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  1. #1
    Join Date
    Jan 2008
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    Default sub panel cable rating

    I have a 125 amp sub panel box in a cabin. I will be feeding it from a 200 amp main box in a house using a 100 amp plug in breakers to feed the sub panel. I have found some #2 copper cable with markings as #4 awg MTW or THHN, or THWN. It will be ran in 2" PVC conduit in ground. Will this cable be heavy enough on 100 amp breakers at max load. Thanks Gary

  2. #2
    Join Date
    Feb 2008
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    North Dakota
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    1

    Default Re: sub panel cable rating

    #2 thhn Cu is rated for 125 amps, because you only need to worry about the wire matching the ampacity of the breaker you could
    get by with #3 thhn Cu for the 2 hot conductors & the neutral, but chances are that home improvement stores don't carry #3. #8 thhn Cu is the minimum equipment grounding conductor you can use
    for a 100 amp circuit. 2" pvc is over kill, but I am all for it,
    because in the future you may need to pull larger or more conductors in this pipe.

    I have based my recommendations on the NEC, your State or local inspectors might have more strigent codes. You could talk your local electrical inspector to come out & give you ideas about what is required for your installation. You may not like what he has to say, but keep in mind codes protect you, your family and the next family to own your home.
    Last edited by Lyle; 02-21-2008 at 10:14 PM. Reason: sp

  3. #3
    Join Date
    Jun 2007
    Location
    Shamokin, Pa.
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    645

    Default Re: sub panel cable rating

    Quote Originally Posted by Lyle View Post
    #2 thhn Cu is rated for 125 amps, because you only need to worry about the wire matching the ampacity of the breaker you could
    get by with #3 thhn Cu for the 2 hot conductors & the neutral, but chances are that home improvement stores don't carry #3. #8 thhn Cu is the minimum equipment grounding conductor you can use
    for a 100 amp circuit. 2" pvc is over kill, but I am all for it,
    because in the future you may need to pull larger or more conductors in this pipe.

    I have based my recommendations on the NEC, your State or local inspectors might have more strigent codes. You could talk your local electrical inspector to come out & give you ideas about what is required for your installation. You may not like what he has to say, but keep in mind codes protect you, your family and the next family to own your home.
    Sound advice indeed, but in these parts all inspectors would rather see 2&1/2" PVC....

  4. #4
    Join Date
    Jun 2007
    Location
    Chicago, IL
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    162

    Default Re: sub panel cable rating

    Quote Originally Posted by garydbarger View Post
    I have found some #2 copper cable with markings as #4 awg MTW or THHN, or THWN.
    If it's marked #4 AWG, then it's #4 AWG, NOT "#2 copper cable".

    What leads you to believe that it's #2?
    HTH

    Dave

  5. #5
    Join Date
    Jan 2008
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    6

    Default Re: sub panel cable rating

    my mistake, it is # 4 copper, I was thinking #2 but it is for sure #4

  6. #6
    Join Date
    Mar 2008
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    2

    Default Re: sub panel cable rating

    Number 4 is fine on a 100 amp breaker for this feeder. Number 8 ground is needed, and number six for the grounding electrode conductors.

  7. #7
    Join Date
    Aug 2007
    Location
    The deep South
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    459

    Default Re: sub panel cable rating

    Welcome back jwhite .

  8. #8
    Join Date
    Jan 2008
    Location
    Lakeland ,MN
    Posts
    356

    Default Re: sub panel cable rating

    Good call Guys. Just remember some areas of the country require this to be considered a new service. Even thoughit is feed from a main panel and feed to a out building. That requires you to bond the nuetral bar to the panel can. and new driven ground.
    Last edited by Ravens53; 04-15-2008 at 07:44 PM.

  9. #9
    Join Date
    Aug 2007
    Posts
    1,131

    Default Re: sub panel cable rating

    Didn't see any mention of distance. Voltage drop could be a concern, not?

    http://www.csgnetwork.com/voltagedropcalc.html

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