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Thread: Electrical Subpanel Feeder Wire

  1. #1
    Join Date
    Jul 2006
    Location
    Monroe, GA
    Posts
    25

    Electrical Subpanel Feeder Wire

    My new house has a 20 X 20 detached workshop about 25ft from the house. Currently is has a small subpanel fed by 8 gauge wire (4 wire) from a 50 amp breaker (which I think is too much for 8 gauge) in the main service panel. I originally planned to try to make this work but I think it would be best to go ahead and bump everything up to 100 amps in the workshop. That way I can seriously think about adding HVAC.

    What size feeder wire do I need for 100 amps? I've heard conflicting info. I'm thinking 2 for copper or 1 for aluminum. Is voltage drop a concern for a 25 ft run? Can the ground be smaller than the hots and neutral?

    Also, the 8 gauge feeder wires are in 3/4" pvc conduit for the entire run. What size conduit is necessary for my new wires for 100 amp service?

  2. #2
    Join Date
    Feb 2007
    Location
    Oglesby,Il.
    Posts
    73
    you'd use 1 1/4" conduit. I'd definately go with the Copper wire. At that short of distance voltage drop isn't an issue. I'm also pretty sure you'll have to drive a ground rod at the 2nd structure as well you'd have to check but around here they require a #4 Solid Copper conductor for that. If I remember right a #6 would be acceptable for the ground connection in the conduit run. yeah #8 is rated good for 40 Amps. Hope this helps alittle.
    I can sure make a mean pile of Saw Dust !!!

  3. #3
    #8 wire is rated to 50 amps per so the present wire is fine.

    To feed a 100 amp subpanel will require #2 copper for the 2 hots and #4 for the neutral.

    The ground for the separate structure will require a separate ground rod installed. Code will not allow you to tie into the ground for the house and run it for the separate structure.

    Hope this helps,
    Gary
    "Chaos is the law of nature. Order is the dream of man."
    Wallace Stegner

  4. #4
    Gary,
    In a sub panel aren't the grounds & neutrals to be separated on separate bars? Or is this different for a sub panel in an unattached structure?

  5. Quote Originally Posted by Gary Redden View Post
    The ground for the separate structure will require a separate ground rod installed. Code will not allow you to tie into the ground for the house and run it for the separate structure.
    If there are any common ground paths between the two buildings, then you MUST consider them to use a common ground and be tied together. The outbuilding would have to be wired as a subpanel with isolated ground and neutral. (The second ground rod is Always a good idea.)

    In the rarer event that the two structures do not share some type of common ground path (no phone, no water, no cable, etc.) then you may treat them separately, but you are not required to do this as Gary stated.

    The deafult answer is that they are treated as a shared system unless you can prove otherwise.

  6. #6
    I would go with #2 for the hots and neutral (a little overkill but I favor a more conservation approach), plus a #6 ground. In an ideal world, all of the 120 volt circuits would be balanced and there would be no current flow on the neutral, but that does not always happen. If you are using metal conduit, the ground wire "could" be eliminate, however I like the reassurance that if anything ever happens to the conduit I still would be protected. (After the 1989 earthquake in California, some conduits moved so much that the pipes pulled out of the connectors, so there was no longer a ground path.)


    As for a panel, get a 42 circuit 200 amp panel (can still use it for 100 amp but you have more spaces available for dedicated circuits), it is not that more expensive.
    I'm such a good good boy,
    I just need a new toy.

  7. #7
    Join Date
    Dec 2005
    Location
    Du Bois, PA
    Posts
    69
    You can buy a sub feed cable called 4-wire dyke. It is designed for direct burial but I normally run it in conduit or even 4 inch PVC pipe between buildings and conduit out of the ground. This is aluminum which all service entrance cable is. You will need to run a seperate ground and neutral. The neutral bar in the sub panel is not to be bonded to the box so you may have to add a seperate ground bar in the sub panel. A second ground rod is not required.

  8. #8
    Join Date
    Oct 2006
    Location
    Mazon, Il
    Posts
    375
    Listen to Rick.

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