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Thread: Service entrance cable size & voltage drop

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    Default Service entrance cable size & voltage drop

    I am about to upsize a service entrance cable to accomodate a new 100 amp panel and subfeed a smaller 60 amp panel next to it. The voltage will be approx. 120-240, single phase, fed from a 200 amp panel that already has a 2 pole 60 amp breaker(ahu), 2 pole 30 amp breaker(ac condensor), 1- 30amp, and about 5- 20 amp single pole breakers. The cable will be approx. 320' long. What size cable should I use for the voltage drop, to maintain less than a 5% drop @ 75C for 80%FLA?

  2. #2
    Electrician ActionDave's Avatar
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    please describe what you mean by service entrance cable.

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    Plumber jimbo's Avatar
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    The service entrance cable is about as far from a DIY project as it gets. Sounds like you mean the cable from the pole to the meter? The electric company and the electrician doing the work will know what size cable to use.

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    Moderator & Master Plumber hj's Avatar
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    I would suspect it is the line from the meter to the panel. The cable from the pole to the meter is usually installed by the utility company.

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by curtherrin View Post
    The cable will be approx. 320' long. What size cable should I use for the voltage drop, to maintain less than a 5% drop @ 75C for 80%FLA?
    FLA = 200A,
    80% of FLA = 160A,
    5% of 240v = 12v
    you need a total resistance of less than 12/160 = 75 mΩ.

    For copper,
    R@20C= [R@T]/[1+.00393(T-20)] with T = 75 C
    so R@20C = 0.82 of the resistance at 75C = 0.82 (75 mΩ) = 62 mΩ
    The total distance is 640' so you need a wire that has 0.097 Ω/1000' @ 20 C.
    According to
    http://en.wikipedia.org/wiki/American_wire_gauge
    0 AWG is within 1% of this value, so use 0 AWG.

    This is for voltage drop only; ampacity is a more complicated issue.
    Last edited by Thatguy; 08-22-2009 at 02:32 PM.

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    In the Trades mattbee24's Avatar
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    From your description, it sounds like you are going to run from your main 200a panel to a 100a sub panel. If that is the case, and I am assuming you will be running this underground and in conduit, you will need 2/0 aluminum 4-wire URD.

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