what is NPT?

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Jraef

Jraef

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Cmdr_Suds said:
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Tapered threads have a slight taper towards the end of the fitting. ...
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Depends on what you mean by "end". The threading die for NPT is tapered, so when you cut the threads, the narrowest part is created AFTER the die finishes passing over the pipe. So the longer you run the threads, the more straight thread you will end up having, the taper will only be at the end closest to the un-threaded part of the pipe.

I was always taught that NPT was for pipe, but the NEC requires NPT threads on RMC and IMC anyway (I'll have to look up a reference later). NPS was only for fittings because it makes it better to plan out lengths etc. if the conduit screws in all the way to the stop shoulder (or another piece of conduit in the case of a coupling) and that the purpose of conduit was mechanical protection, not protection against water entry. That's why cable insulation is still required to be water resistant even though it is pulled into conduit.
 
don_resqcapt19

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Jraef said:
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I was always taught that NPT was for pipe, but the NEC requires NPT threads on RMC and IMC anyway (I'll have to look up a reference later). NPS was only for fittings because it makes it better to plan out lengths etc. if the conduit screws in all the way to the stop shoulder (or another piece of conduit in the case of a coupling) and that the purpose of conduit was mechanical protection, not protection against water entry. That's why cable insulation is still required to be water resistant even though it is pulled into conduit.
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I was always taught that the taper on the male threads is to prevent it from being screwed in far enough to hit the stop or the other pipe. When the pipe end hits something it would be easy to create a ridge that could damage the conductors when they are installed.
 
tom baker

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Water pipe couplings have tapered thread, electrical couplings are straight. I used an electrical coupling for a water pipe and it leaked. That was a long time ago before I became an electrician,,,,
 
Smart $

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Jraef said:
Depends on what you mean by "end". The threading die for NPT is tapered, so when you cut the threads, the narrowest part is created AFTER the die finishes passing over the pipe. So the longer you run the threads, the more straight thread you will end up having, the taper will only be at the end closest to the un-threaded part of the pipe.
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That is true for fixed dies. However, Ridgid geared receding threaders and the 1224 for example use NPT dies to cut both tapered and straight-machine threads, where a simple adjustment locks in taper cuts even on runout.

I was always taught that NPT was for pipe, but the NEC requires NPT threads on RMC and IMC anyway (I'll have to look up a reference later)...
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The NEC requirement is for field cut threads. Manufacturers "Electrical Rigid Steel Conduit" thread criteria fall under ANSI C80.1, last revised 2005 I believe. I do not have a copy of the 2005 revision, but the 1994 version says...

6.6 Threads
The number of threads per inch (25.4 mm), and the length of the threaded portion at each end of each length of conduit, nipple, elbow and bend shall be as indicated in table 1, and shall conform to ANSI/ASME B1.20.1-1983 (R1992). The perfect thread shall be tapered for its entire length, and the taper shall be 3/4 in/ft (62.5
mm/m).

6.7 Couplings
Couplings shall comply with the following requirements.

6.7.1 The exterior surface of couplings shall be protected by means of a zinc coating, which shall comply with the requirements of 6.1. The interior surface shall be treated to inhibit corrosion from taking place prior to installation.

6.7.2 Couplings shall be so made that all threads on the conduit will be covered when the coupling is pulled tight on standard conduit threads.

6.7.3 Both ends of the coupling shall be chamfered to prevent damage to the starting thread.

6.7.4 The outside diameter, length, pitch diameter, and chamfer diameter of couplings shall be as indicated in table 3.

6.7.5 Couplings shall be straight tapped.

6.7.6 Each length of finished conduit shall have one coupling attached.
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W

Wes G

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My understanding is that the taper on the pipe is indeed to control release of hot gases should there be an combustion in a conduit system in a hazardous location. When combined with a streight threaded fitting, it provides a slow release of expanding gases that prevents pipes and fitings from becomming granades while protecting the outside environment from possible combustion / explosion should it contain igniteable levels of gasses / dusts or whatever.
Most power pipe threaders and some hand threaders are adjustable to acomodate more than one size of pipe. Some times you will have to set the cutter to cut a bit deeper than normal to accomodate the fittings you are working with. ( The pipe should have no threads visible with coupler or fitting in place when tightened wrench tite. Fittings, hubs etc need to have at least five threads mateing for EP work. )
 
don_resqcapt19

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If the tapered thread is required for the controlled release of hot gasses in an explosion proof installation, then why, as stated in a previous post, do Class 1, Division 1 push buttons use straight threads for the seal between the push button body and the enclosure?
 
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