Wire Size for Sub-panel Feeder


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Old 11-22-07, 03:24 AM
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Wire Size for Sub-panel Feeder

I'm going to install a sub-panel in my garage so I can run more than one shop tool at a time without overloading the single 20amp circuit that feeds two outlets in my garage as well as the two outdoor outlets, and the outlet next to my main panel. My wife also wants another outdoor circuit for holiday lighting. After adding up the total usage, I determined that I need a 60amp breaker on the main to feed the sub-panel. At the home improvement center they had a chart saying #6 wire is thick enough for a 60amp breaker, so I bought a 100' roll of #6 THHN. When I got home and thought about it a bit more, #6 wire seems a bit small, and Google appears to agree, saying #6 is only good for 30amps at 25'. Unless that's 30amps on each side of a 240v circuit = 60amps total, but that doesn't make sense to me. Then again, I'm not an electrician. I do have a book somewhere on wiring guidelines, but can't seem to locate it right now. So who's right, the store or Google? Thanks in advance
 
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Old 11-22-07, 05:19 AM
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Your wire is good for 60 amps. But you need three, or maybe four wires, for your panel. You need white wire for the neutral. Yiou need green or bare wire for the ground. You need black or red or some color for the hots. You cannot use a single color for all the wires.

It is better to figure out what you need first and then buy everything. There are many, many details here. YTou will make additional mistakes until and unless you understand exactly what you need.

Start at the beginning. Read one or two books on hime wiring, starting with Wiring Simplified. Then give us your plan and we can comment on it.
 
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Old 11-22-07, 08:32 PM
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Can't I use electrical tape to re-identify the wires? I've done this several times in switch loops to re-identify the white wire as hot. My plan was to spiral green, white and red tape down each length of wire, leaving the fourth one black. Or is this not kosher? I've read a few wiring books, including "Wiring Simplified", and I've done my share of wiring too. This will be my first sub-panel, but I wired my entire basement, and passed the rough-in inspection with flying colors. If code does prohibit what I suggested, I'll run down to the store tomorrow and exchange this wire for 4 25' spools in different colors. Otherwise, I have my tape at the ready.

As for the rest of my plan, I bought a Square-D Homeline package that came with a 20 space panel and 5 20amp breakers (I don't need 5 circuits at this time, but the package was cheaper than buying a lone panel and individual breakers). I also grabbed a grounding bar, a section of 1 1/2" PVC conduit, the threaded adapter to connect to the box, and PVC glue. That will take me from the garage wall into the ceiling joists of the basement. From there, the wire makes pretty much a straight run to the main panel. At the main, I have two spaces reserved for the 60amp double pole breaker, with a couple spaces inside on the grounding bar for neutral and ground. The only thing I didn't think of until just now...if I flush mount the panel, I assume I'll have to build some sort of firestop around it. 2x6 bracing with a 5/8" sheetrock liner a few inches above and below the box, with some caulking around the conduit should suffice, right?
 
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Old 11-22-07, 08:59 PM
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The wire must be the proper color for its entire length. Now, as to the legality of wrapping the entire length with tape as a method of reidentifying, I'm not sure about that. I don't see anything in the code that would prevent it, but you should ask your inspector.

Your comparison of this with a switch loop is not applicable. Switch loops have special rules all of their own.

I seem to infer from your post that your garage is attached to the house. Is this true? It makes a difference.

If you are intending to mount your subpanel on a wall shared between the garage and the house, then the fire code will play a large part in your plan. That wall needs to have a specified fire rating, and cutting a hole in the drywall for a panel would greatly affect that. If it is on some other wall, then you probably don't need to worry about the fire code.
 
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Old 11-22-07, 10:41 PM
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Your #6 wire is the magic size where the allowed identification methods change. (Well it actually changes at #4 -- #6 is grouped with the smaller ones.)

For #6 and smaller, the rules for the grounded conductor (neutral) are quite specific: The entire conductor insulation must be solid white or gray finish, or be marked with 3 continuous white stripes along its entire length. A single spiral white tape would not cut it IF the black showed through at all. There are exceptions which don't apply here so I won't list them. And of course the underlying insulation cannot be green if you're using the 3 stripes method.

For #4 and larger you have a bit more leeway. White tape on the termination points alone would suffice.

So no, you cannot simply use tape on your #6 black THHN unless you create a white "continuous finish". Or cut it into reeeeeeeeealy thin, long pieces to form straight stripes. If either of those would even pass in your juristiction. Seems dicey.

The equipment ground has similar rules.

As for the hots, in your case no identification is required whatsoever for differentiating the two. If you want to place red tape on the termination points on one conductor as you said, so be it. But no need to tape the whole thing, or at all.

I'd keep the black spool and buy additional smaller lengths of white and green. Or return the #6 and get #4, in which case you can get away with murder.
 

Last edited by core; 11-22-07 at 11:11 PM.
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Old 11-23-07, 07:24 AM
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I'll go to town today after the black Friday traffic dies down and get some green and white #6 wire. I'll still put red tape on the second hot line, just so I can tell them apart in the future.

As for the panel itself, the garage is attached, and I was going to use a shared wall. I know I'll have to check with the inspector to find out exactly what's needed to meet fire code, but can you give me an idea of what's generally required? Maybe I'd be better off surface mounting the panel.
 
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Old 11-23-07, 11:30 PM
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You made no mention of a raceway for the THHNs.
If you are not planning on a conduit from panel to panel, you can't use THHN.
Better to buy a piece of 6/3 romex and use that for your feeder.
 
 

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