Threading Rigid Galvanised Conduit

NMCB13

Member
Location
Florida
I was taught that when you thread Rigid Galvanised Conduit that you would install stright threads instead as opposed to tapered threads. Yet I noticed the in the NEC and on some manufactures WEB sites that they reference a N.P.T. thread (which is tapered). Can somebody shed some light on this.
 

Smart $

Esteemed Member
Location
Ohio
I was taught that when you thread Rigid Galvanised Conduit that you would install stright threads instead as opposed to tapered threads. Yet I noticed the in the NEC and on some manufactures WEB sites that they reference a N.P.T. thread (which is tapered). Can somebody shed some light on this.
You were taught incorrectly. RMC/IMC field threads are NEC required to have 1 in 16 taper (NPT), same as factory threads. Only factory couplings use straight threads.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We were always taught to use straight threads too but as Smart noted the code requires NPT which are tapered.
 

kwired

Electron manager
Location
NE Nebraska
Straight threads will never be able to be tightened like tapered threads, instead of binding at several points of the taper a straight thread will only tighten when something "bottoms out" or "butts against something else". The straight thread only needs to back off a little bit to become loose, but the tapered thread needs to back off much more before becoming just as loose.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm surprised that this electrical myth has lasted so long. I could ask 50 electricans on the job all would say that we use straight threads. :roll:
 

Smart $

Esteemed Member
Location
Ohio
And all because fittings used with nuts require straight threads....
With RMC/IMC, the only "fittings" I can think of, other than a coupling, that uses straight thread would be KO connectors (hub, threadless).

I imagine some of the straight-thread myth was conceived from using running threads and couplings instead of three-piece couplings.
 
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NMCB13

Member
Location
Florida
Ok, so the electrical thread has a (slight) taper, so what is the difference with the plumbing thread (say for sprinkler piping)?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Ok, so the electrical thread has a (slight) taper, so what is the difference with the plumbing thread (say for sprinkler piping)?

Nothing, NPT is the standard for electrical, plumbing and sprinkler. Even time I use a plumber's or fitter's threader on the job some electrician will tell me that the threads are wrong. :roll:
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Am I wrong in believing that any model of thread cutting machine (except a lathe!) could produce either kind of thread depending on what dies you installed in it?

Tapatalk!
 
Am I wrong in believing that any model of thread cutting machine (except a lathe!) could produce either kind of thread depending on what dies you installed in it?

Tapatalk!
Yes and no.

Straight threads are 'bolt threads'. A UNC is 13 and a UNF is 20. A NPT will be 13 TPI.

Also, the diameters will be off. NPT is based upon trade sizes that use the ID (conduit size), where UNC and UNF use the OD (rod size).

Just think of trying to put a 1/2" machine nut on the end of a 1/2" RMC conduit.
 
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Am I wrong in believing that any model of thread cutting machine (except a lathe!) could produce either kind of thread depending on what dies you installed in it?

Tapatalk!
Yes if you use the correct dies. For years we used NPSM dies for straight threads.
 

kwired

Electron manager
Location
NE Nebraska
Nothing, NPT is the standard for electrical, plumbing and sprinkler. Even time I use a plumber's or fitter's threader on the job some electrician will tell me that the threads are wrong. :roll:
I've had plumbers tell me the thread is not the same as well, NPT is NPT no matter what you are threading.


As far as same machine making straight vs tapered thread, once you thread the width of your die any further threading will result in straight threads from the end of the pipe to wherever the trailing end of the dies at the stopping point. Kind of hard to put more taper on when you are not cutting it anymore, and you can only taper so far anyway or else you eventually get through the wall thickness.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I've had plumbers tell me the thread is not the same as well, NPT is NPT no matter what you are threading.


As far as same machine making straight vs tapered thread, once you thread the width of your die any further threading will result in straight threads from the end of the pipe to wherever the trailing end of the dies at the stopping point. Kind of hard to put more taper on when you are not cutting it anymore, and you can only taper so far anyway or else you eventually get through the wall thickness.

Yes NPT is NPT. :D

And for larger threads such as 2.5"-4" the dies are very small, maybe 1/2" of cutting area. They make the taper by riding on a ramp as the machine advances the thread along the pipe.
 

kwired

Electron manager
Location
NE Nebraska
Yes NPT is NPT. :D

And for larger threads such as 2.5"-4" the dies are very small, maybe 1/2" of cutting area. They make the taper by riding on a ramp as the machine advances the thread along the pipe.
That is true for dies that move as you cut, some of us are used to small simpler hand threaders with fixed dies. The ones that move as you cut often you can only thread so far and then the dies are released giving you consistent threads every time, hand threaders you can keep going and cut threads the whole length of the pipe if you really wanted to, but they would only be tapered the thickness of the die at the stop point if you stop.:cool:
 

jap

Senior Member
Apprentices that thread pipe too far is on my top ten lists of things that boil me over.
It rates right up there with running a drill bits full speed and forcing my cordless to try and do something it cant cause they're too lazy to put a fresh battery in it.
 
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