[John Valdes]

I believe electric threads are tapered while plumbing threads aren't

My understanding was the inside of RMC was smooth, so, the wire isn't damaged. Also, does RIGID make separate electrician & plumber thread dies?

On rigid jobs I never even thought about this. We were threading pipe all day long with no consideration. I have never seen or heard about the dies being different.
Also when working in the plants, we used nipples from the supply room. There was no distiction between electrical and plumbing.

 

[bill39]

The definition of a pipe nipple has a limited length, maybe 24 inches maximum.

People here worrying about difficulties pulling wire thru a plumbing nipple probably are the same ones wearing a mask driving in their car by themselves.

 

[Veteran Sparky]

Rigid Ferrous Metal Conduit (DYIX) UL Mark ANSI/UL6 is the Listing. (electrical rigid conduit)
Everything has a listing. Refer to the 'white book'.
If NOT listed for electrical, it CANNOT be used for electrical.

 

[CAUSA]

Where I work I keep commig across place where there is a mixture of rigde conduit with a plumbing nipples,
and some are in a hazrdous location, where in NFPA 70 or 79, can I find this is violation or unsafe practices.
I do not is see any wiring methods in ch 3 , wher it is ok.

Thank you. for any and all information.

I know a old post but would like to add a toones worth.

For internal smoothness, I have had electrical nipples with poor reaming and poor finishing internally. I need to build a jig with sand cloth to internally ream the nipple to prevent conductor insulation tearing. So it can work for plumbing nipples or electrical rated nipples.

This is a Segway for the issue of dissimilar metals reacting according to the Galvanic table.

This is a big issue for keeping bonding, if one has not been run separately. Not to mention for threading structure integrity for explosion proof sections.

For a sound installation. The view point of, how the materials react with each other will benefit the installation and your peace of mind.

So if the materials are too far on the Galvanic chart and they are joined. This is a accident waiting to happen. More so in a hazardous area.

The rule of thumb,

Electricians: need the piping to be conductive and leak at a regulated controlled rate. No galvanic action destroying the joints made up tight. with smooth interior walls for ease of conductor pulls

Pipe fitters: need the piping to not leak, but cares of conductive properties if spec on plans, for galvanic mitigation. and also no galvanic action destroying the joints made up tight.

So no matter what trade. Check the internal condition of the piping and prepare as required.

Check threading as well no need to have unnecessary galling because of a bent thread.

 

[Dennis Alwon]

On rigid jobs I never even thought about this. We were threading pipe all day long with no consideration. I have never seen or heard about the dies being different.
Also when working in the plants, we used nipples from the supply room. There was no distiction between electrical and plumbing.

Actually the thread is the same - tapered, but the electrical couplings are straight thread so the conduits will leak. I am guessing that the couplings for plumbing are tapered.

 

[John Valdes]

Actually the thread is the same - tapered, but the electrical couplings are straight thread so the conduits will leak. I am guessing that the couplings for plumbing are tapered.

Thankfully we don't need to worry about leaks. Cut, thread, install.

 

[splatz]

This simple topic is comically complicated to me

Not for nothing but Ridgid does make different dies for electrical conduit, they make NPSM dies that you'd use rather than the NPT dies you'd use for plumbing. I wouldn't worry too much about causing a hazard using tapered dies.

Tapered male into tapered male you get a lot of mating surface and the seal is going to be very tight, like pressure tight, OK for plumbing, compressed air, etc.

Straight male into straight male you don't get a seal in the threads. (You might get a seal between a shoulder on the male side and the female part, especially with a gasket or sealing washer.) But conduit isn't really supposed to be water tight, right, that's why you have to use THWN in wet locations / underground inside RMC.

Tapered male into straight female you get a limited seal between the threads, it might be weather resistant or drip proof, especially with a sealant, but it or something but it won't seal water or air under pressure.

The real controversy is, is it code compliant to thread a fitting like say an EMT connector into the threads on an LB? 😰

 

[MechanicalDVR]

My understanding was the inside of RMC was smooth, so, the wire isn't damaged. Also, does RIGID make separate electrician & plumber thread dies?

Never seen more than just standard NPT (National Pipe Thread) at any supply house.

You'd have to special order straight dies.

 

[CAUSA]

This simple topic is comically complicated to me

Not for nothing but Ridgid does make different dies for electrical conduit, they make NPSM dies that you'd use rather than the NPT dies you'd use for plumbing. I wouldn't worry too much about causing a hazard using tapered dies.

Tapered male into tapered male you get a lot of mating surface and the seal is going to be very tight, like pressure tight, OK for plumbing, compressed air, etc.

Straight male into straight male you don't get a seal in the threads. (You might get a seal between a shoulder on the male side and the female part, especially with a gasket or sealing washer.) But conduit isn't really supposed to be water tight, right, that's why you have to use THWN in wet locations / underground inside RMC.

Tapered male into straight female you get a limited seal between the threads, it might be weather resistant or drip proof, especially with a sealant, but it or something but it won't seal water or air under pressure.

The real controversy is, is it code compliant to thread a fitting like say an EMT connector into the threads on an LB? 😰

It has been quite awhile for me to look at a NEC book, so I cannot give you a note for a specific code.

But... yes this is quite the topic.

In Canada.. for the Canadian CEC. Rule 12-1006 states, to reference ANSI/ASME B1.20.1 for threading pipe and fitting joining. The B1 is the code book for the piping trades.

But depending on the classification of the area or conduit service. There are rules that govern threading engagement depth and when straight threads are allowed and not allowed.

An example is 3 complete threads for EMT. But in a Class 1 Division 2. 5 complete thread engagement, and has to be NPT minimum no running threads.

Including a pipe threading chart for all the apprentice's for clarification. with a white paper on threading design.

Also a example of a electrical fitting that uses both.

 

[Dennis Alwon]

The NEC requires tapered threads but the rigid couplings are not tapered.

My understanding is that for high compression type installs like in thermowells you need the straight threads

 

[paulengr]

Never seen more than just standard NPT (National Pipe Thread) at any supply house.

You'd have to special order straight dies.

Everything I can find in NEC and UL related to conduit except couplings calls for NPT. None of it mentions NPSM and this is the first thread I’ve seen it mentioned. ANSI does have a separate spec for NPSC (couplings) which is strangely NPST but this doesn’t make sense. I have heard conduit is NPST too but UL 6 is pretty clear that this is not the case.

Any ANSI thread is going to leak. Even perfectly mated NPT threads have rounded roots and crests so there is a spiral shaped gap. If you made them seat perfectly you could turn the pipe to get it connected. Plumbers fix this with some sort of filler (pipe dope, tape, anaerobic thread sealants). In hydraulics they use either machined taper threads with a steep angle like 45 degrees or machined gasketed faces with bolts to squeeze the joint together or something similar with the SAE and JIC fittings so it seals with dry threads (no pipe dope).

With conduit it’s there as physical protection, fault protection, and inadvertently as electromagnetic shielding (I don’t think they thought about this when it started being used). The wiring is suitable for wet locations so conduit sealing isn’t really a factor.

Not buying the rigidity argument. NPT makes it easier to get the connections together with less friction compared to NPST. You have probably noticed it gets progressively tighter. NPST doesn’t act like that. As to mixing the two, bad idea. You’d have a beveled edge up against a straight edge that is quite literally hanging on by a thread. It would be loose and break loose easy.

At best and this is my speculation here I’ll say that nuts are purposely designed so that the threads stretch and deform so they hold better. The deformation is permanent, This is why you aren’t supposed to reuse nuts and bolts. Of course electricians tend to always reuse and mechanics tend to just lose half of them. So maybe a coupling is NPST (NPSC) but we force it on so that it flares open a bit and gives a more rigid connection than if it is NPT? I don’t know for sure and having a hard time finding anything that talks about what the thread specs are.

 

[CAUSA]

 

[splatz]

 

[kb1jb1]

The NEC requires tapered threads but the rigid couplings are not tapered.

My understanding is that for high compression type installs like in thermowells you need the straight threads

Maybe I missed it but it took 30 posts before someone quoted the NEC requirements of a taper when cutting and threading in the field. That was a question on one of my licensing tests. 3/4 inch tapered per foot. NEC 344.28

 

[VELOCI3]

Plumbing threads are tapered, too (NPT)



That's right, couplings are NPSM (national pipe straight mechanical) which is for mechanical joining not a watertight seal.

Which also means if you use a coupling and a chase bushing on both ends to connect a box and/or enclosure, that fitting is not considered to bond.


Sent from my iPad using Tapatalk

 

[splatz]

Which also means if you use a coupling and a chase bushing on both ends to connect a box and/or enclosure, that fitting is not considered to bond.

I was wrong about the NPSM, I forgot there's an NPSC for couplings that's slightly different, but that wouldn't change what you're saying. But why would that not be good for bonding?

 

[HertzHound]

I don’t think the threads are the issue with it being considered a bond. It’s the fact that you need two locknuts, or a shoulder and a locknut. So an EMT connector would satisfy the rules for bonding over 250v, but a chase nipple into a coupling or LB wouldn’t.

 

[VELOCI3]

I was wrong about the NPSM, I forgot there's an NPSC for couplings that's slightly different, but that wouldn't change what you're saying. But why would that not be good for bonding?

Has to do with the product listing of male fittings. They are tested/listed for use with locknuts and not hub type NPS fittings.


Sent from my iPhone using Tapatalk

 

[paulengr]

View attachment 158597

So that makes it clear that NPSC is “straight” tapered threads so just another word for NPT, not NPST despite the name and the screwy way they explain it.

Looks like NPST for electrical, or NPSM is just a myth.

 

[CAUSA]

So that makes it clear that NPSC is “straight” tapered threads so just another word for NPT, not NPST despite the name and the screwy way they explain it.

Looks like NPST for electrical, or NPSM is just a myth.

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys.