sub panel info

Discussion in 'Electrical Forum discussion & Blog' started by M3, Oct 19, 2007.

  1. M3

    M3 Member

    Messages:
    42
    Location:
    Minnesota
    I'm installing a subpanel about 90' away from the main so I'd like a main breaker set up and a full cover (small child in house), and I anticipate installing about 8-10 household circuits. The problem is that all the breaker style boxes sold at retail stores only come with 100 amp (min) breakers, and I only want to run 40-50 amps. So far not one person at the stores can give solid advice... This will be inspected, so I need to plan it with good information.

    So far I was planning on pulling 3-8 + ground to the sub... I been told that I can buy a main lug and backfeed it that that will take 2 spaces out of the panel.

    Any thoughts on what panel to purchase?
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,561
    Location:
    North Carolina
    It would all depend on where the panel is being installed as to the Main Lug panel.

    The size of the breaker used as a disconnect in the panel (main) wouldn’t matter.

    I can install a 200 amp main panel and feed it with a 60 amp feeder as long as the conductors are #6 and the breaker protecting the feeders is 60 amp. The 200 in the panel is nothing more than a disconnect and its size has no meaning at all.

    If you are installing #8 feeder conductors protect them with a 50 amp breaker and the size of the breaker in the subpanel does not matter.

    The Equipment Grounding Conductor installed with the feeders will need to be at least a #10 and kept separate from the grounded (neutral) in the subpanel.
  3. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The backfed breaker will work but it must be fastened in with an appropriate device so it can't be inadvertantly removed.

    You will have a breaker in the main panel that protects the feeder, so there is nothing wrong with having a 100 Amp main in your subpanel. It will still perform the function of a switch and an overload will trip the breaker in the main panel.

    If you are going to use an 8-3 cable with ground you should try to get an SE R cable to get the full 50 Amps of capacity. If it is NM it is only rated for 40 Amps.

    If you are going to pull conductors in conduit you will get the full 75C ampacity of the wire and can use #10 Cu or #6 Al for the ground up for up to a 60 Amp feeder, and #8 Cu or #6 Al for up to a 100 Amp feeder.

    You can often find #6 Aluminum SE R which would give you 50 Amps. Aluminum is fine for feeder cables and the current materials don't have the problems that arose during the '60s and it is less expensive than copper. I used #2 Al SE R when I did mine. It provides 90 Amp capacity though some erroneously treat it as 100 Amp. The difficult part is to find a 90 Amp breaker for the feeder protection.

    I selected my subpanel to use the same QO breakers as my main. That made it convenient when I wanted to move circuits from the main panel to the subpanel. On the other hand, you can often get a moderate size main panel with a selection of 15 and 20 Amp breakers for about the same money as an empty panel. HD and probably Lowes usually have good deals on complete panels.

    EDIT NOTE: The jwelectric reply that was made while I was writing. I didn't intend to step on his reply but there are some additional things here so I will not delete this.
  4. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,561
    Location:
    North Carolina
    Yes you did give a better detailed post and I for one thank you for taking the time.

    Please never delete a post because it may duplicate something someone else has said. When you repeat something someone else has said it just makes the point more supported. Again thank you for taking the time to post.
  5. M3

    M3 Member

    Messages:
    42
    Location:
    Minnesota
    Thank guys for your replies... and I appreciate your patience dealing with a "handy novice" like me. I've gotten a lot of conflicting info when researching this and want to do it right.

    I was planning to run 3-#8 THHN and a #10 solid THHN ground through my flex conduit and couple it with a Homeline 100 amp panel. Then I read online (different forum) that most preconfigured panels will not accept smaller than a #6 wire, and that if someone uses a 100amp breaker style load center they cannot supply it with less than a #2 wire...

    I did call Tech Service for the Homeline and they said it will take down to a #12... which would support your statements that it can be used as a subpanel...

    I also pointed out at one of my local big box retailers that their wiring signage states #8 copper stranded is rated for 40 amp max in one place, and rated for 50 amp in another place on the same sign. The assistant said "oh, so it does, I guess I don't know". I just decided to err on the safe side, although it would be nice to have the 50.

    Thanks again.
  6. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The NM is 40 Amps but the THHN is 50 Amps.
  7. M3

    M3 Member

    Messages:
    42
    Location:
    Minnesota
    Thanks for the help, I've got just one more question regarding the sub P. I'm in MN - do you know what are the rough inspection requirements are for a subpanel? The inspection "checklist" does not specifically cover that subject.

    On another subject, can the only outlet (GFI) in a bathroom be installed in the side of a vanity?

    Thanks again.
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,561
    Location:
    North Carolina
    removed post .
    Last edited: Oct 26, 2007
  9. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,561
    Location:
    North Carolina
    Yes as long as it is not more than 12 inches down
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