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Old 05-25-2009, 01:16 AM   #1
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Transition from NM to THWN


I am running several new outdoor circuits using PVC conduit and PVC J-boxes. The circuit will involve transitions from interior house wiring NM to THWN and back to underground NM.

When NM enters PVC conduit or a PVC box, is cable clamping necessary? If so, why?, and what type of clamp do I use, I can't find any clamps designed for use with PVC conduit or PVC boxes.

One person told me clamps are not needed where NM enters boxes and just to use silicone where underground NM enters conduit to keep moisture bugs and dirt out. Another person told me clamps are required and to use metal conduit clamps screwed into PVC threaded adapters. Someone else told me you can't use any metal fittings on a PVC system because there is no mechanism for grounding.

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Old 05-25-2009, 01:23 AM   #2
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Transition from NM to THWN


Connectors are required by code.

Why? In some cases, so the gremlins don't pull the cable out.

Drill the back of the box to accept the connector. Use the plastic black button type.

If you are using PVC, stay with PVC fittings.

Edit:

Wait...you are talking about connectors for the underground NM (which is called UF)?

You don't need connectors there.

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Last edited by 220/221; 05-25-2009 at 01:27 AM.
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Old 05-25-2009, 02:25 AM   #3
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Transition from NM to THWN


I have a wierd situation. I have four runs of NM exiting a 1 1/2" hole in an int/ext wall and entering a 1 1/2" hole in the back of a 6x6 PVC J-box surface mounted to the siding. Any suggestions on how to secure the romex where it comes into the box? Inside this box it transitions to THWN and exits through four separate conduits.
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Old 05-25-2009, 03:04 AM   #4
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Transition from NM to THWN


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Originally Posted by Miranda7 View Post
I have a wierd situation. I have four runs of NM exiting a 1 1/2" hole in an int/ext wall and entering a 1 1/2" hole in the back of a 6x6 PVC J-box surface mounted to the siding. Any suggestions on how to secure the romex where it comes into the box? Inside this box it transitions to THWN and exits through four separate conduits.
Well since the job is already done, why are you asking? Seems a little backwards... I guess you could use the idea for device boxes. Those don't need a cable clamp as long as the cable is strapped within 8" of the box. Or you could start over and drill two 7/8" holes for 1/2" plastic "button" connectors like 220 said. Put two cables in each hole (for 12-2, that is).
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Old 05-25-2009, 09:47 AM   #5
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Transition from NM to THWN


Why are you transitiong from nmb to thwn to uf cable?
Take the nmb into the jbox and connect it to the uf and then to where you want the wire to go.
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Old 05-25-2009, 10:07 AM   #6
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Well since the job is already done, why are you asking? Seems a little backwards... I guess you could use the idea for device boxes. Those don't need a cable clamp as long as the cable is strapped within 8" of the box. Or you could start over and drill two 7/8" holes for 1/2" plastic "button" connectors like 220 said. Put two cables in each hole (for 12-2, that is).
I did ask before I started (elsewhere) and I think I got some bad advice which led to this crazy configuration. I am trying to make sure everything is to code before calling for an inspection.

The device box idea sounds interesting, can you explain further what would constitute "strapping" and where? The only other solution I can think of is to borrow some saddle type clamps (the ones that come inside J-boxes that look like this ^-^ with a screw in the center to attach to the box) and screw them into the plastic next to the big hole in the box where the cables come in. OR I could get a a round plastic ceiling fixture box , stick it in the middle of the big box and run the cables through the tab clamps in the round box. Seems kinda silly but it would keep the cables in the box.

What is the usual way of securing NM in these slick PVC boxes?
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Old 05-25-2009, 10:12 AM   #7
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Transition from NM to THWN


I wouldn't worry about the clamps for the NM, but I am curious about the NM-THWN-UF transitions. Why not NM-UF or NM-THWN in conduit?
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Old 05-25-2009, 10:19 AM   #8
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Why are you transitiong from nmb to thwn to uf cable?
Take the nmb into the jbox and connect it to the uf and then to where you want the wire to go.
It is a long story, but even if I did what you are suggesting, the question remains. HOW do I secure the NM or UF cables where they enter this box? This is a plain grey PVC 6x6 box. The 4 line cables come into this box through a 1.5" hole in the back and load wires exit to go underground through individual 1/2" PVC conduits. The box is all slick, no place for clamps.

There has got to be a right way to do this...
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Old 05-25-2009, 10:42 AM   #9
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I wouldn't worry about the clamps for the NM, but I am curious about the NM-THWN-UF transitions. Why not NM-UF or NM-THWN in conduit?
I probably will stick with one or the other, but I wanted to understand all the options. You say you wouldn't worry, but do you think it would pass inspection without clamps?

Last edited by Miranda7; 05-25-2009 at 10:45 AM.
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Old 05-25-2009, 11:21 AM   #10
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Transition from NM to THWN


The back of the box is pretty well protected. NM goes into the back of things like air conditioner disconnects all the time with no clamps. I think you'll be fine.
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Old 05-25-2009, 11:58 AM   #11
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Originally Posted by Miranda7 View Post
The device box idea sounds interesting, can you explain further what would constitute "strapping" and where?
A staple driven into a stud securing the wires within 8" of where it enters the box. I'm not sure if this is exactly compliant, but if you're not going to change the box, it's still better than not securing them.
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Old 05-25-2009, 12:04 PM   #12
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Transition from NM to THWN


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I have four runs of NM exiting a 1 1/2" hole in an int/ext wall and entering a 1 1/2" hole in the back of a 6x6 PVC J-box surface mounted to the siding.
Code requires a connector but you are not going to find a 1 1/2" romex connector. It's a huge hole, in PVC, so it aint gonna damage the cables, but an inspection MAY catch it (not in my area)

If you want to stay legal:

1.Remove the box, drill a couple 1/2" holes and use romex connectors. You have to seal the old hole also.

2. Get a 1" two screw romex connector and two 1 1/2" x 1" reducing washers and use the existing hole.
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Old 05-25-2009, 01:12 PM   #13
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Transition from NM to THWN


Everyone here has been very helpful and patient, and I truly apreciate the time you have taken to help me. I confess that I left out another detail which complicates these solutions (I apologize for holding back but I brought this issue up on another forum and this detail became a point of confusion and contention to the point that I gave up trying to explain it, so I left it out here to avoid the same) Try to picture this:

The 6x6 PVC box is surface mounted to lap siding with a Z profile, leaving a 3/4" gap between the hole in the back of the box and the hole in the side of the house, because of the siding profile.

box hole>|Z|<hole in house

So, to bridge the gap between the box and the house, I used a short piece of conduit, I glued a PVC box bushing into the hole (sticking out of the box), and glued the bell of 2" long piece of conduit over the bushing. This 2" sleeve closes up the gap between the box and the siding, fits snugly into the siding hole and protrudes about 1" into the wall cavity, sealing off all gaps.

Therefore the hole in the back of the box isn't just a simple hole, but has a short sleeve of conduit attached to it, sort of like a giant homemade LB condulet.

That is why I am focused on solutions that secure it within the box itself and not by using holes in the box.
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Old 05-25-2009, 01:32 PM   #14
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Transition from NM to THWN


That makes sense. I know what you mean about the siding. Just staple the wires close to where they enter the conduit nipple. Seal the pipe on both ends with some duct seal or foam. Call it a day.
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Old 05-25-2009, 02:19 PM   #15
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Transition from NM to THWN


I'd be very surprised if an inspector had a problem with this installation. It's how I would have done it.

Just don't say anything during the inspection.

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