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Thread: ohm's law sucks.

  1. #1
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    ohm's law sucks.

    missed a 120 volt receptacle on a bid.

    30' up in the air, feeding a wifi repeater.
    7 amp load.

    490' from the nearest panel.
    #6 copper gives 2.81% voltage drop.

    anyone got any magic math, or did i just
    eat 1,000' of #6 thhn for one outlet?
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  2. #2
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    PROOF

    7 AMPS?
    Take 2 500 foot rolls of #10 and see what the actual voltage drop is. Is it more than 5%?
    Is it functional voltage? ie over 115 from 120?
    Might need a change order for a transformer that wasn't on/in the plans?
    Don't give up yet, make a play for their wallet.

  3. #3
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    Is voltage drop a part of the specs? If not, I would care about the actual voltage at the end, not the drop percentage.

    I used #10 @ 8 amps and got a tad over 110V at 500' starting with 120V at the breaker. That would work IMO.

    If the supply voltage was running high, using 490', and 7A load you could possibly fudge numbers to make #12 work, I am just conservative.
    "Electricity is really just organized lightning." George Carlin


    Derék

  4. #4
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    ..if the NEC is mentioned in the specs, that pretty much covers voltage drop. Looks like you have to eat the cost of the mistake.

  5. #5
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    Quote Originally Posted by jusme123 View Post
    ..if the NEC is mentioned in the specs, that pretty much covers voltage drop. Looks like you have to eat the cost of the mistake.
    Other than for fire pumps and maybe a couple of other things the NEC does not have voltage drop limitations.

  6. #6
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    Quote Originally Posted by Fulthrotl View Post
    missed a 120 volt receptacle on a bid.

    30' up in the air, feeding a wifi repeater.
    7 amp load.

    490' from the nearest panel.
    #6 copper gives 2.81% voltage drop.

    It's hard to believe that this was missed if it was shown correctly on the drawings/plans.

    What's actually called for on the plans? I would think a dedicated circuit would be needed and not just shown as a receptacle.

    If all they are showing on the plans is a receptacle I wouldn't worry about how far to the panel I would connect to the nearest receptacle circuit ( In theory) and ask for a change order to run a dedicated circuit (what's needed ).
    90% of doctors don't graduate in the top 10% of the class.

  7. #7
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    Quote Originally Posted by Fulthrotl View Post
    missed a 120 volt receptacle on a bid.

    30' up in the air, feeding a wifi repeater.
    7 amp load.

    490' from the nearest panel.
    #6 copper gives 2.81% voltage drop.

    anyone got any magic math, or did i just
    eat 1,000' of #6 thhn for one outlet?
    My magic math says 3 wires multiplied by 490' is more than 1000'.......

    In all seriousness, most electronics can handle a pretty wide voltage tolerance. Since this a dedicated circuit just for the repeater I wouldn't have a problem using #8 with a 5.4% drop.

  8. #8
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    Quote Originally Posted by Cow View Post
    My magic math says 3 wires multiplied by 490' is more than 1000'.......

    Randy is on a big EMT job right now, he is working all weekend, probably using EMT as EGC.
    "Electricity is really just organized lightning." George Carlin


    Derék

  9. #9
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    Quote Originally Posted by Fulthrotl View Post
    missed a 120 volt receptacle on a bid.

    30' up in the air, feeding a wifi repeater.
    7 amp load.

    490' from the nearest panel.
    #6 copper gives 2.81% voltage drop.

    anyone got any magic math, or did i just
    eat 1,000' of #6 thhn for one outlet?
    What is the actual delivered voltage at the panel? 126 volts will be a #8 with a 4.6 volt drop... Still 121.4 volts under load
    # 10 at 126 volts will be 119 volts under load... Is this load constant?

  10. #10
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    Quote Originally Posted by jumper View Post
    Is voltage drop a part of the specs? If not, I would care about the actual voltage at the end, not the drop percentage.

    I used #10 @ 8 amps and got a tad over 110V at 500' starting with 120V at the breaker. That would work IMO.

    If the supply voltage was running high, using 490', and 7A load you could possibly fudge numbers to make #12 work, I am just conservative.
    Actually using this chart, depending on the supply voltage, #12 might work.

    "Electricity is really just organized lightning." George Carlin


    Derék

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