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Thread: ohm's law sucks.

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  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?
    “Give a person a fish and you feed them for a day;
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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 iwire View Post
    Other than for fire pumps and maybe a couple of other things the NEC does not have voltage drop limitations.
    210.19 Conductors — Minimum Ampacity and Size.
    (A) Branch Circuits Not More Than 600 Volts.
    FPN No. 4: Conductors for branch circuits as defined inArticle 100, sized to prevent a voltage drop exceeding3 percent at the farthest outlet of power, heating, and lightingloads, or combinations of such loads, and where themaximum total voltage drop on both feeders and branchcircuits to the farthest outlet does not exceed 5 percent,provide reasonable efficiency of operation. See FPN No. 2of 215.2(A)(3) for voltage drop on feeder conductors.

    NEC has specified percentages of voltage drop!
    Transforming it should be cheaper.
    Last edited by Daniel malack; 05-26-12 at 10:28 PM.

  7. #7
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    Quote Originally Posted by iwire View Post
    Other than for fire pumps and maybe a couple of other things the NEC does not have voltage drop limitations.
    But local code may. The Florida building code limits voltage drop throughout the state for example.


    I know what I don't know, and I know where to go to find it!

  8. #8
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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

  9. #9
    Join Date
    Dec 2004
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    Alternate solutions

    The required distance is fixed but there are other variables. One option might be to install a small buck-boost transformer at the source. The cost savings is substantial when compared to 1,500 ft of #6 or # 8 copper and the increased raceway and labor.
    You can begin with 120V, step this up to about 140V, allow for a 10% VD and end up with 126V.

    Keep in mind that the maximum VD on the branch circuit is 3%. This 3% is the difference between the voltage at the OCPD and the outlet, with the load connected.

  10. #10
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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 ).
    "A sight for sore eyes to the blind would be awful magestic"---Wax Fang

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