Going to the Big Box Store

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mattsilkwood

Senior Member
Location
missouri
they have exactaly the same thread, npt, its the couplings that are different.

there is such a thing as a straight thread, our threader will do either one, but im not for sure what its used for. something like a garden hose thread comes to mind.

why is anytime someone says ridgid pipe we have this disscusion?
 
tkb

tkb

Senior Member
Location
MA
480sparky said:
Then 'splain to me what the NEC says in 342.28 and 355.28?
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IMC 342.28 says "taper of 1 in 16 (3/4in. taper per foot) shall be used.
RMC 344.28 says "taper of 1 in 16 (3/4in. taper per foot) shall be used.

Both of these are NPT same as plumbing.

355.28 is Reinforced Thermosetting Resin Conduit Type RTRC and does not get threaded. Must be a typo or previous code section for IMC or RMC.

See ANSI/ASME B.1.20.1-1983
 
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tkb

tkb

Senior Member
Location
MA
hillbilly1 said:
Just went to my shop, pulled out a stick of 1/2" ridgid, and a stick of 1/2" galvanized water pipe, took a 1/2" galvanized water fitting and put it on the water pipe, two full turns and it was hand tight. Used the same fitting on the ridgid conduit, and it took three full turns to get it hand tight. Apparently the manufactures are not meeting the taper requirement as if it was the same taper, it should have taken close to the same number of turns.
Click to expand...

The taper is the same, the difference was probably the depth of the cut.
 
ceb58

ceb58

Senior Member
Location
Raeford, NC
jeremysterling said:
Exactly, the couplings are different, the nipples are threaded the same. That is why you can get a 3000 psi plumbers cplg and you will not find psi on an electrical cplg.
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The difference is the elect. coupling is "non tested" for pressure. I have bought non tested pipe for making hand rails and racks for trucks. Much cheaper than tested pipe.
 
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ctmike

Senior Member
I was told plumbers couplings were tapered to ensure that a piece off pipe could be threaded in more than 1/2 way to insure that the pipe on the other side would be threaded in fully . also the "couplings" shipped on plumbing pipe are not couplings but thread protectors.
 
Fulthrotl

Fulthrotl

~Autocorrect is My Worst Enema.~
Location
HB, CA (19 Hrs. 22 Min. from Winged Horses)
Occupation
E
mattsilkwood said:
they have exactaly the same thread, npt, its the couplings that are different.

there is such a thing as a straight thread, our threader will do either one, but im not for sure what its used for. something like a garden hose thread comes to mind.

why is anytime someone says ridgid pipe we have this disscusion?
Click to expand...

because you didn't have a frustrated ex machinist with a need to be right
butting in.... allow me to introduce myself.

a 3/4 npt is the same for plumbing, and for electrical.
it is a mechanical standard, not an electrical or plumbing standard.

when you cut threads that are tapered, and set up your equipment,
you use a go/no go gauge to set the PITCH DIAMETER CORRECTLY.

electricians conduit is cut with the pitch diameter at minimum, so
locknuts and couplings spin all the way on.

plumbers pipe is cut with the pitch diameter at maximum, so the
INTERFERENCE FIT OF THE MATCHING THREADS IS AT IT'S GREATEST.
this way the piping can withstand pressure without leaks.

the same dies and taps are used to create both. it is how they are set up.

sloppy and loose = sparky
tight and right = sprinkie

hope this helps.
 
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Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
ctmike said:
also the "couplings" shipped on plumbing pipe are not couplings but thread protectors.
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One of my old coworkers found out about that the hard way when he was loaned to our heating crew to install some black pipe for gas lines. I can only imagine how much fun that must of been taking it all back apart.:rolleyes:

For the record, learning from other people's mistakes is the best way.:)
 
iaov

iaov

Senior Member
Location
Rhinelander WI
tkb said:
The taper is the same, the difference was probably the depth of the cut.
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Thats my guess too. On many threaders you are able to very the depth of the cut. Make the cut deeper and you can turn the coupling/fitting on farther before it tightens up.
 
nolabama

nolabama

Senior Member
Location
new orleans la
hillbilly1 said:
Just went to my shop, pulled out a stick of 1/2" ridgid, and a stick of 1/2" galvanized water pipe, took a 1/2" galvanized water fitting and put it on the water pipe, two full turns and it was hand tight. Used the same fitting on the ridgid conduit, and it took three full turns to get it hand tight. Apparently the manufactures are not meeting the taper requirement as if it was the same taper, it should have taken close to the same number of turns.
Click to expand...

this is not true - meaning it should take about the same number of turns to tighten - differnent countries of origin on the pipe fitting and different factory wears on the dies could easily account for this - in my last line of work we had to quite using stainless threads from the factory - they were to inconsistent to do what we needed them to do - i personally feel that this is an internet only phenomonon - npt is npt is npt - we are not sealing a pipe to a pressure rating and odds are we are not using some kind of thread compond on the pipe anyway - lastly i would have a long talk to an inspector who wanted me to take my pipe apart to put a pitch gauge on it - i doubt that many electrical inspectors would have such an animal and if they did .... who is to say that the compression from putting the pipe together and taking it apart did in some way change the overall shape of the thread - overtighting can deform threads

edit ; thanks fultrhtl for putting that into the words that i could not
 
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cal1947

Member
Location
waldorf,md
retired

retired

Cow said:
What?

Their both NPT. Why do people always think it's a different thread?

EDIT: Forgot to say, before we got a big threader, we used to use an irrigation supply house to thread 3" and above.
Click to expand...

its the plumbing couplings that are different
 
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satcom

Senior Member
Electrical Rigid was always the straight thread, and the taper was for plumbing, the reason for the straight thread was so the lock nuts would tighten up, then the major manufactures decided they were loosing money on the two taper system, and changed the rules. Many of the younger guys never seen the stright thread system.
 
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macmikeman

Senior Member
Location
Planet macmikeman
All this does is remind me how hard it is to get a 2" rigid conduit threaded end to screw up tight into a Cuttler Hammer cast hub for the top of the meter sockets. Seems like you can never get more than a few threads in before it locks up tight on you.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
macmikeman said:
All this does is remind me how hard it is to get a 2" rigid conduit threaded end to screw up tight into a Cuttler Hammer cast hub for the top of the meter sockets. Seems like you can never get more than a few threads in before it locks up tight on you.
Click to expand...
You could always try running the threads a bit farther up the pipe.
 
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