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Thread: What gauge and type of wire for running between main panel and sub panel in garage?

  1. #1
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    What gauge and type of wire for running between main panel and sub panel in garage?

    The subject kind of says it all. I have been reading here and elsewhere about how to get power out to my sub-panel. I have put in breakers before and wired lights, switches outlets etc... So I have no fear of that part. But I have never put in a panel before.

    So I am wondering about the new elements involved in that. I have spoken to a couple of neighborhood electricians. I have spoken to a couple, read a lot, of "experts" down at Home Depot and Lowes. I am just trying to figure out what to do for 1 simple piece.

    I was planning a 50 amp line out to the garage with 6/3 copper wire. Now I am thinking that perhaps a 100 amp line is a better call. But wire to use? I have read recommendations here and elsewhere that 2/2/2/2 4 wire aluminum is enough for 100 amp. Then recently a post that it didn't pass inspection. So do I got for the much larger 2/0 aluminum wire?

    I have also read tons about aluminum wires and fires and problems. So I started planning copper. Well the the 6/3 I was looking at won't go about 50. So that takes us down the 2/3 copper wire that HD sells. More than 2x what the 6/3 cost but the kid helping me had no idea what the max amps on it was... They had a chart that was very hard to read and it is either 65 amps or 125. Well that is a big enough swing to burn the house down with...

    Now the panels are 21' apart on base measurements. But given the oddities that happen figure it is going to be more like... 25' of cable. My intention is to buy 30' just in case.

    So I thought I would pose the question here. Given that I am just talking about the link between the panels themselves would you:
    1- Go with copper or aluminum wire?
    2- And based on #1 what gauge would you use?
    3- A 50amp line for a 1-man hobby garage or a 100?
    4- Anything else you can think of that I haven't asked about in regards to connecting the 2 panels...

    Any advice or information would be great. I just don't want to burn the house down.

    Thanks!!!
    Joshua

  2. #2
    Aluminum if installed correctly is fine, but if you have the money copper is still the conductor of choice.

    #2 AL is only good for 90 amperes unless it meets certain conditions which your shop feed will not meet.

    As for 50 or 100A, you can always upgrade if you oversize your conduit.

    There are more issues but am too lazy to post anymore...

  3. #3
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    Joshua, with me being in the Navy it goes without saying that I move a lot (about every 2-3 years). Each time I have to reestablish my shop.

    I have a subpanel that I wire into the main panel using 6/3 copper and then run surface mounted conduit (10/3 and 12/2) for all the outlets. The subpanel is a 100 amp main panel that is typically used in household applications (I replaced the 100 amp main breaker with a 50 amp breaker). I use that size panel for the space - I have separate circuits for each 220 tool and four separate 110 circuits. This does not include the 110 circuits that are typically already in the garage (typically both the outlets and lighting are on the same circuit).

    I hook that subpanel into a 50 amp breaker in the household main. In 15 years I have NEVER tripped that 50 amp breaker. At most, I am drawing about 40 amps (startup) at one time. That's the dust collector and combo jointer/planer both starting at the same time (which NEVER happens). Of course there are miscellaneous loads running all the time (like chargers) and I always wire in a bank of lights off of the subpanel as well (I like having the lights fed by two separate circuits). But again, for a one-man hobbyist shop, I've never tripped the 50 amp breaker.

    Be well,

    Doc

  4. #4
    Quote Originally Posted by Joshua Dinerstein View Post

    So I thought I would pose the question here. Given that I am just talking about the link between the panels themselves would you:
    1- Go with copper or aluminum wire?
    2- And based on #1 what gauge would you use?
    3- A 50amp line for a 1-man hobby garage or a 100?
    4- Anything else you can think of that I haven't asked about in regards to connecting the 2 panels...
    1 - Cu, unless you find Al to be HUGELY less expensive.

    2 - For 50A, 6AWG; for 100A, 2AWG.

    3 - Compare the costs of the wire. Like Don said, you'd likely never exceed 50A, but maybe someday you'll add an electric heater, welder, or decide that panel is a good place to hook up a hot tub. You'd hate to have to re-do that main feed, so if the cost difference between 2AWG and 6AWG is small, you might decide that it's worth the assurance that you won't have to re-do it, no matter what. OTOH, if it's expensive and wouldn't be difficult to pull a new wire someday (if you needed it), maybe you just risk having to redo it.

    4 - Unlike main breaker panels, subpanels have SEPARATE ground and neutral busses, so you'll need to remove the tie bar that goes between them. That is basically the only thing you need to know that you might not have already been aware of

  5. #5
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    Quote Originally Posted by Joshua Dinerstein View Post
    1- Go with copper or aluminum wire?
    2- And based on #1 what gauge would you use?
    3- A 50amp line for a 1-man hobby garage or a 100?
    4- Anything else you can think of that I haven't asked about in regards to connecting the 2 panels...
    A civilian like you and me should always use copper, in my opinion. Aluminum requires too much fussy getting it exactly right. Copper's reliable.

    Wire size is a combination of three factors: voltage, amperage, and distance. Your goal is to prevent a voltage drop below a certain minimum amount - low voltage is hard on motors and other components. For the distance you've listed, I believe you should be able to use No. 6 wire for 50 amps - the old book I've got indicates that size has sufficient ampacity, and minimal voltage drop for such a short distance.

    I've gotten by for years with a single 110-volt 20-amp circuit, though I'm thrilled that I'll have a 220-volt, 60-amp panel when I finish hooking it up this summer. But I don't run 5 hp saws. If your ambitions are large, you might consider springing for a 100 amp panel. You'd want to go to No. 2 wire, I believe. If money is a consideration, 50 amps will probably do fine for years or forever.

    If you're burying in conduit, by the way, and if you're using plastic, upgrade to Schedule 80 conduit. The extra wall thickness provides extra protection against future digging disasters. For some reason, it's sometimes hard to find Schedule 80 connectors (elbows and suchlike), but they're out there too.

    Other threads have suggested that any electrical work should be inspected, primarily so that, if there's a fire, the homeowner's insurance will pay out. There's wisdom in this argument; if you adopt the idea, you may find your inspector is helpful (or you may not - inspectors vary).

    You referred to 2/3 wire - is that 3 conductors plus a ground? If you're wiring a 220-volt panel, you need four wires - two hots, a neutral, and an equipment ground (this assumes your main panel is grounded). In this area, any subpanel in a separate building also requires a ground rod, which will provide double protection on the equipment ground.

    Go to your local library and find the books on residential wiring. These will discuss the additional issues involved in putting in a subpanel. There aren't too many, but you want to include them all. For instance, the neutral and the equipment ground are on two separate bus bars in subpanels (although the neutral is bonded to the panel in the main panel). The equipment ground should, of course, be tied to the panel itself, so that the panel box is grounded, but the neutral should be isolated from the panel.

    Use GFCI outlets wherever you can.

    Note: I am NOT an electrician; just someone who's done some wiring

  6. #6
    I would not use aluminium wiring unless it is installed by a certified electrician. There are a number of steps that you have to take with aluminium wiring to make it safe.
    In addition, I would check your local building codes to make sure that aluminium wiring is permitted. Most municipalities require that you have a building or electrical permit for the type of instillation you are planning. If you do not obtain the necessary permits, your home owners insurance policy may be voided. When the work is completed, it will be inspected by the electrical inspector from your municipality and a certificate of completion will be issued. In my 30 years as a firefighter, I have seen many homes go up in flames due to faulty electrical instillations. In advance, you need to find out what the electrical code is in your area. Many municipalities require metal conduit for external instillations. Once again, if you do not follow the code requirments, your insurance carrier may refuse to pay the policy in the event of a fire.
    Last edited by Don Selke; 07-18-2009 at 1:20 AM.
    Good Luck:
    Don Selke

    Julius A. Dooman & Son Woodworking
    My Mentor, My teacher. "Gone but not forgotton"

  7. #7
    Stranded feeder cables are fine in aluminum as long is it is sized correctly for the service and length of run. Most municipalities allow stranded feeder cables in aluminum. The only fussy thing that needs to be done is anti-oxident paste at the lugs to prevent corrosion of the wire. All solid wire branch circuits should be in copper and sized for the appropriate circuit.

    I prefer copper for feeder cables because it can be of a smaller gauge and is easier to pull in conduit IMHO.

    There are some good books that will help with a DIY installation. However if you are not comfortable with installation or if it is against local oridnance to DIY I would strongly suggest you hire an electrcian.

    Good Luck

    Alan

  8. #8
    Aluminum is fine if your running it outside ie buried from one structure to another. Once inside the structure, most residential codes want you to terminate the aluminum and run copper inside. It doesnt take any special skills or experience to use an aluminum feeder. Anti oxidant pastes are recommended, but Id use that on copper too.

    If you are running the feeder indoors, go with copper. Id compare the cost on 50 or 100 amps. Shop around for the cable, other than the big box stores, the cost difference might not be that much.

  9. #9
    Quote Originally Posted by Dave Westover View Post
    Aluminum is fine if your running it outside ie buried from one structure to another. Once inside the structure, most residential codes want you to terminate the aluminum and run copper inside. It doesnt take any special skills or experience to use an aluminum feeder. Anti oxidant pastes are recommended, but Id use that on copper too.

    If you are running the feeder indoors, go with copper. Id compare the cost on 50 or 100 amps. Shop around for the cable, other than the big box stores, the cost difference might not be that much.

    If you wished to use 10 AWG aluminum for your branch circuits there is nothing in the NEC to prohibit it, the good news is that no cable manufacturer makes 10 & 12 AWG aluminum anymore. HUD standards for manufactured/mobile homes ("trailers", 'cause a trailer by any other name is still a trailer) and any local amendments would be the only thing to prohibit AL conductors, that being said I would not use aluminum for any circuit under 100A, which would mean a 1 AWG minimum for a shop feed since #2 AWG is only good for 90 amperes, a shop feed is not a dwelling service covered by NEC table 310.15(B)(6). Copper is still the first choice IMO, 50-60A would be adequate for a single user shop, one can only run 1 machine at a time other then dust collection & a compressor.

  10. #10
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    I'm not going to speak to the size of wiring to use, as I'm not an electrician, but personally I would stick with copper.

    I did have an electrician install a 50 amp subpanel for my shop off the main circuit panel for our house. The electrician made a suggestion that I found to be useful. Instead of running the circuit for the lights off the subpanel, he ran the lighting circuit off the main panel as well. This way, should something major happen with the subpanel, you'll still have light, and that is one less circuit your subpanel has to deal with.

  11. #11
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    Stick with copper since it is easier to work with. To guess the cable length you should measure the distance up/down the wall, to the panel, back up or down the wall, add a few feet for routing in both panels and add a few feet just incase you overlooked an obstacle. The wire size is dependant on the amp rating on the sub and distance of the run. You will be ok with #6 for 50A or #2 for 100A for your run distance.

  12. #12
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    I spoke with a Friend-of-a-friend who actually is an electrician and I was once again over-engineering the doggone thing. I am a software engineer and I guess it is just a pitfall I am going to fall into over and over and over.

    He explained what kinds of power I really needed for what I am doing. And had a few simple fixes for some stuff. I was going to have re-route a line in order to get into my breaker box. Or so I thought. Turns out I was wrong.

    Since my panel is full I thought I needed to reroute a line to a dryer that I don't use but don't want to lose for resale purposes out to the garage. Instead he pointed me to a doubled 220v breaker. So that both 30 amp circuits can be plugged into 1 slot. Freeing up a slot for the 50amp to go into the garage.

    Given that I don't need to send more power out just to bring it back I am going to go with a 6/3 copper 50amp circuit. With the other line it is at least twice what I currently need so I should be golden for years to come.

    Since it is only 50amp I am definitely going with Copper. It just isn't that expensive at that size. So it makes everything easier.

    I have checked the local city code. And it doesn't say I need a sub-panel inspected. But it also doesn't say I don't. I have to call them on Monday and try to get a direct answer. I don't know what they charge but the good news is that I have a great many answers and I am now past the design stage into the getting-it-done stage. I hope.

    Thanks everyone. I think I would have gone Aluminum and paid a pro to put it in for 100amp. But now I don't need too. Go aluminum that is.

    I have talked to 2 electricians so far and I am gather bids. I had thought to do it myself. But in the end I thought naw... Better to pay someone and have no sleepless nights. Thought of my 4 month old daughter caught in a house fire would keep me up fro the rest of my life.

    As always you guys are great! Thanks so much for the help, experience and advice!

    Joshua
    Last edited by Rob Russell; 07-20-2009 at 12:06 PM.

  13. #13
    I ran #6 copper for about 40" from panel to sub-panel on 50amp breakers. As in your case this was recommended by a couple licensed electricians in different conversations.
    “If liberty means anything at all, it means the right to tell people what they do not want to hear.” -- George Orwell


  14. #14
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    Joshua,

    I second what Doc did. I am in the process of doing the same thing. Ran 6/3 - 80 ft and plan for a 50 amp panel in the garage and a breaker in the main box.

    If you know an electrican that can do the hook up at main box and is willing to show you have to hook up the wires in the garage then you can wire the garage at you leisure and save your self some dough. You just kill the power at the main.

    Just a tip. I'm going to run all circuits at 20A with 12g except the 30A and lights. It will make it easier so I don't have to guess what is 20 and what is 15.

    Don

  15. #15
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    Wire size per length of run question

    Don't mean to hijack this thread, but where does one find the wire size requirements for a certain distance being run (from the main to sub in particular).

    Ed

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