Transition from NM to THWN with pvc conduit

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Old 03-27-10, 06:07 AM
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Transition from NM to THWN with pvc conduit

This forum has been really helpful so far! I will run four #10 THWN wires in pvc conduit underground to a disconnect switch in my detached garage. This will be a multiwire branch circuit on a 20-amp double-pole GFCI breaker at the main panel. I plan on using 10/3 NM cable from the main panel to the exterior wall of the unfinished basement and then transition from there to the THWN in conduit.

Do I make this transition in a j-box right inside the basement before exiting? If so, does the pvc conduit need to attach to the box? Or, do I make the splices in an outdoor weatherproof box on the exterior of the wall or in an ell fitting on the conduit? (My understanding is that I can use 1/2" pvc conduit for my four #10 THWN conductors.)

Thanks!
 
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Old 03-27-10, 07:13 AM
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Yes, you will need a box to make the transition. I find putting the j-box on the outside of the house to be better for two reasons:

1) With the box on the outside it will not interfere with any thing on the inside of the house if you ever decide to finish the basement.

2) It will be easier to pull in your wires because you can run the pipe to the garage out of the bottom of the box. Also feeding the Romex into the back of the box will be easier than trying to pipe to a box in the inside of the box.
 
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Old 03-27-10, 08:03 AM
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(My understanding is that I can use 1/2" pvc conduit for my four #10 THWN conductors.)
Yes, you can, but I wouldn't. I think you'll find it is much easier to pull wire through 3/4" conduit unless it's a very short distance. As a rule-of-thumb, I would never put 1/2" PVC conduit in the ground, with a few exceptions, and usually consider 3/4" PVC a minimum size for a feeder.
 
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Old 03-27-10, 10:55 AM
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Thanks for your replies! Okay, I'll go with 3/4" conduit.

What's the best way to run it under a brick path? If I hit it with a sledgehammer, will it crack?
 
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Old 03-27-10, 11:18 AM
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Probably be best to remove enough bricks for the excavation and replace them later. If they are on top of a solid concrete sidewalk and it isn't too wide, you could probably dig under from both sides (as far as possible) and then drive a steel 1" pipe under the dirt which remains under the walk to open a hole in the dirt and then slide the conduit through the opening. If you wallow the steel pipe around, you'll likely open a hole larger than the 1" pipe.
 
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Old 03-27-10, 11:33 AM
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If your problem is routing the PVC conduit under brickwork without disturbing the brickwork , then try smashing a 1-1/4" steel pipe sleeve thru the soil under the B-W.

I base this suggestion on your mention of using a sledgehammer .
 
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Old 03-27-10, 11:44 AM
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Using a pipe attached to a hose you can tunnel under a sidewalk fairly easily but it is quite messy.
 
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Old 03-27-10, 12:52 PM
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Lots of ways to do it, use old long woodworking bit to bore hole even, drive a pipe sleeve and clear the dirt out, run conduit thru, compressed air or water, as was said, drive a large rod or pipe, pull it and run conduit, good place to use some imagination.
 
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Old 03-27-10, 04:03 PM
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I can hire out my pup to you.. She'd be more than happy to dig however deep you need!

I had to tunnel under my walkway to run power and water to the garden. A simple power-jet style hose nozzle attached to a 3-4' length of PVC works very well. But as Tolyn said, it is messy, and not something to do on a cold day.
 
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Old 03-27-10, 04:55 PM
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Today was cold in NJ, wasn't it, JerseyMatt? We also have a pup who loves to dig, so I think we're covered!

Thanks a lot for all of the good solutions here, everyone!
 
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Old 04-03-10, 07:24 AM
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What size external j-box do I need if I'm joining 10/3 NM and #10 THWN (red, black, white, green)? I'm trying to calculate box fill but can't find all of the appropriate info.

Does the threaded pvc adapter screwed into the box count in the fill calculation? Do I need something in the back of the box like a clamp where the NM comes in and does that count?

Thanks for your help!
 
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Old 04-03-10, 07:31 AM
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A 1 gang or 2 gang Bell box would be acceptable and yes, you need a properly sized romex connector to enter the back of the box. Another way to do it would be to use an inexpensive A-C pullout disconnect.
 
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