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Thread: Wire size for sub-panel

  1. #1
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    Aug 2010
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    Wire size for sub-panel

    I have 200 amp service in my basement. I want to put a 100 amp subpanel in my garage. It's about 60 to 75 feet run for the wire in between. I'm going to run it in conduit, what size copper wire should I be using? And I need two 50 amp breakers in my main panel correct? Also I should have 2 hots, a neutral, and a ground?

  2. #2
    Aaron
    I assume you will want 220V available in the garage. Usually the rating given for a panel/subpanel is it's max 220V input. A given panel can be used for a lower capacity, usually isn't, unless reserving space for future upgrades. If you want 100A of 220V capacity in your garage subpanel, then you need a 100A 220V breaker in the main panel, which will have a 100A breaker on each hot leg, which are mechanically tied together into a single unit. An electrician would probably run a 1/0 (One Ought) aluminum for 100A service.

    If you want 100A of 110V service, then you could use a 50A 220V breaker in the main, with possibly 8ga, but more likely 6ga leads.

    Strongly recommend you have a licensed electrician do this work, if only for the fact that 1/0 wiring is a bear to work with without the proper tools!!!

    Good luck.

  3. #3
    Join Date
    Mar 2010
    Location
    Williamsburg, Virginia
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    I second the previous comment about getting a licensed electrican to run the primary feed to the sub panel. This past summer I needed to get 100 amps to my detached shop about 25' from the house. I rented a ditch witch and had a 24" deep trench ready for the electrician. He came out prior and made measurements and brought out the materials (I believe 3/0 aluminum in conduit for the feed) and he installed a 100 amp breaker on my main breaker and energized the sub panel. Although not required, I had a main breaker installed on the sub panel. I agreed to be on site to help pull the 3/0 wire as it was very stubbon. Not including the rental for the ditch witch, I spend about $800 for parts (nearly 100' of 3/0 to go breaker to sub) and labor to get the sub box hot. I have been wiring the shop myself and have just one line to complete before I get it inspected. I never wired before but got a couple of good electrical wiring books and talked with several "experts" to gain knowledge and confidence.

    I probably saved $800 running the circuits myself and by taking my time I have it layed out just as I want including outlets for a future air cleaner, 5 x 220v circuits for current and future equipment. I saved some additional money by getting a great deal on a used 125 amp breaker panel at the local Habitat for Humanity store for $10 and it was filled with breakers! My box has 20 slots and I've just about filled it out so a larger box would be highly recommended.

    Good luck.

    Don

  4. #4
    Join Date
    Nov 2007
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    Saratoga Springs, Utah
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    863
    I ran 6-0 for mine. 100 AMP capable panel - 50 AMP main breaker. Not going to ever need more in this current shop
    Sawdust is some of the best learning material!

  5. #5
    Join Date
    Nov 2007
    Location
    Milwaukee, WI
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    Quote Originally Posted by Aaron Neve View Post
    I have 200 amp service in my basement. I want to put a 100 amp subpanel in my garage. It's about 60 to 75 feet run for the wire in between. I'm going to run it in conduit, what size copper wire should I be using? And I need two 50 amp breakers in my main panel correct? Also I should have 2 hots, a neutral, and a ground?
    4-4-4-6 copper and a single 100A breaker in the main. 2 hots, neutral, and ground is correct. Make sure that you separate the neutral and ground bus in the sub-panel. PS: Not an electrician, there are several members who are, so maybe thye can chime in. But I think this is accurate to wire size.

  6. #6
    Join Date
    Mar 2007
    Location
    Corvallis, OR
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    86
    A 100 A sub-panel requires a two-pole 100 A breaker - if you want 100 A. You can use #4 AWG 75 deg C or 90 Deg C copper wire in conduit for a 100 A breaker provided the breaker is labeled for use with 75 deg C wire. A #8 copper ground is adequate for a 100 A feeder If running NM cable or SE cable, you would need #2 copper, I believe.

    The sub-panel does not require a main breaker at the sub-panel, since there will be a breaker protecting it and the feeder in the main panel. You can put a main breaker in if you want to.

    As mentioned already, you can buy a 100 A sub-panel and feed it with a smaller 50 A or 60 A circuit as long as you use the proper size breaker in the main panel.

  7. #7
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    Aug 2010
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    Joliet, IL
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    Ok thanks for the info guys!!! A couple more questions. should I be using stranded thhn cable? Home depot dosent carry the aluminum cable. Some one told me (who has electrical knowledge) that i dont need a ground from the main panel to the sub panel if i use conduit, because the conduit bonds it. Is this true? He said I only need 2 hots and 1 Neutral, He also told me to use #2 guage thhn cable, (Is that overkill?)The home depot guy said (which I take with a grain of salt) That I need to put a grounding rod outside and tie to that, I have not heard that before. He also said I could get away with #4 Thhn cable. Hiring a electrition is out of the question ($), I have no problem adding new circuts and outlets and switches. But I just want to make sure I go big enough so to speak with the sub panel. I have read alot of information on installing a sub-panel, But it seems everyone has a different opinion. I plan on having 5 220 outlets and a couple 110 outlets and some shop lights. It seems to me that I need a 100 amp breaker in the main panel, 2 hot cables a neutral cable and a ground going to the sub-panel? Or is the ground optional with conduit? Once again thanks for your time and your knowledge!!!
    Last edited by Aaron Neve; 02-14-2011 at 9:54 PM.

  8. #8
    Join Date
    Jul 2009
    Location
    Sandwich, MA
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    134
    Hi Aaron,

    I ran a 100 amp service out to my detached garage/workshop last summer, a total distance of 170' from my service panel. I recommend you discuss this with your electrical inspector. I found my inspector a bit more conservative than the NEC requires, but on the plus side, he gave me lots of useful advice that made the job easier. Plus, since he had essentially blessed the design (with his added on conservative elements) this avoided the risk that he would not signoff on the final installation.

    He required a 4 wire conductor from house panel to workshop subpanel (2 hots, a neutral and a ground). He also required a separate ground at the garage/workshop. Everything is in 2 inch PVC conduit. NEC required only 1 or 1.25 inch PVC (I forgot which), but he suggested I use 2" since the cost of PVC is cheap and it made pulling the 2-2-2-4 AWG copper much easier. He also recommended installing a second PVC conduit for future cable installation since the trench was already open. I put in a 1 inch spare conduit for very little money. You will probably need to use expansion fittings on the PVC where it comes out of the ground to counter frost heaves, since you're in Illinois.

    I also suggest you ask around for where the contractors buy their large cable. A friend of mine directed me to a place 50 miles away where the price was half of that at Home Depot.

    Good luck.

    Bob DeRoeck

  9. #9
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    Aug 2010
    Location
    Joliet, IL
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    Thanks Bob... I think I will see what the inspector gadget says. When you are refering to 2-2-2-4 are you implying #2 guage for the 2 hots and the nuetral and #4 guage for the ground?

  10. #10
    Aaron, I don't know if you have more than one Home Depot within reasonable distance to you but if you do, you might call another one and see if they have 2-2-2-4 aluminum cable. I have 4 home depots that I frequent around me and 2 of them had the cable and 2 of them didn't. I just had a 100 amp sub panel put in the shop by an electrician and then I wired in a bunch of new circuits. The 2 electricians I had helping me both suggested using 2-2-2-4 aluminum since it was much cheaper than copper and would do the job no problem (I bought all the material and they installed). I didn't use conduit (mine was all inside so it wasn't needed) so I can't comment on not needing the ground (though it doesn't sound correct). They also put a 100 amp breaker in the main.

  11. #11
    Aaron.... Any particular reason for conduit? Can you run and secure romex, or is everything going to be surface-mounted, hence the conduit? I just did the almost the same distance/amperage as you are looking at, and I'd strongly suggest 2-2-2-4 aluminum romex cable if you can. No reason for copper unless you're pulling and want (slightly) easier pulls and smaller conduit.

  12. #12
    Quote Originally Posted by Aaron Neve View Post
    Thanks Bob... I think I will see what the inspector gadget says. When you are refering to 2-2-2-4 are you implying #2 guage for the 2 hots and the nuetral and #4 guage for the ground?
    Yes, 2-2-2-4 means 2 gauge hots and neutral and 4 gauge ground.

  13. #13
    and.... 2-2-2-4 refers to romex cable, not individual wires. A great big version of the 14/2 or 12/2 or 12/3 stuff you run to your lights and outlets.

  14. #14
    Join Date
    Aug 2010
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    Joliet, IL
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    In the garage everything will be surface mounted... But my basement is unfinished, and I guess I just thought romex was a no-no. Everything they ran when they built the house was in conduit in the basement. I.e. Basement lights and any runs to the kitchen or other rooms. If i could do romex that would obviously make it alot easier. Is it fine to just attach it to the underside of the joist?

  15. #15
    You can't use romex in an exposed position on the surface. It has to be in between or through the joists or walls, and secured every four feet in most jurisdictions. Do you see "regular" romex runs when you look up in the basement?

    Potentially, you could run the 4-4-4-2 aluminum through the joists and in the garage wall to get into the back of a surface-mounted subpanel, then run your surface conduits out of the subpanel as needed. Just FYI: You can't run romex in conduit, even briefly to enter the subpanel.

    When you figure out how it can be done, I'd suggest going to the Building Dept that covers you and run your idea by them and clarify any areas that you have questions about. The inspectors here were VERY helpful in guiding me in my project.

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