Cowie
    Oct 16, 2016 at 17:34

If you moved one of the circuits to the other phase, you could build up a 240V circuit. This assumes 240V GFCI sockets exist. And, of course, you'd have to use a step-down transformer for the charger (messy), or find a 240V charger (much better). But you'd have twice the power available without exceeding 15A.

In an ideal world, your cart would have a 240V charger with a J1772 connection. Charging equipment for that already has the GFCI (and more) built in.

  • 240V is not so common here in USA for residential use other than for large appliances such as stoves, clothes dryers, water heaters.... Also we use 60 Hz here and the 240V chargers I have seen are 50 Hz so that is not a perfect match.
    – David
    Oct 17, 2016 at 3:43
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    There's 240V in your fusebox. That's how they wire up the dryers. The breakers for 240 have two linked handles, to make sure both phases trip together. I'm going to have to accept the fact that golf cart chargers are nothing like EV chargers. But someday your car will be electric, and all this will suddenly make sense.
    – gbarry
    Oct 17, 2016 at 4:11
  • Yes I am well aware of how split phase 120V combines for 240V using vertically adjacent breakers, allowing for a dual bay switch to be "stacked" vertically on adjacent bays. I have these for my water heater for example which is 240V. What I have never seen is a "skip stack" where 2 in phase breakers are "married" together, thus ensuring they are turned on and off in tandem. In my case it would not really matter cuz I am the only one using it and only on a temporary basis (using a special wiring harness only for the charger). I don't even have this charger yet I am contemplating. @gbarry
    – David
    Oct 18, 2016 at 3:02

To answer the question raised repeatedly by OP in comments of "why is this 120V x 2 any different from 240V x 1" (essentially):

The key difference is safety if a wire goes bad (falls out of a connection, chewed through by an animal, whatever).

With 240V x 1, if either of the two hot wires is broken, the circuit stops. Fundamentally no different from turning off a switch. Not great, because you may have a hot wire somewhere waiting to cause problems, but the circuit stops conducting current.

With 120V x 2, you are relying on two wires in each half of the circuit - two hots sharing current and two neutral sharing current. It fundamentally doesn't work with GFCI because due to even minor variations the current flowing one each wire in a pair will not be exactly the same. But if you didn't have a GFCI then nominally it would work. It wouldn't matter if you had 8A on one wire and 10A on the other wire because 15A is the max on each wire.

However, if any one wire breaks (i.e., one of the two hots or one of the two neutrals) then you have that entire half of the circuit (hot or neutral) going on one wire that is not rated for that level current. Which can lead to overheating, fire, no more house.

The modern solution is EVSE - a.k.a., car chargers. But for circuits that can't use standard (today standard, 2016 they were still a bit unusual) EVSE, the usual solution is either to configure for a 240V circuit or for a 30A 120V circuit to use a 30A breaker, 10 AWG (or larger) wire and a TT-30 "travel trailer" 30A 120V receptacle.


You may expect to have some differential resistance between pairs of power cable joined at both ends to separate breakers and GCFI's hoping the current sharing stays in perfect balance in L-N in each GCFI when the outputs are combined to one load.

I think this is only possible if the panel breaker contact and cable resistances are matched perfectly. An unlikely but possible scenario.

  • It seems very difficult with GFCIs but it is an interesting scenario. I suspect some EE person could make it work but a tinkerer like me cannot. An easy test I can do for the limit of the single receptacle is to load it up with my 1200 watt battery charger and then also use three 300 watt lights, turning them on one at a time (so 1500W, 1800W, 2100W). The outlet (if it is truly 15A at 125V rated = 1875 watts max) , should trip when I turn the 3rd 300W lamp on. The 3 lamps are incandescent so they are a nice (perfect) 1.00 power factor but the charger is not so is that a concern?
    – David
    Oct 16, 2016 at 17:27
  • Also, assuming both GFCI receptacles were perfectly balanced, when applying a single load to both of them as a combined source of power, do you think they would stay in balance? Might it be the case they are balance at some (or no) load but when a heavy load is put on them they go out of balance? I am not an EE person otherwise I would probably know the answer to this. Assuming they were perfectly balance, would the load be split exactly in half?
    – David
    Oct 16, 2016 at 17:31
  • I think it is an unlikely solution to balance the heavier loads causing more imbalance. Does your heavy load need a GCFI? Oct 16, 2016 at 17:47

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