Tools and Art. 344

[jfragola]

Just a quick question about article 344 Rigid Metal Conduit. I have a Ridgid power threader with with standard NPT dies. I need replacement dies for the heads. I check out the tool catalog and they make "electrician's threads" replacement dies, NPSM. According to 344.28, you need a 1 in 16 taper (3/4"taper per foot). My question, are NPSM dies legal for threading IMC/Rigid by code ?

BTW, I asked Ridgid's customer service dept, who were kind of worthless.

Any opinions ??? Thanks.

Joe

 

[renosteinke]

Re: Tools and Art. 344

NPT means "national pipe taper."
NPS means "national pipe straight."
I suspect that the "NPSM" dies would be used for making a running, or continuous, thread. (like you also see on garden hose spigots!)

 

[cm]

Re: Tools and Art. 344

The dyes I have from rigid and collins now rothenberger have nct or nc stamped on them. You would think it stood for national conduit thread

 

[bennie]

Re: Tools and Art. 344

Straight threads can only be used with lock nut connections.

Taper thread must be used with threaded fittings.

 

[cm]

Re: Tools and Art. 344

Conduit threads are straight so when you screw two peices together in a coupling or in a fitting they butt or seat all the way and the wire has a smooth continious raceway so the insulation isnt damaged when the conductors are drawned into the raceway.Some of the nipples I have seen with ul lables on them looked like they were threaded and by plumbers and not reemed.

[ March 25, 2003, 08:54 PM: Message edited by: cm ]

 

[bennie]

Re: Tools and Art. 344

Running thread is not permitted for threaded couplings.

[ March 25, 2003, 09:08 PM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: Tools and Art. 344

cm,
Conduit does not have straight threads. It has the same 3/4" per foot of taper as plumbing pipe. This is required in 342.28 and 344.28 for field cut threads and by the product standards for factory cut threads. If the ends of the conduit were to touch in the coupling and you tightened it up wrenchtight, you would create a ridge that would damage the conductors. One of the reasons that conduit thread is required to be tapered is to prevent the ends of the conduit from touching within the couplings.
Don

 

[don_resqcapt19]

Re: Tools and Art. 344

Here is a conduit installation guide from the Steel Tube Institute.
Don

 

[jfragola]

Re: Tools and Art. 344

Thanks everyone for your comments.

Don,

Thanks for the info. I thought I was right using the NPT dies. After talking to Ridgid's technical support division, who told me all electricians use NPSM (Straight) dies, I kind of doubted myself.

Joe

 

[don_resqcapt19]

Re: Tools and Art. 344

Joe,
Many electricians think that conduit threads are straight threads and use straight thread dies in violation of the code. As I stated in another thread, there are at least 2 manufacturers of conduit threading machines that are not capable of cutting tapered threads. The use of these machines for conduit threads is a code violation. The use of straight threads on conduit has a serious impact on the fault return path when the conduit is used as the equipment grounding conductor.
Don

 

[cm]

Re: Tools and Art. 344

Don I learned something here ,but I think the reason a lot of guys think you use straight dyes is because when I did my apprenticeship in 1975 that was the school of thought ,that wording didnt appear in the nec untill 1978 look in the 75 and previous codes its not there.like I said before my dyes say nc and nct I wonder what the pitch is? on them, no inspector has ever called me on it and never checked it either.Ive gone to all the code change seminars and this was never mentioned wonder why