The tabs take the place of a regular cable clamp.

How did you transition the NM into the conduit?
There should be a clamp right there.

Why did you transfer the NM to conduit?
Run it directly into the box with a cable clamp on the box.
Inside the wall is not considered a wet location.

To install cable and clamps blind without access to the back of a box, you can attach the clamp to the cable, remove the lock ring, feed the cable and clamp into the box, then attach the lock ring.

brickeye,
I am transitioning the romex into conduit to change to THWN because the circuits are going underground outside from there in conduit.

I'm not sure I understand which clamps and lock rings you are referring to, so I will clarify. This is a 6x6x4 grey weathertight PVC box. It has no openings, clamps, etc. I had to drill holes in the box and insert 1/2" PVC threaded conduit fittings for the THWN circuits to exit the box, and I drilled a hole in the back and used a 1 1/2 slip fitting and a short piece of conduit to route the romex from the interior/exterior wall into the box.

"...used a 1 1/2 slip fitting and a short piece of conduit to route the romex from the interior/exterior wall into the box."

You could have just punched the correct hole for the romex cable clamp into the back of the box.
No need fort the conduit.

Now you are left with trying to attach an NM clamp to the end of the 1/2 inch PVC.
The fittings are liable to be found at an electric supply house, but not the big box stores.

I needed the short piece of conduit to keep the back of the box weathertight, as the box is surface mounted to vinyl lap siding. Without the conduit there would be a gaping hole in the back of the box, and in the side of the house. The short (2") piece of conduit fits snugly into the hole, protruding into the wall about an inch. This allows me to caulk around it. I suppose I could have cut away the siding and mounted the box flush against the house, but that would have left a lot more potential openings to caulk and seal around the box.

I guess the gist of my question is whether clamps are even necessary in PVC conduit boxes. Isn't the purpose of the clamp to prevent cutting of the cable in the opening?

Neither the slip nor the threaded fittings made to connect PVC conduit and boxes together include clamps, indeed there seem to be no clamps made for PVC boxes at all. Are you suggesting that I not use the PVC fittings, but instead screw metal fittings with NM clamps into my PVC boxes? Wouldn't that create a potential grounding problem for the fittings?

I've got an answer to Q#1 and Q#3, but the jury is still out on #2, so I will reword it more simply:

When using PVC conduit and PVC conduit boxes, is wire clamping required? If so how is this best accomplished (since there seem to be no PVC fittings designed for this purpose)
Is clamping required for THWN as well as NM? Or just NM? Or neither?

You do not need the conduit on the back.

Any box on the exterior of a house is going to get water in it, if only from condensation.

Wires in a conduit system are not required to be clamped.

You do NOT have a conduit system, but are changing wiring methods between junction boxes.

The NM requires a cable clamp where it enters the end of the conduit.

I wish I could post a picture, because you are not visualizing what I have here. The back of the box is flat/plumb. The siding (lap siding, Z profile) is not. There is NO J-box inside the wall. There is a hole in the siding/wall (1/12" round) through which the wires pass, and a corresponding hole in the back of the 6x6 box. Because of the siding profile, this leaves a gap of 3/4" between the back of the box and the hole in the wall. If I do not use the short piece of conduit on the back of the box, any water that runs down the side of the house will run directly into the gaping round hole in the wall and fill the stud cavity, not to mention the hole in the box. The wires would be completely exposed for 3/4" as they pass out of the hole in the wall and into the hole in the box.

So...I definately need the conduit, or SOMETHING to bridge that small gap. The conduit just seemed the easiest thing, didn't know it would change all the rules.

So are you saying that if NM exits a wall and enters a PVC j-box through a hole it does not need clamping, but if it enters through a 2" piece of conduit it does? If it needs clamping even if it enters through a hole, how do you properly accomplish this since the PVC J box does not have any accomodation for clamps? I have seen no cable clamps whatsoever that are designed to be used with PVC conduit or PVC J-boxes.

"So...I definately need the conduit, or SOMETHING to bridge that small gap."

Why do you think that?
Is soemthing going to get into the small gap and damage the NM?

"So are you saying that if NM exits a wall and enters a PVC j-box through a hole it does not need clamping, but if it enters through a 2" piece of conduit it does?"

That is not what was said.
If NM enters a box OR a conduit section it needs a clamp.
You have created a problem by using a very large conduit that you will have a hard tome finding a clamp for s single small NM.

A piece of 1/2 inch conduit would have been more than adequate, though still not required.

I very much appreciate the time you have taken to reply to my posts, and I am sorry I can't seem to explain the situation adequately so that you can understand. I will try again.

Imagine the PVC box is 3/4" from the wall. There is a hole in the wall and a corresponding hole in the back of the box. Four circuits passing through this opening, not one NM. Without conduit the wires would be completely visible and exposed to weather for 3/4" between the house and box and there is a gaping hole in the back of the box and a gaping hole in the side of the house hole to leave wide open in the side of a house. The holes have to be covered, the gap has to be bridged. Please take my word for it if you can't visualize what I am describing. Enough water can enter that opening in one rain storm to fill my wall cavity with water several inches deep. We aren't talking condensation here, we are talking deluge.

Nevertheless, even if I could use 1/2" conduit, I have never seen a wire clamp designed for PVC conduit of any size. Can you point me in the right direction? I've been told not to attach metal fittings to PVC conduit because there is no way to ground them.

Does anyone else understand what I am talking about?

I know this thread is 2.5 years old, but it shows up in Google searches and brickeyee gave some bad information, so I'm going to correct it now anyway so that future searchers get the right info.

The original poster is right: You *do* need a conduit stub running from the box back through the wall, for two reasons: NEC requires that all boxes must be protected against water entry. Also, NM cable is *not* rated for getting wet, so it needs to be protected. This would be true even if the house siding was flat; NEC requires that exterior boxes are mounted slightly above the house surface (if not embedded flush) to prevent water from wicking between the house surface and the box and staying there. Many weatherproof boxes have that spacing built in.

You would drill a hole in the box rear, or use a knockout if available, and attach a conduit stub just as pamela1863 thought. You can install a clamp on the interior side of the stub, but you don't need it as long as you secure the cable (staple it to a joist or stud) within one foot of the end of the conduit. Make sure that if the conduit is metal that the end has a fitting or plastic cap to prevent snagging on the metal edge, or that if it is PVC that you've reamed off any rough edges.

You could easily caulks the back of the box to the siding and not need conduit for anything.

It would be even easier if you can get the back of the box to lay flat on the siding (less caulk to make a seal).