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Old 01-27-2010, 10:06 PM   #1
 
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subpanel feeder question


Hi Everyone,

I have a quick question about how to wire my subpanel. My main panel is on the east side of my house. I have a shop about 25ft off the west side (house is about 35ft long). In the shop is a 100amp panel. I do no have a basement, just a crawl space under the house.

My question is what cable should I use and how should I run it. My first thoughts were conduit and THHN but now I'm thinking URD.

Would this comply with code:

From main panel in the house run URD from panel, through wall into crawl space. Hang URD directly from floor joists in crawl space to west side of the house. From there, directly burry cable to shop and come out of the ground below the panel, entering a length of conduit straight up through the floot into the bottom of the panel.

Would this be alright? The only possible concern I have is whether or not the aluminum cable will fit under the 100A breaker in the main panel. If not, I might not have a choice but to go with copper.

Thoughts?

Thanks everyone!
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Old 01-28-2010, 06:45 AM   #2
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Al 100 amp wire (#2/0) will fit fine. You cannot run exposed URD, it can be direct buried but not just stapled under the floor joists. Conduit and THHN will be allowed there, but when buried you need to use PVC
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Old 01-28-2010, 07:33 AM   #3
 
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Hi Bob,

Thanks for the reply. From what your saying, it sounds like I'd be better off with conduit and THHN the entire way. With Cu I can run a smaller guage on the hots and a much smaller guage on the ground. With the URD I'll need a team of horses to pull it through multiple bends of conduit.

I was looking for an easier solution but it doesn't look like there is one. Conduit and THHN it is.

I'm thinking #1 Cu for the hots & neutral + #8 Cu for the ground.

Last question, conduit fill calculators are reporting a 1 1/4" conduit is required. My experience with smaller size conduit is you cant even get close to fill capacity before you start running into major jamming.

What size conduit would you recommend for this cabling? Total conduit length will be apx. 70 ft with 4 90* bends. The #2 bend will be where it leaves the side of the crawl space and an LB could (and will likely have to) be used. Four bends is ALOT isn't it? thats a fill 360*.

Scott
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Old 01-28-2010, 07:59 AM   #4
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360 is maximum allowed. But you may need some offsets which would make this an illegal installation. Normally you would use aluminum wire in 2" pvc. With two 90 below grade and two LB to enter each building. And at each building use an entrance sleeve which will allow the movement required when you experience frost heaves.
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Old 01-28-2010, 09:29 AM   #5
 
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I didn't consider an LB at the main and sub panels... that adds another two 90*'s. I was thinking of coming straight down out of the box through the wall but I suppose going out out the back of the panel through the exterior wall into an LB would be an easier way to do it.

I'd have an LB at the main panel, a 90* to turn it under the crawl space... an LB at the other end of the house where the conduit pops out, a 90 once it's below grade, a 90 to come up from grade and an LB to go into the rear of the sub panel.

Any suggestions on how I can do this? not only is this install illegal, it's impossible. What do electrician do in situations like this? What are my other options? Is there any cable I can run exposed in the crawl space? service line or something? Then I could convert to THHN or URD and enter conduit where it will be going below grade.
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