Home Improvement Asked by bigchief on March 31, 2021
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.
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.
Correct answer by ThreePhaseEel on March 31, 2021
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.
Answered by Ed Beal on March 31, 2021
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)
Answered by Ted Mittelstaedt on March 31, 2021
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.
Answered by Greg Hill on March 31, 2021
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:
For a 9" CHASE I wouldn't worry about it, and the tapered threads will not be much of an issue.
Answered by JRaef on March 31, 2021
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