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Thread: Double pole breaker voltage differential

  1. #1

    Default Double pole breaker voltage differential

    This is driving me crazy! 30A double pole via 10-2/g to subpanel in detached garage (put in 20 years ago so 10-3/g not required, neutrals and grounds share an insulated bar, nothing bonded). Voltage at all points on the white wire is good, voltage at all points on the black wire is less than half what it should be. I've checked all connections at the main box and at the sub panel, and the one junction in between. I even tried a different breaker albeit 20A, and got the same results. The main box is new. While checking a different breaker, I did discover I had a colony of ants had set up residence in the main box, which has been erradicated. What am I missing???

  2. #2


    I should add that I do run some 220V equipment, so merely jumpering is not an option.

  3. #3
    Join Date
    Feb 2012
    Kent, WA


    What kind of cable was run to the garage (NM, UF, SE, Triplex) and how (overhead or underground)? 10-2 NM or UF is not acceptable if you want 240V. I don't think a bare neutral was ever allowed for a subpanel feeder, only main services. The white in this case is really hot, so it needs to be recolored (red would be a good choice).

    Generally, the only thing that will give you half voltage is a broken or poorly connected wire. Putting that wire under load will show for sure -- the voltage will probably drop to near 0 under a decent load and the bad connection will get hot.

    What is the voltage white-black (should be 240)?
    What is the voltage white-bare (should be 120)?
    What is the voltage black-bare (should be 120)?
    Use a space heater as a load on an outlet from the black sourced circuit. Remeasure the black-bare voltage.
    Use a space heater as a load on an outlet from the white sourced circuit. Remeasure the white-bare voltage.
    At the house panel, are the black and white on the 2 poles of the 30A double pole breaker?
    Are you sure you have a double pole breaker and not a tandem, or that the breaker is in the correct set of bus stabs?

    With this type of a feeder, the neutral (bare) MUST be bonded to the panel chassis (insert green screw), and grounds and neutrals share the same bus. Do you have a ground electrode in that garage?

  4. #4


    Thanks, Mark. I didn't install the garage electrical, but now I wish I had. When I opened the box, I found the neutral busbar free-floating - insulating clips were broken on both ends - and there were two wires under each screw on the bar. I secured the busbar as best I could, and separated the wires.

    It's type UF in PVC conduit.

    Everything worked fine until a few days ago when the garage door opener failed to function. I have yet to determine whether the motor has gone out, or i it just doesn't have enough juice to kick-start it.

    When I plugged in my jigsaw, the lights dimmed severely and the saw ran fast enough to cut butter - if it was really soft butter. The bench grinder won't function - again, not enough juice to kick-start the motor. Same thing with the space heater.

    With no load, I'm getting about 50V from the black wire, and about 130V from the white/hot. With load, the black drops to about 20V and the white/hot ramps up to near 150V.

    It's a double pole, not a tandem. I'm not sure about the position of the breaker in the bus stabs - it should work anywhere in the box, shouldn't it? I can move it to the other stabs side easily enough and test it that way.

    The bare neutral is on the main lug on the busbar. I thought if I bonded it to the panel chassis, I'd create a backflow situation.

    Since there is not ground wire per se, there is no ground electrode. As I said, I didn't wire the sucker.

  5. #5


    Posted By: AGT

    Crap! I just noticed that the bare neutral in the main box is on the ground busbar. Could it be that simple????

  6. #6


    Posted By: AGT

    Okay, I feel stupid or not noticing that earlier, but at least I have good voltage in the garage again. Thanks again, Mark!

    P.S. - Yea, Seahawks!!!! It's good to have the "real" Wilson back. Glad he's taking off when he has good yardage to gain.

  7. #7
    Join Date
    Feb 2012
    Kent, WA


    Was going to say that voltage going down on one leg and up on another is a classic symptom of a loose neutral. However, your ground bus bar and neutral bus bar in the main panel should be pretty much equal in function as to where the bare wire from the UF cable was landed. If the ground bar can't take current from a neutral load, how's it going to do when a fault occurs and 10K amps try to flow? I'd check the bonding of your main panel ground bar. It should be hard bonded to the neutral bar (usually a green screw and a flat bar to the other bus bar) unless there is yet another breaker box upstream of the house one that is feeding the garage.

    In some panels, not every stab is the same. The GE ones come to mind where they have thin 1/2" breakers and the adjacent slots are on the same bus stab. Most modern panels (except GE) are difficult to do wrong. I'd double check your grounding/bonding situation in all panels. You should be able to run a heavy motor load from hot to ground as a way to test that ground path (maybe make a test extension cord, but using the ground instead of neutral won't work on GFCI protected circuits). If it won't do run that motorized item, something is not bonded well somewhere.

    A ground electrode in the garage doesn't seem useful to me, but the code requires it in any detached structure with a breaker panel.

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