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    cruiser2000's Avatar
    cruiser2000 Posts: 5, Reputation: 1
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    #1

    Sep 30, 2010, 10:47 PM
    Why does the code require a #2 wire for a 100 amp double-pole
    1) I have wired a subpanel feed with #2 ( or #3) wires protected with a 2 pole 100amp breaker. I am confused on why the code requres this. Doesn't each wire only carry half the load (50 A) max.

    2) Does a subpanel on a second floor require a grounding rod (or water pipe) connection, or just the ground wire back to the main panel?

    3) Is the bonding lug ( connected to grounding wire) sufficient to ground the sub-panel, or does the ground bar have to be bonded?

    4) Will a 1-1/4 inch flex conduit be large enough for 3-#2 wires and a #8 ground wire
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #2

    Oct 1, 2010, 05:16 AM
    My answers follow your questions.


    I have wired a subpanel feed with #2 ( or #3) wires protected with a 2 pole 100amp breaker. I am confused on why the code requres this. Doesn't each wire only carry half the load (50 A) max.

    Each pole of a 100 amp breaker can handle 100 amps, so the wire feeding it must be rated to handle 100 amps.

    2) Does a subpanel on a second floor require a grounding rod (or water pipe) connection, or just the ground wire back to the main panel?

    Any panel that is remote from the Main Breaker must be supplied with a four wire feeder, which includes an equipment ground that you mention.

    3) Is the bonding lug ( connected to grounding wire) sufficient to ground the sub-panel, or does the ground bar have to be bonded?

    An equipment ground bar must be bolted directly to the metal box of a panel with machine thread screws. The Neutral bar in a panel that doens not contain the Main Service Disconnecting Means does not get bonded to equipment ground.

    4) Will a 1-1/4 inch flex conduit be large enough for 3-#2 wires and a #8 ground wire

    Assuming THHN or XHHW insulation on the #2, yes, it will fit.
    cruiser2000's Avatar
    cruiser2000 Posts: 5, Reputation: 1
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    #3

    Oct 6, 2010, 02:05 AM
    Thank you for your answer.. Sorry still confused.

    If each pole can deliver 100 amps isn't that 200 amps, or 120v x 200amps = 2400Kva?

    A double pole breaker is made up of 2-50 amp breakers physically connected with a common trip bar labeled 100 amps.

    I know it may just be a naming convention but it's confusing to an old engineer like me.
    cruiser2000's Avatar
    cruiser2000 Posts: 5, Reputation: 1
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    #4

    Oct 6, 2010, 02:18 AM
    Comment on cruiser2000's post
    Sorry for calculation error 24 Kva or 24000 watts
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #5

    Oct 6, 2010, 02:45 AM
    If you were to cut a 2 pole 50 amp breaker in half, you would have two separate single pole breakers each rated 50 amps.

    A 100 amp service is 240 volts, 100 x 240 = 24000 VA.
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