I am in the process of updating the wiring for my dock. It was originally wired with a single 15-amp GFCI outlet into which the two motors for the boat lift were plugged into. The problem I’ve had is that I can only operate one motor at a time. If I try to run both motors at the same time the 15-amp circuit breaker back in the sub-panel, located at the beginning of the dock, trips.

I have two 15-amp circuits coming out to the subpanel, from a double-pole (tandem) 15-amp breaker in the house. This is a standard 3-wire with ground format (black, red, white, ground). I’m in the process of putting in a 2nd GFCI outlet, in a larger box along with the original outlet. One outlet will be powered by the black circuit, and the other will be powered by the red circuit. The motors will then plug into the separate outlets. So far so good, I think.

Question(s) are:

  1. How to handle the white (neutral) line. Can this be shared between the 2 GFCIs?

  2. When both motors are running, the current in the white return line will be close to zero, since the current through the 2 motors will be 180 degrees out of phase. Won’t this cause the GFCIs to trip?

Final Results 3/18/2020

Here's what I ended up with:

  • Two GFCI outlets (WR/TR), one on each 120V phase.

  • The load (protected) side of each GFCI feeds one of the lift motors via a switch for controlling lift direction

  • Everything works OK, no GFCI or ckt breaker tripping.

Waiting to get the boat back from winter storage.

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  • Is this a true double pole breaker that takes up 2 spaces? Or a tandem breaker (2 small switches that takes only one space in the panel? If one space you can't do it safely...both breakers will take power from the same phase and overload the neutral. If 2 spaces, it should work fine. Each individual GFCI breaker measures the power going out and the power returning, any tiny variation will cause them to trip. Just because that return current goes to the other phase via the neutral shouldn't cause any problems. I've wired MWBC the same way and it's fine. Commented Mar 2, 2020 at 0:55
  • @George Anderson - It's a true double pole breaker, with each side taking power from a different input phase. If that's different than a tandem, I apologize.
    – SteveSh
    Commented Mar 2, 2020 at 2:01
  • No problem SteveSh. The term "tandem breaker" has a specific meaning as they tap power from the same buss (phase). Using them to power a MWBC is dangerous. Glad you clarified this. But others here have given better answers: A GFCI protected 15 amp double pole breaker is your best bet. But if it trips, it will mean a "trip" (no pun intended) to the main panel to reset. That's why I like GFCI outlets instead as outdoor outlets often trip without a good cause. Yeah, they are more prone to failure outside, but it's a trade off. Commented Mar 2, 2020 at 2:09

3 Answers 3


To start with, this is a shared neutral aka multi-wire branch circuit. One rule of MWBCs is that neutrals must be pigtailed when the other hot is near; you can't daisy-chain neutral through a device like a receptacle, because that would sever neutral for the other side if you removed it.

Keep in mind on a plain recep, all screws are LINE. There is no LOAD on a plain recep, because it doesn't do anything special like provide GFCI protection, dim, etc.

If you leave the warning tape on the LOAD terminals and don't use it, then you don't have any worries about GFCIs tripping from miswiring. The only time you'd get in trouble is if you did that thing with daisy chaining neutrals, that thing you're not allowed to do anyway, and tapped a recep's LOAD terminal for that purpose - in that case, you are correct, the GFCI would have a problem with that.

However, there's a safer way to wire your dock

Delivering GFCI protection at the socket is better than nothing, but there are all sorts of wire failures before the receps that could leak current into the ground around the lake. And many miswirings could cause the same effect.

Further, outdoors is a terrible place to put a GFCI device; they fail all the time there.

So you are actually better off fitting a 2-pole GFCI circuit breaker back at the service panel... And then you don't need any additional protection dockside. The's no need to put a GFCI on a GFCI. This will protect the entire cabling run, as well as all wiring dockside.

  • I'm using the load side of the GFCIs or at least that was my plan. Wiring would be sub-panel->GFCI->Motor switch->Motors. The 2 motors are going to be hard-wired to the 2 switches.
    – SteveSh
    Commented Mar 2, 2020 at 2:05
  • Also, back in the house the 2-pole breakers are 30-amps. That feeds out to the dock sub-panel through 3 #10 UF. At the dock sub-panel there are 2 15-amp breakers (non-GFCI) for the lights, outlets, and motors.
    – SteveSh
    Commented Mar 2, 2020 at 2:09

"Simple" solution would be to use a two-pole GFCI breaker.

But, AFAIK, there's no problem with running the single white to a pigtail to each GFCI, and that just happens to "solve" the "problem" of both motors running at once - there won't be any current to speak of running back to the panel, but exactly the correct amount of current will run from one GFCI's neutral, through the pigtail, to the other GFCIs neutral, and any imbalance between motors will run back to the panel, with each GFCI seeing the same amount in from its hot and out from its neutral, unless there is a fault.


I've just had a similar question myself. In an older house with additions, someone used that old 2 wire (hot, neutral, no ground) to more outlets or rooms than should be. I was curious if I could pull a new 3-wire romex rather than 2 2-wire romex. Turns out the way the wire is ran I cannot pull the new wire very far anyway. But what I found from recently searching is it's about the 'phases' from the different blocks at your breaker. If you use the the left side in your panel for black and the right sight of the same row.. then the phases are 'adjacent' to each other or out of phase. The black (hot) wire receives current, and then the red (also hot or 'Neutral') receives current. But because each receives current in different phases.. then the white wire is able to carry both currents back to the breaker. I read this is popular in newer Kitchens so the top and bottom of the outlets are independent from each other and on separate breakers. But I also read that because GFCI's are required in kitchen's it's not a perfect or easy solution.

  • Your 'answer' is more of a comment which should be posted as such. If you're asking a question you would like answered you should do it in a separate post. You also might want to take our tour to learn how the site operates.
    – HoneyDo
    Commented Mar 2, 2020 at 17:20

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