NMB inside conduit suggestions

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Old 04-03-18, 09:47 AM
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NMB inside conduit suggestions

i know that NMB isnt meant for wet locations but please offer some suggestions in my case

- I will have a switch in my garage which I will have controlling an outdoor light.

- Conduit will exit the exterior wall and go up for about 2’ where the light is mounted. Light will be 10’ above the ground and conduit runs 2’ under it perpendicular.

The problem is, there is no place for me to transition from NMB to THWN unless I introduce a junction box outside and that will look really ugly. I just want to use a 90 degree pvc elbow directly. Whats the worst that can happen if I just ran NMB in that conduit? An individual THWN conductor looks a lot like the conductor inside NMB. I dont get whats the real world explanation of why this isnt allowed?

or I could run UFB, but that is so hard to work with
 
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Old 04-03-18, 10:12 AM
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Transition on the inside of the wall .
 
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Old 04-03-18, 10:23 AM
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How? Its a finished garage. There is only light switch box there. Are you saying I run 3xTHWN conductors directly into the light switch via the standard NMB knockout? Is that allowed?
 
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Old 04-03-18, 10:59 AM
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Individual conductors need to be in a complete conduit system between boxes. You may need to patch drywall .
 
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Old 04-03-18, 11:40 AM
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NEC wise run a piece of UF-b from switch box to light. The thing is in your case that short run in conduit using NM-b I wouldn't loose sleep over using NM-b as a home owner.
 
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Old 04-03-18, 11:44 AM
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Maybe I should just directly run UFB in that 2’ of conduit from the light switch.

I m guessing its not a problem to run UFB from the light switch inside to outside through the conduit?
 
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Old 04-03-18, 11:49 AM
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Pattnp sorry did not see your response, thanks!
 
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Old 04-03-18, 11:58 AM
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I'd probably use UF. It's a little harder to strip than NM, but not really that big of deal.
 
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Old 04-03-18, 04:50 PM
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there is no place for me to transition from NMB to THWN unless I introduce a junction box outside and that will look really ugly. I just want to use a 90 degree pvc elbow directly. Whats the worst that can happen if I just ran NMB in that conduit?
Why would it be ugly? I don't think it will any worse than a PVC conduit exiting the wall.
If you want the conduit flush against the wall, you cannot use 90 elbow. Turning radius of the elbow will cause your conduit to sit 2~3" further out from the wall. You will have to use a weather proof junction box or a pull elbow or LB pull elbow.

You can use UF-b as others suggested as well. You can even bury this without conduit if you bury it 2ft deep, but I'd still use conduit for better protection.
 
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Old 04-03-18, 06:25 PM
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Thats a fair point about the turn radius. The only issue I have is the more I read the more I realize the problem with NMB isnt just it cannot handle physical abuse (which is one of the problems with it outside its just that not the problem I was concerned about), but the paper cover inside when get wet will keep the water like sponge causing other issues. Which means, I probably wouldnt even want it on that LB junction box because technically even that can get moisture inside. I think the only safe way is to directly run UFB from inside.

Whether its 6” of length or 2’ or 10’, I think as long as a stripped NMB cable is anywhere that can get wet, its just not a good idea. So I think the only option is UFB.

now I need to figure out how hard it is to strip that thing
 
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Old 04-03-18, 07:28 PM
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As long as the junction box does not get filled with water, NMb is ok up to surface of the outside wall. The conduit outside will eventually have some water pooled inside at the low spot especially if it runs underground. THWN's insulation is waterproof and oil resistant where as NMb may or may not be.
As far as I know most manufacturer's current model NM-b actually contains the same THHN/THWN, but cannot be guaranteed.


UF-b is much harder to strip compared to NM-b, but not too bad. But, it may be harder to pull compared to THWN if it is a long run.
 
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Old 04-03-18, 07:37 PM
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Thats the part I dont understand. How can it be okay for NMB to be in exterior junctionbox but not in conduit? In my case conduit runs UP so the junctionbox is actually lower than the conduit itself. And I know NMB’s conductors look identical to THWN.

i m not disagreeing that NMB is not suitable for wet. I just dontnunderstand how its okay in the box. As far as i m concerned that box is just as exposed if not worse than the conduit (because whatever water accummulates in the pipe will pool into that box).
 
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Old 04-03-18, 08:18 PM
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It is ok for junction box on exterior wall because that still is considered inside of the building.
Normally junction box on the exterior wall should not get pooled with water. If it does, it is not installed right. In your case, wire in the conduit may be not be exposed to the conditions worse than in the junction box, but that is always the case.

Drilling a small hole at the bottom of weatherproof junction box helps draining any water that may enter the junction box.

Most, if not all, modern NM-b has THHN/THWN inside, but that was not the case always. Older NM-b had different insulation from modern ones. It probably was cheaper for the manufacture to use THHN inside NM-b.
 
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