100 amp box wiring

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Old 09-16-17, 03:07 PM
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100 amp box wiring

Quick question I hope. I've been researching this and can't find a answer for my particular question.

Feeding a 100 amp box in a detached garage. Pull from the house will be roughly 72 feet. This garage is just that, a garage. It's going to house cars and nothing more. That said it will have 10 outlets on a breaker, two garage doors on a breaker, 3 led lights on a breaker. All 20 amp breakers.

Question on is what size wire do I need? I keep going back and forth between #4 copper and something like a 6-3.
 
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Old 09-16-17, 03:17 PM
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Any possibility of buying an electric car in the future?

#4 copper is good for 90 amps
 
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Old 09-16-17, 06:36 PM
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This garage is just that, a garage. It's going to house cars and nothing more.
If no electric car or heat then at most you probably need 20 amp multi wire circuit using 12-3 UF or #12 THWN in conduit from either two handle tied 20 amp single pole breakers or one double pole 20 amp breaker. That will give you two 20 amp 120 volt circuits. No need for a breaker panel. Why did you want a sub panel?
 
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Old 09-17-17, 07:38 AM
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Sub panel is just the way I did it. Rather then have a basic on/off switch I wanted the ability to shut off certain items just in case in the future.
 
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Old 09-17-17, 08:03 AM
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Do you want to install a direct bury style cable or bury conduit for wire to be pulled?
 
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Old 09-17-17, 08:39 AM
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Either, whatever is best. I just want to do it once and the way that is correct.
 
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Old 09-17-17, 10:44 AM
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It's better to use conduit. To future proof power needs to the garage I suggest the subpanel with at least a 50A feed. My suggestion on feeder materials is to run 1" PVC conduit as a minimum or 1.25" is better using four pieces #4 Aluminum XHHW-2. #4 Al is good to 65A and by using #4 you don't need to worry with the correct color wires and can use black and tape the neutral ends as white and the ground ends as green. Wire sizes #6 and smaller need to be in the appropriate colors. So I'm suggesting that the ground also to be a #4. If you use a green for the ground then you can use a #8 Al if you don't go over 60A for the feeding breaker.
 
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Old 09-17-17, 03:40 PM
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Late getting back, but why aluminum? Why not copper?
 
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Old 09-17-17, 05:29 PM
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If you want to pay 3 to 4 times the cost of aluminum by all means use copper. Use copper thwn three #6 and a #10 ground for 50A and 3/4" conduit.
 
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Old 09-18-17, 06:58 AM
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In my previous post I said #6 Cu THHN for 50A. Actually #6 Cu THHN is good for 65A, but with the #10 ground you are limited to 60A.
 
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Old 09-18-17, 11:06 AM
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If it is just money, no biggie. I didn't think aluminum and copper were to be mixed like that.
 
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Old 09-18-17, 11:30 AM
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There is no mixing of copper and aluminum. Use either all copper for the feeder or all aluminum for the feeder. Not sure where you are coming up with mixing of copper and aluminum.
 
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Old 09-18-17, 01:55 PM
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Aluminum feeder to copper outlet lines. Being as how they react different to heat, I didn't think they could mix like that.

Wither everything copper, or everything aluminum.
 
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Old 09-18-17, 03:55 PM
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Aluminum feeder to copper outlet lines.
The aluminum wire goes to the panel lugs and bars. They never touch the copper branch circuit wires. The lugs and bars are rated for both aluminum and copper. It is recommended you use antioxidant where you connect the aluminum wires.
 
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Old 09-18-17, 04:56 PM
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As Ray pointed out the aluminum is for the subpanel feeder. There is no contact with the copper branch circuit wiring. Most likely your current main service panel is fed with aluminum wire.
 
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Old 10-01-17, 11:29 AM
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I'm getting lost and need to figure this out. You guys are great with your advice, but I get conflicting advice from others.

So, I can run #6 if I use appropriate colors? One black, red, white. Then a ground wire...covered or uncovered?

I have to run a second ground at the sub panel to a ground bar, so do I still need to run a ground from the main panel?

Sorry for the repetition, but I bought wire and bought the wrong stuff and had to return. It's to expensive to do wrong and find out after the fact and get failed on inspection.
 
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Old 10-01-17, 11:45 AM
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Here is what I have

My new box and what is feeding it. Yes that is a 30 amp double pole. It's what fed the original garage.
 
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Old 10-01-17, 12:53 PM
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So, I can run #6 if I use appropriate colors? One black, red, white. Then a ground wire...covered or uncovered?
It can be two blacks. Sometimes that is cheaper because you can buy a whole roll at a discount and cut it in half. Neutral must be white (or gray). Usually an insulated ground is used but it can be bare if it doesn't feed a swimming pool.
 
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