I need to connect a steel junction box on the inside of a building with an outdoor rated disconnect/switch on the outside of the building. The disconnect will be used to power a piece of outdoor equipment and be connected with appropriate cordage to the disconnect.

The wall is 8.5" thick solid concrete. I was planning on drilling a 1" hole through the concrete and connecting the inside box into the back of the outside disconnect using a piece of 3/4" galvanized rigid conduit. I don't, however, have the tools to cut and thread the conduit to length so that the threads can be tightened with conduit locknuts. I asked an electrician if he could do this for me, and he told me to just get a piece of black iron or galvanized plumbing pipe (which I can get pre-threaded in 10-in lengths) and use that instead. Since we are only running THWN wire through this short length of pipe he thought the risk of snagging on the inside of the rough piece of pipe would be minimal. Is this sound advice or should I find another electrician that can cut me a piece of rigid conduit? I thought about using other types of conduit, but I don't see how I could run directly into the back of the disconnect and have room for a conduit connector or clamp unless I make a significantly bigger hole in the concrete. Any advice would be appreciated.

I am also curious as to what standard practice is for caulking sealing outdoor boxes like this when connecting directly through the wall into the back of the box.

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    I never thought of going to an electrician to cut and thread conduit. Rigid is the same size as water pipe, so I'd go to my friendly neighborhood hardware store who cuts and threads pipe (and shoplifters). Commented Dec 9, 2020 at 19:30
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    That was my first thought too, but the big box store told me they only thread pipe, not conduit. Maybe I'll go back and try again. The person I talked too probably didn't understand what type of conduit I was talking about.
    – bigchief
    Commented Dec 9, 2020 at 19:55
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    I don't think you are suppose to use rigid conduit that way (it should go into approved fittings, not simply be held in with conduit locknuts.. Drill or chip out enough space for the right PVC glue on fittings and use a short length of PVC conduit.
    – mfarver
    Commented Dec 9, 2020 at 19:58
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    @mfarver -- no, locknut on inside + outside is fine, even if you need bonding, given that we're dealing with non-service-entrance wiring running at <250V to ground. (See NECA-NEIS 101 section 4.7.6.) Commented Dec 10, 2020 at 0:29

5 Answers 5


3/4" by 10" is a standard nipple size, it turns out

If you're looking for a pre-threaded 10" length of GRC, you're in luck! As it turns out, 10" is a standard length for conduit nipples, so any electrical supply house, or even a big-box store, should be able to hook you up with a 10" nipple of 3/4" GRC/RMC. Given that you'll need a bit of length outside the wall to get locknuts on the conduit anyway, this should do the trick.


You can use rigid as the ground it is done absolutely all the time. However both black iron and water pipe are code violations (how would anyone know?) The water pipe has a seam that is not smooth and the black iron is not galvanized. Rigid conduit uses the same dies as regular pipe if they have a machine there is no difference. The big deal is the center needs to be chamfered, some don’t do this on water and gas.

you can connect the boxes and use the pipe as your ground. from the pipe size why not pull the ground in the pipe the nuts are cheaper than a short run of copper.


Rigid Galvanized Conduit (RGC) and plumbing pipe starts out in the factory as the exact same thing, then there are two differences that take place in the final steps:

  1. The INSIDE of RGC is finished (honed) to be smooth so as to not cause wires to chafe. Plumbing pipe can have a ridge down the middle where it is welded and it can have galvanizing slag protruding into it. RGC can have neither.
  2. The threads on conduit are not tapered, they are straight.

For a 9" CHASE I wouldn't worry about it, and the tapered threads will not be much of an issue.

  • You need to read NEC 344.28 the code requires 3/4” per foot or a 1 in 16 taper. ruining Or straight threads are a code violation.
    – Ed Beal
    Commented Dec 10, 2020 at 7:05

I contemplated something very much like this just this week.. I needed 4 inches to go from an L-body outside into a 10x10x4 junction box inside, mounted in a 2x6 wall. The longest rigid conduit nipple I could find was 2 inches long. Pain in the behind to use regular connectors on EMT in that spot, don't have a way to cut threads on rigid pipe..

I might or might not have used a 4 inch galvanized plumbing nipple to do the job (and checked its interior for burrs, a ridge along the weld joint, etc). But I can say that it seemed the plumbing nipple had a tapered thread whereas the rigid conduit had a straight thread, or just less taper. The lock nut would spin only about 3/8-1/2 inch onto the end of the plumbing nipple and it went further up the threads on the rigid conduit nipple.

Now, just a little hearsay: seems like I asked somebody at the Orange Home Improvement store about threading rigid conduit and was told they won't do it because rigid conduit is harder than plumbing pipe and wears out the dies on the threading machine. I didn't do any additional work to verify the claim.

  • I have heard the rumor of harder pipe, I don’t think that is true. I think we set the dies deeper on universal machines (this allows the lock nuts to go further). But the fixed hand threading dies are the same as pipe I have both R11 and R12 sets the R11 dies fit my hand held power threader. When I thread black iron it is tougher but it’s for air pressure lines and the walls are thicker so I end up double cutting to keep from shattering my dies. Remember conduit doesn't have to pass a pressure leak test but we need to get the nuts and insulating bushings on so we thread longer or deeper.
    – Ed Beal
    Commented Dec 10, 2020 at 7:01
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    @EdBeal Thanks for the clarification. I had another look at a rigid conduit nipple at the store last night and did notice its thread had a bit of taper. I didn't measure, but I believe you're right that cut depth is the difference between plumbing and electrical threads.
    – Greg Hill
    Commented Dec 10, 2020 at 16:18

You can drill a 1" hole then pound in a 1" black iron or galvanized or PVC, then slip your 3/4 steel electrical conduit through that. As the other poster said in the comments you don't thread electrical steel conduit (at least not the thin stuff)

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