Yet another subpanel wiring question

Chris_M · 06-10-2010, 04:04 PM

Hello folks.

What a great site! Please be kind to me as I have done a search but sure would appreciate someone knowledgeable to confirm what I think I've already figured out.

I'm installing a subpanel in an attached garage. I've looked at the planned loads and am quite certain I will not pull more than 40A at any time.

Main service is 200A, and will require about 100' of wire to feed the subpanel.

I already have some components that I would like to use--a used, 10 year old Square D panel with a 100A main, and a 110' coil of #6 AL SER.

The feed wire will travel through the attic on a wood raceway attached to the framing, then back down the interior wall.

My concern is the wire run length. I calculate a 4.35% voltage drop for this wire run length. At less than 5%, will I be OK?

First question is whether I should protect the sub feed with a 50A or a 60A breaker in the main panel.

Second question is whether I must replace the 100A main in the subpanel with a lesser-amperage breaker. My understanding is that a subpanel doesn't even require a main disconnect, so in my mind it should not matter whether the main disconnect is 100A.

And yes, I know that the neutral must be isolated in the subpanel, and about the antioxidant stuff for the lugs.

Thanks in advance!

Chris

WaldenL · 06-10-2010, 04:31 PM

I'll leave the AL wire and run length to someone else, but as for the breaker and disconnect. Yes, you'd add a breaker in the main box appropriate to the size of the cable (and sub panel) you're running, no harm undersizing it. A 50 or 60 would be fine. The "main" breaker in the subpanel then just becomes a disconnect and not an OC device as the upstream breaker would trip first. You could in theory pull it and put in lugs, but it's nice to have a local disconnect, so I'd leave it in place.

brric · 06-10-2010, 04:41 PM

#6 Al SER is only rated for 40 amps.

Chris_M · 06-10-2010, 05:26 PM

Where are you getting the 40A number from?
The wire is manufactured by Southwire...#6, 3 conductor with ground, and rated at 90 C. The NEC 310.16 table rates aluminum SE type cable of this temp rating at 60A. But there are two correction factors. One is heat and one is run length. This will cross through an attic where it could easily get into the 130s in the summer. The ambient temp adjustment factor for that temp is .76, which gives 45.6A. Are you factoring in something for the run length to come up with the 40A number?

brric · 06-10-2010, 05:45 PM

2008 NEC 338.10(B)(4) states SER must be installed the same as NM cable when used for interior wiring. 334.80 limits NM cable, and therefore SER as well to 60 deg. C.

secutanudu · 06-10-2010, 05:52 PM

The applicable section:

Chris_M · 06-10-2010, 06:12 PM

Well crap. I would have liked to have had a 50A capacity but 40 will do.

Thanks guys!