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NEC article 346-8 reaming + threading #76867 03/26/01 08:23 PM
Joined: Mar 2001
Posts: 1
P
pottenger Offline OP
Junior Member
is the standard cutting die with a 3/4-inch taper per foot (1 in 16) requirement + the FPN note referencing the ANSI/ASME B.1.20.1-1983 an NPT (national pipe taper) as found on a typical plumbing threader/die or is it an NPSM (national pipe straight mechanical) which usually has to be specified + special ordered? should conduit threaded in the field have a taper thread or a straight thread? factory threads on conduit look tapered to me. i have heard NPSM (straight thread dies) also called conduit dies ; but is this really true? thanks


mike p
2017 / 2014 NEC & Related Books and Study Guides
Re: NEC article 346-8 reaming + threading #76868 03/26/01 08:47 PM
Joined: Nov 2000
Posts: 72
B
Bennie R. Palmer Offline
Member
The standard thread for connecting conduit to threaded fittings, is the same as that used by pipe fitters. The straight cutting die is for lock nut connections only.
The length of the taper thread must not exceed the dimensions as stated in the ANSI standards. A thread longer than the standard dimension is a running thread, and a code violation, when making threaded connections.
Two, or more threads exposed, indicates a loose connection.

Re: NEC article 346-8 reaming + threading #76869 05/19/01 08:41 AM
Joined: May 2001
Posts: 129
F
frodo Offline
Member
i was under the impression that conduit threads were made to drain...

npt is a taper thread....pipe fitters, who use black pipe and galvanized etc., i thought used a straight thread to acheive a seal....

Re: NEC article 346-8 reaming + threading #76870 05/19/01 10:12 AM
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Posts: 4,283
electure Offline
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The tapered thread on black pipe, etc. allows for a greater surface area to be in contact between the pipe and fitting. B.I. pipe comes with a "thread protector" on one end that is straight thread but looks like a coupling. If they're used, they usually leak like crazy. (A plumber I know had to redo the gas on 5 complete houses when the guy he sent out came back with a whole box of unused couplings on his truck)

Re: NEC article 346-8 reaming + threading #76871 05/21/01 07:11 AM
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Redsy Offline
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Hazardous Location work and fittings require tapered threads. These must engage for at least 5 threads and also be wrench-tight without use of locknuts. This helps to minimize spread of flame into the surrounding(explosive) atmosphere by cooling the flame to the point of extinguishment by the time it would work its way through the path of the threads. Running threads (Electrical allthread?) are not permitted in these applications.

Re: NEC article 346-8 reaming + threading #76872 05/21/01 08:55 AM
Joined: Nov 2000
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resqcapt19 Offline
Member
Redsy,
All field cut and threaded conduit requires taper threads, not just those in calssifed areas. Also because 346-8, the only time running thread time you can use running thread is if it is "factory" cut.
Don(resqcapt19)


Don(resqcapt19)
Re: NEC article 346-8 reaming + threading #76873 05/21/01 11:50 AM
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Redsy Offline
Member
Thanks.
I don't have much experience with running threads myself, but I have seen all-threaded nipples of various lengths used with double locknuts & bushings at each end. Would this violate 346-8, if they were used in accordance with 346-9(b)? Or only if they were field-made? Also,in what lengths are these nipples commercially available.
Quote
Originally posted by resqcapt19:
Redsy,
All field cut and threaded conduit requires taper threads, not just those in calssifed areas. Also because 346-8, the only time running thread time you can use running thread is if it is "factory" cut.
Don(resqcapt19)

Re: NEC article 346-8 reaming + threading #76874 05/21/01 12:36 PM
Joined: Nov 2000
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resqcapt19 Offline
Member
I think that all thread comes in 36" lenghts. They can be used with double lock nuts and bushings. I've seen them many times used between panels.
Don(resqcapt19)


Don(resqcapt19)
Re: NEC article 346-8 reaming + threading #76875 05/21/01 07:55 PM
Joined: Nov 2000
Posts: 123
D
doc Offline
Member
well I swear that a few years back we had a contractor do a igid pipe job for us and the pipe dies he used had 3/4 re and which stood for electric rigid and the box that the dies were in had for rigid electric pipe only not to be used for regular piping or something along that line and when you put them up aganist regular pipe dies they were different and when using a thread guage on them u could see the difference in the threads I have always understood that rigid conduit threads and regular pipe threads are different not much but some now am I wrong


MAY THE SUN SHINE ON YOUR FACE IN THE MORNING AND YOU AWAKE WITH A SMILE
Re: NEC article 346-8 reaming + threading #76876 05/21/01 11:18 PM
Joined: Nov 2000
Posts: 2,148
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resqcapt19 Offline
Member
Doc,
The code requires them to be the same. I am aware of at least two manufacturers of small threading machines for conduit that can only cut straight threads. They say that they are code leagle, but I don't see how.
Don


Don(resqcapt19)
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