Rmc

[Djelite]

Article 344 states that couplings arent allowed with running threads. I understand this is due to not being able to adequately tighten both sides. Does it also has to do withe that fact the coupling isnt tappered and conduit is?

 

[tom baker]

I wold say yes on your reason.
I once used an electrical coupling in place of a plumbing coupling…when I first started as a maintenance man (it leaked)

 

[infinity]

Article 344 states that couplings arent allowed with running threads. I understand this is due to not being able to adequately tighten both sides. Does it also has to do withe that fact the coupling isnt tappered and conduit is?

I'm not so sure that the taper has anything to do with it. When using the coupling at the running thread the RMC is weakened. Back in the day when everything was RMC this was called a depression coupling because 3-piece couplings (Erickson's) were too expensive. Looked like this "depression coupling":

 

[qcroanoke]

I may have done that when supply house was 40 miles away and i needed to get it done

 

[tom baker]

I'm not so sure that the taper has anything to do with it. When using the coupling at the running thread the RMC is weakened. Back in the day when everything was RMC this was called a depression coupling because 3-piece couplings (Erickson's) were too expensive. Looked like this "depression coupling":




View attachment 2559989 View attachment 2559990

Sounds reasonable. A lot of the whys are lost to history.

 

[synchro]

I'm not so sure that the taper has anything to do with it. When using the coupling at the running thread the RMC is weakened. Back in the day when everything was RMC this was called a depression coupling because 3-piece couplings (Erickson's) were too expensive. Looked like this "depression coupling"

Labor was cheaper way back too.
In this excerpt from the 1914 American Electrician's Handbook they show the use of a cut off section of another coupling instead of a normal locknut to make the joint a little more rigid. But it's still weakened compared to a full taper on the conduit.
The extra cost of an Erickson was also mentioned in the book as you've indicated.

 

[Djelite]

Labor was cheaper way back too.
In this excerpt from the 1914 American Electrician's Handbook they show the use of a cut off section of another coupling instead of a normal locknut to make the joint a little more rigid. But it's still weakened compared to a full taper on the conduit.
The extra cost of an Erickson was also mentioned in the book as you've indicated.


View attachment 2559991

Awesome.

 

[djd]

Article 344 states that couplings arent allowed with running threads. I understand this is due to not being able to adequately tighten both sides. Does it also has to do withe that fact the coupling isnt tappered and conduit is?

the threads , the conduits smaller , at the threads I mean .

 

[kwired]

the threads , the conduits smaller , at the threads I mean .

The "running thread" portion is not tapered, and is consistent OD until you get to where it starts to taper. It will not tighten in the side with running thread other than should you put a jam nut on it or let it bottom out, but only takes slight turning and it is just as loose as if you made three full turns. With tapered thread in the socket once it starts to tighten up, it always has a reasonable amount of contact to assure fair electrical continuity. Wrench tight isn't really a torque spec, but kind of has some unwritten acceptable variance to it and could easily be acceptably tight enough even though is fairly easy to get another turn or two out of it.

 

[Djelite]

Sounds reasonable. A lot of the whys are lost to

The "running thread" portion is not tapered, and is consistent OD until you get to where it starts to taper. It will not tighten in the side with running thread other than should you put a jam nut on it or let it bottom out, but only takes slight turning and it is just as loose as if you made three full turns. With tapered thread in the socket once it starts to tighten up, it always has a reasonable amount of contact to assure fair electrical continuity. Wrench tight isn't really a torque spec, but kind of has some unwritten acceptable variance to it and could easily be acceptably tight enough even though is fairly easy to get another turn or two out of it.

The running threads arent tappered and neither is the coupling threads, right?

 

[infinity]

The running threads arent tappered and neither is the coupling threads, right?

That's correct, only the last portion of the running thread is tapered (farthest from the end) which the normal thread length. Electrical couplings are not tapered.