What wire for 100amp subpanel?

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  #1  
Old 04-06-11, 07:21 PM
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What wire for 100amp subpanel?

I'm putting a 100amp subpanel in the next stud bay over from the existing main panel. The cable run should be <4 feet between the two. What gauge wire do I need to use?

I've asked at both Home Depot and Lowe's and been frightened by the answers I received. A guy at Lowe's actually told me to use 10 gauge! I pretty much left the store at that point. I know that they can't always have truly qualified people there all the time, but they need to teach them to say "I don't know." I may not know what the right wire is but I know enough to know that 10 gauge would be dangerous.

At Lowe's they have some 2-2-2-4 SEL. Would that be sufficient?

Thanks much.
 
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Old 04-06-11, 08:09 PM
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If you can run conduit between them I would suggest three #3 THHN copper (1 white, 2 blacks) and one #8 green in 1-1/4" Conduit.
 
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Old 04-07-11, 09:19 AM
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Originally Posted by dukescotts View Post
but they need to teach them to say "I don't know."
That should absolutely be their policy, or a simple "we can't give advice because aren't licensed professionals". The fact that it is a business relationship between the customer and the store makes me feel that they are skirting on the edge of practicing illegally.

At Lowe's they have some 2-2-2-4 SEL. Would that be sufficient?
If copper, yes. If aluminum, no. For a cable you need at least #2 copper or #1/0 aluminum. For individual conductors in conduit you need at least #3 copper or #1 aluminum.
 
  #4  
Old 04-07-11, 10:00 AM
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2-2-2-4 SER aluminum is rated for "residential dwellings" for 100 amp service use, so Lowes and even the company I work for as well as local competitors sell that cable. However, I chose to run # 2 copper on mine. Of course that was well before copper went to $4.30 a lb!
Local codes may differ of course.
 
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Old 04-07-11, 10:10 AM
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Originally Posted by JimElectric View Post
2-2-2-4 SER aluminum is rated for "residential dwellings" for 100 amp service
But only when it's the main panel for the dwelling, and the service is entirely or mostly outdoors. For subpanels and other uses the 310.16 table must be used which provides for either 75A or 90A depending on specifics of the installation.
 
  #6  
Old 04-07-11, 12:00 PM
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I will check further into that. I know though maybe out of old habits we have sold 2-2-2-4 for decades for sub feeds and never had anyone turn it down. Of course our state is far behind the curve on some things anyway as far as codes go. Usually 1-2 editions back..... Perhaps I can switch to number 1-1-1-3 and drop the 2 guage.
 
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Old 04-07-11, 12:42 PM
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Yeah Jim in the older codes it was very common to use #2 AL or #4 CU for a 100A subpanel, but starting at 2005 NEC and moreso in 2008 NEC they clarified the language to make it specific that table 310.15(B)(6) which sets the ampacity for services could only be used for the dwelling's main service, and anywhere else that aluminum cable was used it had to be restricted to table 310.16 ampacities and temp ratings.

The #2 is still a useful size though, especially for something like feeding mobile homes (USE/MHF) or apartment units (SER). I think it's still a good size for subs too -- nobody really notices the difference between 80A, 90A or 100A subpanels. The round number 100 just sticks in their head even though they don't need anywhere near that big. The #1/0 cable is a bear to work with compared to #2 for the extra 20A that nobody will ever use.
 
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Old 04-07-11, 07:47 PM
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Thanks for the help everyone. So, it sounds like I need #1/0 if I'm going to use aluminum or #2 for copper. I suppose I could do conduit and the #3 THHN but I'm not sure I feel like dealing with the conduit.

Originally Posted by ibpooks View Post
nobody really notices the difference between 80A, 90A or 100A subpanels. The round number 100 just sticks in their head even though they don't need anywhere near that big. The #1/0 cable is a bear to work with compared to #2 for the extra 20A that nobody will ever use.
OK, so that begs the question as to whether I really need 100A. The reason I'm adding the subpanel is that I'm finishing the basement. The basement will be about 1800 sq. ft finished. However, it's going to have a few unusual power needs. First, I work for a company that makes home automation equipment so I've got a fair amount of nice home theater equipment that will go in a rack in the basement (I love my industry discounts). Also, my wife is a photographer, so we're setting up a photo studio down there with the requisite needs for studio lighting. There will be a bar area with a mini fridge and/or wine fridge. Finally, I plan on stubbing out some wire from the subpanel to the backyard to handle any future landscape needs like lighting or a waterfall. Other than that, it's just a bedroom and a rec room area.

So, what are your thoughts? Do I need 100A? Can I get by with 80 or 90? (Not sure I've seen a 90 amp breaker.)

Thanks again.
 
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Old 04-07-11, 08:30 PM
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but I'm not sure I feel like dealing with the conduit
PVC conduit is easy to work with.

You wrote:
I'm putting a 100amp subpanel in the next stud bay over from the existing main panel. The cable run should be <4 feet between the two.
Four feet is almost side by side. Just be sure when you mount the sub a hole in the side of it lines up with a hole in the side of the main panel. A straight run with a threaded adapter glued on each end.
 
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Old 04-07-11, 09:27 PM
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Originally Posted by ray2047 View Post
Four feet is almost side by side. Just be sure when you mount the sub a hole in the side of it lines up with a hole in the side of the main panel. A straight run with a threaded adapter glued on each end.
It's exactly side by side. Sorry if I wasn't clear on that. The main panel is flush mounted in one stud bay and I'm putting the subpanel directly next to it in the next stud bay. So, it's only 1.5 inches from the right side of the main to the left side of the sub. If I were to do conduit, I'd need to run it down from the bottom, make a 90 degree right, through the stud, up another 90, and then into the bottom of the sub. Definitely possible, though.
 
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Old 04-07-11, 11:19 PM
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Why do you need to go down-over-up with the conduit? This is how I did my sub-panel. One-inch PVC nipple, three #4 and one #6 type THHN conductors. 125 ampere sub-panel with an 80 ampere feeder circuit breaker. I could have gone to a 100 ampere circuit breaker if I had needed.

 
  #12  
Old 04-08-11, 06:00 AM
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I like the idea of that. The subpanel I bought occupies the full stud width space, so I guess I'd just need to line up the holes and go directly from one to the other through the stud. I'm assuming I'd still need a 1.5" length piece of conduit to go through the stud.
 
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Old 04-08-11, 07:14 AM
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I'm assuming I'd still need a 1.5" length piece of conduit to go through the stud.
No. Just use a threaded conduit nipple. You would need a nut on the inside agains the box and a plastic bushing on the end to protect the wire.

 
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Old 04-08-11, 07:24 AM
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Makes sense. Thanks much.

Here's a stupid question, though. With the main panel already installed between the studs, what's the best way to get to the knockout? Is it possible to "pull" it out from the inside? I've never tried that. I suppose I could just drill from the other side of the stud until I hit it.

For 3 #3 and 1 #8, what size conduit nipple would you recommend?
 
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Old 04-08-11, 07:44 AM
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1-1/4" nipple.
I could just drill from the other side of the stud until I hit it.
Assuming Main panel is against the stud: Use a deep hole saw so you can remove the entire wood plug. TURN OFF main breaker.Choose a spot where there is NOT a knock out. Remove the wood plug and continue deilling through the box. Use a brand new bi-metal hole saw. Dull ones are a beatch to use.

If the main panel is not against the stud line the hole in the stud with a knock out and going through the hole loosen the knockout with a hammer and screwdriver.

Pros may have better ideas.
 
  #16  
Old 04-08-11, 08:21 AM
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If you can please send us a picture of the panel and the area. Then we can see what you have to deal with.

Thanks!
Jim

 
  #17  
Old 04-08-11, 08:40 AM
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Choose a spot where there is NOT a knock out. Remove the wood plug and continue deilling through the box.
Interesting. So you're saying the nipple should NOT go through a knockout but that I should just drill the appropriate sized hole in the main panel. Hadn't thought of that. I'll have to see if there's any space where there isn't a knockout.

If you can please send us a picture of the panel and the area. Then we can see what you have to deal with.
I'll do that tonight when I get home.
 
  #18  
Old 04-08-11, 09:13 AM
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Yes that is one option. If you don't want to bother going directly into the side of the panel, you can use factory KOs on the bottom or top of the panels and make a "U" out of conduit using some prefab elbows and nipples. Or you could use a flexible conduit like FMC, ENT, Liquidtight to make a "U". Lots of options to do it right.
 
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