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  • Installing electric sub panel in a new detached garage.

    Hello everyone, I'm new to the forum and I need some expert advice from you folks here. I have been lurking around on the forum for a few weeks now and have learned a few things about the task I am about to tackle.

    I have built a new 20X24 garage 39 feet behind my home and I would like to run electric service to the garage from my 200amp service panel inside the house, The meter box faces the garage and the service panel is directly behind the meter box.

    First, I am not an electrician but I do have some back ground in the electrical field and I am aware that this does not make me a professional by any means. Therefore when I complete my part of the electrical installation I will have a local electrician check everything I do, tie into the main service panel and handle all permits as the codes inspector we have here is a complete JERK and wish not to deal with him whatsoever.

    I need some advice in a few areas and that's why I am here and hopefully you guys can get me going.

    I will start off by listing the number of outlets, lights and equipment that will be used in my garage.

    I have mounted a 125amp 12 slot sub panel in the garage and have completed most of the electrical installation. Below is a list of what I have.

    1. 17 outlets
    2. 8 ceiling lights
    3. 2 work bench lights
    4. 3 outside security lights and 1 light on photocell under attached car port
    5. 2 switched lights under car port and 1 entry door light as requested by the codes inspector
    6. 220V compressor @ 17amps on 20 amp breaker
    7. 220V ac/heat window unit with auto feature @ 23amps on 30amp breaker

    I have 12/2 with ground for all 110V outlets/lights
    I have 10/2 with ground for all 220V outlets

    Sub panel layout. Note that I am installing 20amp breakers

    Left row of breakers are:

    1. 4 left ceiling lights, carport lights(switched)
    3. 4 right ceiling lights, ceiling fan
    5. 4 east wall outlets with gfci @ #1, carport light(on photocell)
    7. 4 west wall outlets with gfci @ #1
    9. 5 work bench outlets with gfci @ # 1
    11. Garage door opener, exterior security lights, carport outlet with gfci protection

    Right row of breakers are:

    2. AC/Heat unit 30amp 2 pole
    4. AC/Heat unit 30amp 2 pole
    6. Compressor 20amp 2 pole
    8. Compressor 20amp 2 pole
    10. Spare circuit in attic junction box
    12. Spare circuit in attic junction box

    That's about all I will have in the garage.

    The 220V compressor will take 15amps ? on a 20amp breaker
    The 220V ac/heat unit will take 23amps on a 30amp breaker

    The garage is 39 feet from the meter box, add 8 feet up the wall at meter box and 6 feet up the wall at sub panel.

    What wire do I need for this installation. From the 200amp service panel to the sub panel, I am going to estimate the entire run to be no more than 60 feet. I was told to use 6/3 with ground on 60amp breakers and an additional ground rod at the garage, keep grounds and neutrals separated in sub panel and everything would be fine, but I am not convinced that the inspector will fall for 6/3 in this case.

    This will be buried in pvc conduit. I hope this is enough information to get a few reply's but if not I can post more.

  • #2
    Re: Installing electric sub panel in a new detached garage.

    I would not run the panel in the garage on a 60 amp feed. Not that you can't, but why not take full advantage of the panel you installed? You can run #4 copper from a 100A breaker in the main panel to the panel in the garage. Is the panel in the garage a "main lug only" or does it have a breaker in it? Doesn't matter though. If you want to save some money you can run aluminum wire. When you go to the supply house they can tell you what size the aluminum needs to be. it will be a little bit larger than the #4 copper. I would run the 2 hots and the neutral at all the same size wire whichever you use. You will need a #6 bond wire as well larger for al. Some years ago it was permissable to run only the 2 hots and the neutral and sink a ground rod at the garage and create the bond seperately. Now you must also run a bond as well and also use a ground rod, MAYBE TWO. It depends on the inspector. Many like to see 2 rods even though it is 100A or less. I find it is ALWAYS better to ask the local inspector or someone who knows what he wants to see beforehand so when he checks it, you won't be surprised that you have to change something. Run it in conduit and make sure it is 18 inches or more below grade. If your panel has a seperate bonding bar, put all the bonds on it and all the neutrals on the nuetral bar. Keep them seperated. if the inspector wants them together all you need to do is install the bonding screw. This rule changed a few years ago and I suppose it depends on which year NEC book your state is working off of. BTW, 39 feet is not that far that you need to upsize for distance.

    Comment


    • #3
      Re: Installing electric sub panel in a new detached garage.

      You need a main disconnect at the garage since you have more than 6 handle throws to disconnect all power.

      Comment


      • #4
        Re: Installing electric sub panel in a new detached garage.

        I would run 10/3 for the air compressor & use a 30 amp breaker. 17 amps load is to close for me for a 20 circuit. Air compressors pull a lot of amps when they start up with pressure in the tank. Spend a couple extra bucks & do it once.

        Comment


        • #5
          Re: Installing electric sub panel in a new detached garage.

          Originally posted by MR.FUDD View Post
          I would run 10/3 for the air compressor & use a 30 amp breaker. 17 amps load is to close for me for a 20 circuit. Air compressors pull a lot of amps when they start up with pressure in the tank. Spend a couple extra bucks & do it once.
          A 240 volt air compressor does not require a 3 wire cable,as it does not require a neutral. 12-2 on a 20 amp breaker for your compressor is fine. If you tried to use a 30 amp breaker with #10 wire, you would have to hard wire it as a 20 amp receptacle should not be attached to a 30 amp circuit.

          Comment


          • #6
            Re: Installing electric sub panel in a new detached garage.

            Better check with local code, here we are required to run #3 CU or #2 AL for 100A sub

            Comment


            • #7
              Re: Installing electric sub panel in a new detached garage.

              Originally posted by rjniles View Post
              A 240 volt air compressor does not require a 3 wire cable,as it does not require a neutral. 12-2 on a 20 amp breaker for your compressor is fine. If you tried to use a 30 amp breaker with #10 wire, you would have to hard wire it as a 20 amp receptacle should not be attached to a 30 amp circuit.
              In this area you are only allowed to load a breaker 80% Thats 16 amps on a 20 amp breaker. Than hard wire the compressor or change plug

              Comment


              • #8
                Re: Installing electric sub panel in a new detached garage.

                Originally posted by wbrooks View Post
                Better check with local code, here we are required to run #3 CU or #2 AL for 100A sub
                You better check your code #2 for 100 amp excep entrence service only

                Comment


                • #9
                  Re: Installing electric sub panel in a new detached garage.

                  A air compressor is not a continuous load and does not fall under the 80% loading rule.

                  Comment


                  • #10
                    Re: Installing electric sub panel in a new detached garage.

                    Thanks everyone for the replies, I have since completed the work I mentioned above and hired a professional electrician to check out and or correct anything that I might have messed up. However I have decided not to invite the inspector out here again nor will I purchase a rough in/final permit but I do have power to the garage and it is in compliance with the NEC. The codes inspector in my area is a total jerk but it's all good now.

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