Question about long wire run

Discussion in 'Electrical Forum discussion & Blog' started by rowdy235, Nov 17, 2020.

  1. rowdy235

    rowdy235 Member

    Joined:
    Mar 23, 2016
    Location:
    Oregon
    Hey guys! Searched around but couldn’t find the answer, so hoping y’all can help out.

    I have an outbuilding that’s about 80’ away from my house. In addition, the panel is on the opposite end of the house, making my wire run about 125’ or so. The previous owner had installed some direct bury 12-2 wire out to the building, which is disconnected currently but I am wanting to reconnect. I checked the wire with my multimeter and all appears okay.

    I did a voltage drop calculation and appears that I will be good up to about 10 amps, but anything more will have a pretty significant drop. I’m not planning on running anything too high of amperage, likely just lights and maybe a battery charger or shop vac so I am not overly concerned with that in of itself.

    However, I am concerned if there is a safety concern with the long run? Right now it is on a 20a breaker, would I be better off to replace with a smaller breaker? My long term plans include a new shop with better service, but hoping I can make this work in the meantime.

    Thanks in advance!
     
  2. fitter30

    fitter30 Well-Known Member

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    Feb 2, 2020
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    Retired service tech
    Location:
    Peace valley missouri
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  4. rowdy235

    rowdy235 Member

    Joined:
    Mar 23, 2016
    Location:
    Oregon
    Thanks for the link. I had used a similar one and it looks like to stay under 5% drop I would have to keep the load to 10 amps or less.

    Essentially, my question is would there be a safety concern to leaving the 20a breaker or should I downgrade to a smaller breaker (do they even make 10a breakers?)
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Technically, with 12g wire, the 20A breaker is okay. I'd consider just putting a 15A breaker on it. If the receptacles are 20A, I'd switch it to a 15A one (straight slots rather than one T-slot). Whatever you plug in wouldn't damage the wire, but low voltage could damage some devices. If it's just lights, they might dim a bit. A typical vac doesn't draw enough to exceed your 10A in most cases. This assumes that the wire is suitable for the way it was installed.
     
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  6. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    I would keep the 20 amp breaker.

    My voltage is supplied usually at over 5% above nominal, so a 10% drop at a shed for me would be unnoticed.
     
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  7. DIYorBust

    DIYorBust Active Member

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    Mar 5, 2019
    Location:
    Long Island, New York
    Conceivably, you could plug in a 15a device and test the voltage at a second receptacle, or using an extension cord or similar with multiple outlets. You could try different combinations if devices and assess if the voltage drop is acceptable to you. I'd run the load for a while and wait for the wire to get warm.
     
  8. JerryR

    JerryR Active Member

    Joined:
    Jul 27, 2011
    Location:
    Florida
    The biggest concern I would have is low voltage with a high load causing damage to the device you have plugged into. I personally would install 15 amp breaker to limit potential for appliance damage and measure voltage at the end of the run with a load.

    My friend has a similar length run with 10/2 to a barn with a 30 amp breaker. He leaves his RV plugged into it with the ac set to cycle just enough to maintain humidity at a reasonable level. Last week he had his second ac unit fail. I measured the voltage and it varied between 104-108 vac wt the ac running. Once the ac compressor starts up I’m sure voltage dropped below 100.

    When I visited him with my RV my EMS in my RV would disconnect power when my ac tried to run because the voltage dropped below 104 volts.
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Keep in mind that as the voltage drops, some devices will still try to consume the same amount of power, so the amps will go up. At some point, the current could trip the breaker where that wouldn't happen if the voltage was nominal, or, you could exceed the capacity of the device, and it will heat up and potentially fail. A purely resistive load will just drop the power consumption as the voltage drops, but not that many devices are simply resistive loads.
     
  10. rowdy235

    rowdy235 Member

    Joined:
    Mar 23, 2016
    Location:
    Oregon
    Thanks guys for the input. Sounds like I am good to go, voltage at the panel is around 125v, so probably wouldn't be too affected by the voltage drop.
     
  11. rowdy235

    rowdy235 Member

    Joined:
    Mar 23, 2016
    Location:
    Oregon
    Thanks guys for the additional input. I'm not planning on running any sensitive loads. Other than lighting I may run a shop vac or skil saw at the most, all of which use universal motors. I need to get a new breaker anyway so may switch to a 15a just for piece of mind. Of course, the first time I trip it and have to walk clear up to the house I am sure I will regret my decision :)
     
  12. DIYorBust

    DIYorBust Active Member

    Joined:
    Mar 5, 2019
    Location:
    Long Island, New York
    The outbuilding is supposed to it's own shut off mechanism as close as possible to where power enters the building. A simple switch would suffice, but instead you could put in a 15A breaker.
     
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