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Thread: Wire sizes for 100 amp service

  1. #1
    Join Date
    Feb 2003
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    Evansville, IN
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    Question Wire sizes for 100 amp service

    I am running a 100 amp circuit from an existing building to my new garage/shop. The feeder panel has two hots, a neutral buss and a separate equipment ground. Total length between panels will be a maximum of 65 feet. I plan to use #2 copper for the two hot wires (red and black). The neutral (white) will be #_____copper. The ground (green) will be #_____ copper.

    Fill in the blanks for me.

    Not looking to "get by," but don't want to spend more $$ on wire than necessary. Not looking to use aluminum - almost tragic experience.:eek

    Thanks for the help!
    "God does not deduct from a man's lifespan the time spent fishing."

  2. #2
    Join Date
    Feb 2003
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    West Michigan
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    You need 2 more blanks. I believe #2 is only good for 60 amps.

  3. #3
    Join Date
    Jan 2010
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    westchester cty, NY
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    #2, according to this chart, will do for your application:

    http://industrial-equipment.biz/asse...ngth_chart.pdf

    #6 is generally used for 60A circuits.

    if i'm reading this correctly, you would be good up to 389'. my licensed electrician has mentioned to me several times that, on subpanel installs, they don't even think about voltage drops until the run exceeds 100'. i have also seen him downsize the ground wire, but not the neutral. in the interests of safety, i would use #2 for the hots, the neutral and the ground.

  4. #4
    Technically this is a feeder. The equipment grounding conductor may be #8 copper or #6 aluminum. The grounded conductor (the neutral) can be sized as big as the hot legs or as small as the grounding sizes previously mentioned depending on what loads you are expecting. The hot legs will need to be #2 copper and either #2 or #1 (depending on the conductor temperature rating, but probably #1 for most of the stuff you're likely to be using) for aluminum.

    At 65 feet I wouldn't be overly concerned about voltage drop either.

    Don't forget you will need a grounding system (rods) at the new structure.

  5. #5
    Join Date
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    Hebron, KY
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    Doesn't the wire size/selection depend on how it is being run? I.E. underground direct buriel, overhead free air, or in conduit? Maybe it doesn't matter on this distance and amperage?

    I have a 100A subpanel and the inspector made me change the breaker or upgrade the wire because the electrician ran wire rated for 100A at a main panel. The inspector said the amp rating for the wire was derated since this was a subpanel. I eventually changed the breaker to 90A instead of changing the wire. The panels are next to each other, not separate buildings, I think this matters as well.

    I'm not an electrician, but have come across this on projects at work when running power for 3 phase motors (design by others) and making sure we have the wire sized appropriately for the money. My knowledge is second hand, so don't take it as gospel.

    Mike

  6. #6
    Doesn't the wire size/selection depend on how it is being run? I.E. underground direct buriel, overhead free air, or in conduit? Maybe it doesn't matter on this distance and amperage?
    As long as you are running it in a listed way and you're not putting more than three current carrying conductors in the raceway/conduit, you don't have to derate the ampacity. The hot conductors come out of the tables that list the ampacities for 60C, 75C, and 90C conductors in both copper and aluminum (or copper plated aluminum). The grounded conductor (the neutral) and the grounding conductor have different rules that might allow them to be smaller (they will never be larger than the table ampacity for the conductors listed).

    I stand by what I said before. #2 copper for the hots, #2 copper for the neutral (would be safe but might be allowed to be smaller) and #8 copper for the ground will be acceptable for the circuit defined.

  7. #7
    Join Date
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    Thanks to all. I appreciate the input.

    When in doubt re: size of wire, go bigger. It can't hurt!

    Thanks again.

    Joe
    "God does not deduct from a man's lifespan the time spent fishing."

  8. #8
    Join Date
    Apr 2010
    Location
    Hebron, KY
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    1,210
    Thanks for your clarification Ron.

  9. #9
    Quote Originally Posted by Joe Pack View Post
    Thanks to all. I appreciate the input.

    When in doubt re: size of wire, go bigger. It can't hurt!

    Thanks again.

    Joe
    You run afoul of NEC article 250.122(B) here is a copy & paste from the 2008 NEC:

    (B) Increased in Size.
    Where ungrounded conductors are

    increased in size, equipment grounding conductors, where

    installed, shall be increased in size proportionately according

    to the circular mil area of the ungrounded conductors.

    It used to be only when increased for voltage drop, but that was removed a few code cycles ago, now it does not matter what reason, a grounding conductor will be required to be upsized if the ungrounded conductors are increased in size.

    BTW, the 90 degree column cannot be used to size conductors the only thing you can use it for is when derating, a whole topic on it's own.....

  10. #10
    I'm assuming they did that because at some point, someone ran oversized hots/neutrals, but a properly sized ground. Then someone else came back sometime later, saw the big conductors and decided to pop in a bigger breaker, not realizing that they just turned their ground into a fusible link...

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