Rigid Threader

Status
Not open for further replies.
iwire said:
Me too, but Marc's right, Thanks Marc. :cool:
Click to expand...
Actually, it stands for American National Standard Taper Pipe Threads (NPT), and has been abridged in its written form through the years.
 
The problem is that NEC article 344.2, B is in error for anything other than classified locations. It?s obvious that the people on the code-making panel of the NEC know as little about pipe and conduit as the average electrician.
All factory electrical RMC, RNMC [PVC], EMT and liquid tight fittings, couplings, locknuts and WP bell boxes are NPSM threads. Stop buying your rigid pipe at the plumbing supply house. I have both types of threader dies for my threading equipment.
The only place in the NEC where NPT threads are actually required is for hazardous locations where the explosion proof boxes and fittings will also have factory NPT threaded entries. Also with NPT threads, locknuts are not required.
Screwing an NPT field threaded pipe into a factory NPSM threaded coupling or box will result in only partial engagement of the threads. It will not be secure, liquid tight, or provide the same level of grounding as a proper NPSM to NPSM or NPT to NPT connection. THE NEC IS WRONG!
 
KJay said:
The problem is that NEC article 344.2, B is in error for anything other than classified locations. It?s obvious that the people on the code-making panel of the NEC know as little about pipe and conduit as the average electrician.
All factory electrical RMC, RNMC [PVC], EMT and liquid tight fittings, couplings, locknuts and WP bell boxes are NPSM threads. Stop buying your rigid pipe at the plumbing supply house. I have both types of threader dies for my threading equipment.
The only place in the NEC where NPT threads are actually required is for hazardous locations where the explosion proof boxes and fittings will also have factory NPT threaded entries. Also with NPT threads, locknuts are not required.
Screwing an NPT field threaded pipe into a factory NPSM threaded coupling or box will result in only partial engagement of the threads. It will not be secure, liquid tight, or provide the same level of grounding as a proper NPSM to NPSM or NPT to NPT connection. THE NEC IS WRONG!
Click to expand...

Kjay there is no requirement or expectation that RMC couplings will be water tight, the couplings are straight thread. Conduit threads are in fact NPT.
 
KJay,
I suggest that you submitt a proposal for the 2011 code to delete the tapered thread rule. The proposals are due at the NFPA by 5pm November 7. 2008.
 
iwire said:
Kjay, can you provide a anything more then your say so?
Click to expand...

What?. you mean my sanctimonious word is not good enough? :grin: :grin:

I think the easiest thing to do is go to your local plumbing supply house or hardware store and get a piece of 1/2? rigid galv or black iron pipe with standard 3/4? taper [NPT] threads that are required in 344.28. Then got to the electrical supply house and get a piece of standard 1/2? RMC. Notice any difference in the pitch of the threads?
Now thread each piece into a separate round WP 1/2? box entry and see how far it will go with your hand. The NPT pipe will only go a couple of threads before it stops dead. This is not the same as the smooth, gradual thread compression that takes place with an NPT threaded entry as it is tightened down. How much of the total pipe thread available in the entry is actually in contact with the walls and threads of the entry?
On the other hand, the piece of RMC will keep going in until it either bottoms out or runs out of threads because the entry has the same NPSM thread. All of the available conduit threads in the entry are in contact with walls and threads of the entry.

You can do the same thing with a Myers hub, meter socket hub, coupling, threaded LB, etc.
By the way, I believe you can special order RMC with NPT threads.
 
KJay said:
I think the easiest thing to do is go to your local plumbing supply house or hardware store .
Click to expand...

Or just check the spec sheet above from the maker and NPT is not special order it is the standard item.

BUT......... I agree, connectors are straight thread.:)
 
Not sure what to make of this. I just went out and grabbed a piece of 3/4? Columbia MBF RMC off the rack and unthreaded the coupling, by hand I might add. Straight factory threads with no taper, so is my conduit is bad? Granted this conduit is from Canada and is a few years old, but is UL listed, so I tried looking it up on their website, but could find no info on threads.

I tried chasing the threads with my NPT die. No go. Tried my NPSM die, perfect fit. I use NPT and NPSM factory threaded pipe to set my dies and also have several pieces of field-threaded NPT conduit and pipe lying around. This looks much different. My pitch gauge, dies and other actual female NPT and IPS pipe fittings all tell me this particular thread is not NPT.
:confused:
 
iwire said:
Or just check the spec sheet above from the maker and NPT is not special order it is the standard item.

BUT......... I agree, connectors are straight thread.:)
Click to expand...
Here's a little tidbit to throw in the fire...

On a job a few weeks back, running 3/4" GS-RMC from Wheatland, I would swear that one end, the coupling end, was NPSM, and the other end, the one with the reddish plastic thread protector, was NPT threads. The reason I noticed this was because one end would thread into the hub of Crouse-Hinds malleable iron, Form 7 condulets in its entirety, while the other end would only thread in, wrench tight, a little over half way, leaving several threads exposed... and it was not that one end's threads were cut just a little deeper.

I noticed this "phenomenon" only on the 3/4" conduit. We ran comparable amounts of 3/4", 1", 1 1/2" and 2" on the job.
 
Last edited:
According to literature from Allied Tube, "...The 3/4" taper NPT threads (ANSI B1.20.1) are full cut...".
 
Status
Not open for further replies.
Top