Sub panel confirmation

Discussion in 'Electrical Forum discussion & Blog' started by M3, Jul 25, 2007.

  1. M3

    M3 New Member

    Messages:
    39
    Location:
    Minnesota
    I am am planning to put in a sub panel via an old 3/4 flex conduit that housed an old dryer feed to make completion of a basement bath and two other rooms easier. The house has aluminum wiring which I plan to replace as I go, but I don't want to tie in to it for my new areas.

    I've got four question/concerns: 1) the run length and drop in amps, 2) the max size conductors that I can should run to make the pull manageable, 3) using a twist-in couple on the flex, and 4) what size ground should be pulled back to the main.

    My main panel is 100 amp located in the garage and I'd like to run the sub using #6 or #8 copper.... (3+G) I want to extend the flex conduit (total length 90') with a twist-in coupler.

    Any information would be greatly appreciated. Thanks!
  2. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    It is permissible to run four #6 THHN copper through a 3/4" flexible metal conduit (FMC). That will give you a 65 Amp circuit. If you use #8 it will be rated for 50 Amps.

    It is very difficult to pull 90 ft of it through FMC, and I expect impossible to pull it through the INSERT connectors. You will need pulling lubricant.

    It is not a "code" practice, but if faced with the problem I would pull through the FMC and then put the connectors on the ends and connect to the box after the wire is pulled. You will need to fasten the conduit in place while doing the pull.

    If you have a balanced load such as a dryer or water heater circuit you should be able to run #8 copper for the neutral and #10 for the ground for up to a 60 Amp breaker. If the breaker on the feeder is more than 60 Amps then you need a #8 ground.

    If you are not attached to using the old FMC you can run aluminum SE R which is a 4-wire cable that needs no conduit. #2 is rated at 90 Amps and #4 at 65 Amps. The aluminum is fine for a subpanel feeder.
  3. kd

    kd New Member

    Messages:
    207
    I like to use the connectors that attach to the outside of the flex. Be sure to cut or grind off sharp protruding ends of the flex. I recommend pulling into conduit that is securely fastened in place. You could run # 8 for the neutral and for the grounding wire if you have the wire, but it may be cheaper to buy a roll of #6 and use it for all four wires. Either a 60 or 70 amp breaker is acceptable for feeding #6 THHN. Assuming the old run has less than 4 90 degree bends in it, lots of lube should do it.
  4. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The problem with buying a roll of #6 and using it for all four conductors is that it is a violation of the code to reidentify conductors that are not larger than 6 AWG as a neutral or ground. See 200.6(B) and 250.119(A).

    If you are don't need it immediately you can shop on that auction site.
  5. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,714
    Location:
    Central Florida
    I wonder what the UPS charges would be on 360 feet of #6...

    Bob, do you have the preamble text or whatever describes the reasoning (if any) behind the prohibition of reidentifying #6 and smaller?
  6. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    There is no preamble text and the NEC is short on reasoning behind any requirement. It is clearly an economic issue as stocking larger size wire in green and white could be expensive.

    I have bought quite a bit of wire on the auction site. Even with shipping it is quite a bit less than HD. For example, I got 100 ft of 2-2-2-4 Aluminum SE R which made a 90 Amp feeder for a subpanel at $0.45 per ft. I also needed about 40 ft of #4 copper ground for my service upgrade, which I got for about $0.40 per foot. I had to strip the red insulation off it which was quite easy.
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