Subpanel Feeder question


Old 02-12-13, 08:53 AM
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Subpanel Feeder question

I am wiring a subpanel in my garage, about 20' from the main panel. I'd like to run wire from the main panel to the subpanel through the attic without using conduit, and I'd like to use a 100A breaker. (Even though the intellectual part of me knows that 100A is overkill.)

What are my wiring options?

I was initially going to go with three runs of THHN plus ground in conduit, but after looking at the problem, conduit is going to be too problematic and I'd rather run cable through the attic.

I think I can use 2/3 (w/ground) NM copper (Romex).

I think I can use 2/3 (w/ground) SER copper.

Both of these are $7/foot or more.

I think I can run 2-2-2-4 AL SER through the attic without conduit, but would be limited to 90A.

Am I thinking right in these three cases?

Are there other practical wiring options that would get me 100A (or thereabouts) at a reasonable cost?
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Old 02-12-13, 01:07 PM
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The 2-3/G is the cable to use. It should be run inside finished walls or attached to the framing if the walls are open. Or it can be sleeved in a conduit stub on the face of a finished wall.

Why do you want a 100A subpanel in your garage? What are the loads you're planning to supply from it?
Old 02-12-13, 04:54 PM
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There is no rational need for 100A. As I said, it's overkill. I pulled that number out of thin air and have been just going with it.

I'll be using garden variety hand tools in the shop, a 20A air compressor, occasionally a stick welder, a computer with 2-3 monitors, a laser cutter, a largescreen LCD television... I'll have a mini-split air conditioner in there. Eventually I hope to build a CNC router, and I hope to one day add a plasma cutter. But of course these won't all be on at the same time.

100 amps is definitely overkill.

What ampacity would the 2-2-2-4 aluminum Service Entrance cable get me (assuming that this is an acceptable use for that type of cable)? The 2/3 copper is costly stuff.
Old 02-12-13, 06:08 PM
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For the same ampacity.....aluminum must be one size bigger than copper.

2/2/2/4 SER cable is good for 90 amps. (That's what I fuse it at)
Although according to the 2008 code it may only be good for 75 amps.
Depends whose interpretation of the code you go by.

The copper would be a full 100 amps.
Old 02-12-13, 06:31 PM
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Since you just pulled 100 amps out of the thin air, why not use a 90 amp 2 pole breaker in your main panel and feed the subpanel with the 2-2-2-4 SER cable you mentioned already. You'll never exceed the 90 amps with the equipment you listed.
Old 02-18-13, 02:45 PM
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Thanks, all! I pulled 2-2-2-4 aluminum service entrance cable and the AHJ green-tagged my work and OK'd a 90 amp breaker.

Now on to the insulation and hanging drywall...
Old 02-18-13, 03:14 PM
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Very good. Thanks for letting us know the outcome.
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