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    Second sub panel and conduit question

    Asked Oct 9, 2006, 06:44 PM 3 Answers
    Hi all,

    New to this forum and finding lots of great advice. I'm planning on adding a 100A sub panel to my garage workshop.

    The house has 200A service and the main panel is located in the garage. The plan is to use a 125A main lug panel fed by two #2 copper for the hot feeder legs, a #6 copper neutral, and a #6 or #8 copper ground. Does this sound appropriate?

    The sub will be located one stud bay away from the main (16") so would it be best to run the wiring through conduit as opposed to cable (which I assume would have to be covered with drywall)?

    Lastly, the house is already wired for a 100A sub panel in the basement. Does code permit another 100A sub in the garage? The basement is currently unfinished, but even if I finish it, I don't anticipate using all of the circuits in the house, basement and garage at the same time.

    I did a bit more reading and realized it might help to state that the house is a 2000sf single family residence, 1 level ranch. Standard household appliances with A/C and forced hot air heating. If I finish the basement it will have standard lighting fixtures, and outlets - no hot tubs or anything exotic.

    I did look at the NEC 2005 to try and determine amp load for this size residence, but it's very confusing.

    Thanks in advance for any advice...

    Lou N

    Last edited by Lou N; Oct 10, 2006 at 07:55 PM.
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    tkrussell's Avatar
    tkrussell Posts: 9,638, Reputation: 722
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    Oct 14, 2006, 06:04 AM
    Sorry for missing your question until now, not sure how it got missed by everyone.

    #2 copper THHN for the hot feeders is fine, but to reduce a neutral,calculations of the unbalanced load needs to be done, and a neutral can only be reduced one third of the feeder rating, so not really worth it for such a small feeder, and is not done normally in everyday practice.

    To save some money, you can use #1/0 Aluminum for the three feeder conductors to a 125 amp subpanel, with a #4 Aluminum or a #6 copper equipment ground. But considering the distance is so short, copper will not be too expensive.

    Since the subpanel will be right next to the main panel, perhaps a conduit nipple with locknuts and bushings will work, otherwise you can use flexible conduit, cable is not necessary.

    There is no limit to the quantity of subpanels, the limit is the connected load on the main service.

    Yes, calculating total connected load can be confusing, this is why I do not show how to do the calcs here in the forum.

    Without knowing the exact types of appliances, based on "standard" electric appliances being range, dryer, water heater, and 10 KVA AC load larger than heat, 2000 sq foot of livable space, assuming this includes the basement renovated space, and gas or oil heat, and using Example D2b of the load calcs, I arrive at 115 amps of total connected load on the 200 Amp service.

    If any of the individual loads are different, then let me know and I can tweak the calcs.

    I hope this helps, and sorry for the delay.
    Lou N's Avatar
    Lou N Posts: 4, Reputation: 1
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    Oct 14, 2006, 07:48 AM
    Thank you very much, that clarifies things a great deal.

    Lou N's Avatar
    Lou N Posts: 4, Reputation: 1
    New Member

    Oct 15, 2006, 09:48 AM

    I went back and reread your answer and did some additional research. The 2000sf figure did not include the basement, so I added and additional 2000sf for that and another 1000sf for the garage.

    Taking the 5000sf figure I calculated 3W per sq ft for general lighting and outlets. Using some examples found in books at the library and a pocket guide to the NEC 2005 I added in 3 appliance circuits, a dryer and laundry circuit, the range, water heater, disposal, microwave, furnace fan (forced air), and the A/C unit which is rated at 29A @ 240v (I figured 100% of the volt-amps for the A/C unit).

    For the garage I added in a 240 circuit for my table saw and dust collector (taking the full current values for each) and threw in a 50A welding circuit for good measure. I added up all of the volt-ampere ratings, subtracted the A/C load and used the calculations for dwellings with service over 100A listed in the NEC. After adding the A/C circuit back in (at 100%) I came up with a load of 155A. I was generous with the ratings when putting the list together, so it appears I should be well within the 200A main service rating.

    Obviously not being an electrician my numbers are ballpark, but I think there should be plenty of power to safely added the 100A subpanel in the garage.

    On a related note - there are several circuits in the main service panel that are not being used. For example, the builder installed a circuit for a spa tub motor in the master bath. The previous owners opted for a regular tub so there is no motor for the tub and no access to the wiring, and therefore no use for that branch at this time. Can I disconnect those wires, cap and label them, and use that breaker slot for something else? If so, would it be appropriate to leave the capped wires inside the service panel tucked out of the way?

    Thanks you for all the excellent advice...

    Lou N

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