# RMC vs black pipe/galvanized



## dielectricunion (Nov 29, 2012)

I'm not asking to justify using plumbing pipe for electrical installs... But... I'm wondering, aside from being UL listed, what is the difference between rigid conduit and gas or water pipe?

I've heard that the inside of gas and water pipe has a small ridge that isn't acceptable for pulling wire, but I'm pretty sure RMC has just as many rough surfaces (not many on either pipe).

Regardless, when I re-run my gas lines, I'm totally going to use the bender on those MOFO's! No elbows


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## NC Plc (Mar 24, 2014)

You put black tape on your wire nuts dont you.


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## RIVETER (Sep 26, 2009)

Black iron is not approved for the purpose but maybe it should be. If it can keep gas IN...maybe it can keep water OUT.


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## dielectricunion (Nov 29, 2012)

NC Plc said:


> You put black tape on your wire nuts dont you.


Funny


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## Switched (Dec 23, 2012)

RIVETER said:


> Black iron is not approved for the purpose but maybe it should be. If it can keep gas IN...maybe it can keep water OUT.


A little pipe dope might solve the issue too....


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## dielectricunion (Nov 29, 2012)

Switched said:


> RIVETER said:
> 
> 
> > Black iron is not approved for the purpose but maybe it should be. If it can keep gas IN...maybe it can keep water OUT.
> ...


Not too much or you could break the electrical bond


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## backstay (Feb 3, 2011)

I think conduit threads are cut deeper so the fit isn't as tight as water pipe. I know I set my threaded up that way.


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## Jay82304 (May 12, 2015)

The inside of electrical conduit is paid more attention to while being produced and made sure to be free of nicks and other things that could potentially damage the cable. The threading is different too.


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## acro (May 3, 2011)

dielectricunion said:


> Regardless, when I re-run my gas lines, I'm totally going to use the bender on those MOFO's! No elbows


Yes, the seam in the RMC is much smoother. Not sure of the actual contrustion differences between the two, other than RMS is intended to be bent. Not sure if the plumbing pipe can stand the same stresses that a bender would introduce.

The seam you mention could possibly split open when you bend it in the black pipe. That could be a problem with gas service.


Other than that, why not.


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## Mountain Electrician (Jan 22, 2007)

backstay said:


> I think conduit threads are cut deeper so the fit isn't as tight as water pipe. I know I set my threaded up that way.


Rigid conduit systems are designed to contain internal explosions and the threads allow the burning gas to cool by expansion through the threaded connections as it escapes, so as to not blow more stuff up when it comes out. The Code says 5 threads are sufficient to serve that purpose.


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## Mountain Electrician (Jan 22, 2007)

dielectricunion said:


> Not too much or you could break the electrical bond


I've been on jobs where we had to put conductive anti-seize on all the connections.


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## don_resqcapt19 (Jul 18, 2010)

RIVETER said:


> Black iron is not approved for the purpose but maybe it should be. If it can keep gas IN...maybe it can keep water OUT.


It is not the conduit that leaks...it is the fact that conduit uses straight thread couplings. The couplings on the gas pipe are tapered thread couplings.


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## don_resqcapt19 (Jul 18, 2010)

Mountain Electrician said:


> Rigid conduit systems are designed to contain internal explosions and the threads allow the burning gas to cool by expansion through the threaded connections as it escapes, so as to not blow more stuff up when it comes out. The Code says 5 threads are sufficient to serve that purpose.


I am not aware of any place in the code that permits the use of a conduit coupling between an enclosure that is required to be explosion proof and the seal fitting. Conduit couplings do not provide a seal that is tight enough to reduce the temperature of the escaping hot gasses so as not to ignite an flammable atmosphere outside of the conduit.


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## dielectricunion (Nov 29, 2012)

backstay said:


> I think conduit threads are cut deeper so the fit isn't as tight as water pipe. I know I set my threaded up that way.


Code says that RMC is to be field threaded with a 3/4" per foot taper thread (344.28), which is the same as gas and steel water pipe. It seems like the factory threads are usually different though.


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## NacBooster29 (Oct 25, 2010)

Black iron seems more rigid, Not as maliable. 
I would think if you tried to bend it, it would break the seem.


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## Nom Deplume (Jul 21, 2013)

NacBooster29 said:


> Black iron seems more rigid, Not as maliable.
> I would think if you tried to bend it, it would break the seem.


Not to mention, it may be against the gas code to bend the pipe instead of using elbows.


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## NacBooster29 (Oct 25, 2010)

Ive never seen black iron bent its always threaded into fittings. In my experience.


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## JRaef (Mar 23, 2009)

Funny. When I worked at US Steel years ago, a sister company of ours was Triangle PWC who made pipe and conduit. They had been part of our plant, but Triangle bought that part, yet we were contractually responsible for maintaining it because our utilities were too intermingled. So I worked in both places as an electrician. At that time, I thought EVERYONE knew the differences. I guess I was getting a skewed education without knowing it.

Although the OD is the same, the ID is not, even though the "nominal" ID we use is the same. By that, I mean that when we call conduit or pipe "1/2 inch", we are referring to the ID of it being at least 1/2 inch nominally. But in reality, the wall thickness of conduit is less than that of pipe. Pipe, be it water or gas, is a PRESSURE vessel, conduit is not. At the same time, conduit has a specific requirement of not having more than a certain percentage of interior surface deformations, basically the "smoothness", so as to avoid damaging wire as much as possible. So in the manufacturing process they start out the same, but the product destined to become conduit is given an extra deburring treatment, which reduces the wall thickness. That makes conduit unsuitable for use as "pipe" because it reduces the tensile strength and thus the ability to hold the specified pressure, yet at the same time makes it more expensive because it needs more processing in the manufacturing.

Black iron pipe, fugedaboutit... Black iron pipe can be made with a welded seam and left that way with a ridge in the middle, and true black iron (as opposed to black steel) is brittle and would break if you tried to bend it.


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## dielectricunion (Nov 29, 2012)

Awesome, thanks jraef


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## Wirenuting (Sep 12, 2010)

The 1909 era hospital I'm rehabbing now had 1/2" black iron pipe for conduit. They used a jig to bend it before pouring concrete. It all seemed to be in fair shape including the boxes. Now the square lock nuts and the angles were off at times but over all it was a quality job. Removing the old wire was easy even on the long runs. 
The pipe located in the sub basement and steam service tunnels was mostly rotted.


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## dielectricunion (Nov 29, 2012)

Wirenuting said:


> The 1909 era hospital I'm rehabbing now had 1/2" black iron pipe for conduit. They used a jig to bend it before pouring concrete. It all seemed to be in fair shape including the boxes. Now the square lock nuts and the angles were off at times but over all it was a quality job. Removing the old wire was easy even on the long runs.
> The pipe located in the sub basement and steam service tunnels was mostly rotted.


A few weeks ago, in an old Chicago two flat, I removed some old black pipe, same square lock nuts, old style. I also pulled new wire to replace the old cloth cables. Those things do pull smooth even after probably 100 years! I love seeing the old fixtures, fittings and pipe... Hope I can find some old vintage catalogs or code books, because I'm really interested to learn more history


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## John Valdes (May 17, 2007)

backstay said:


> I think conduit threads are cut deeper so the fit isn't as tight as water pipe. I know I set my threaded up that way.





don_resqcapt19 said:


> It is not the conduit that leaks...it is the fact that conduit uses straight thread couplings. The couplings on the gas pipe are tapered thread couplings.


Maybe I was doing it wrong all those years, but when I used the threading machine, I used the same dies as everyone else used.
I could be running conduit one day and have a pipe fitter come right behind me and use the same die.
So, either my conduit ends were tapered, or the pipe fitters (mechanics) had to use extra tape. :laughing:


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## Wirenuting (Sep 12, 2010)

dielectricunion said:


> A few weeks ago, in an old Chicago two flat, I removed some old black pipe, same square lock nuts, old style. I also pulled new wire to replace the old cloth cables. Those things do pull smooth even after probably 100 years! I love seeing the old fixtures, fittings and pipe... Hope I can find some old vintage catalogs or code books, because I'm really interested to learn more history


I had to make some take off 1900 boxes to get out of the old ceilings. 
The boxes had one screw and a notch on the other side. What a PITA that was. 
Last week I was standing outside my old shop and reliesed that there were the right rings still in the ceiling there. But we abandoned the building 8 years ago and it's a moldy mess in there now. Not worth the salvage effort,,,, yet.


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## don_resqcapt19 (Jul 18, 2010)

John Valdes said:


> Maybe I was doing it wrong all those years, but when I used the threading machine, I used the same dies as everyone else used.
> I could be running conduit one day and have a pipe fitter come right behind me and use the same die.
> So, either my conduit ends were tapered, or the pipe fitters (mechanics) had to use extra tape. :laughing:


I said our conduit uses straight thread *couplings*, not that the conduit itself has straight threads.


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## Mountain Electrician (Jan 22, 2007)

don_resqcapt19 said:


> I am not aware of any place in the code that permits the use of a conduit coupling between an enclosure that is required to be explosion proof and the seal fitting. Conduit couplings do not provide a seal that is tight enough to reduce the temperature of the escaping hot gasses so as not to ignite an flammable atmosphere outside of the conduit.


For Class 1 Div 1 you can use couplings, along with GUA fittings, unions, etc. between the seal and enclosure as long as they don't exceed the trade size of the conduit. 501.15(A)(1) 










You cannot put couplings or other fittings except explosion proof reducers between the place where the conduit leaves a classification boundary and the seal for that conduit, which must be no more than 10 feet from the boundary. 501.15(A)(4)

Class 1 Div 2 is about the same for enclosure seals with the exception of 501.15(A)(1)(2) and (A)(2), but has quite a few exceptions for boundary sealing.


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## don_resqcapt19 (Jul 18, 2010)

Mountain Electrician said:


> For Class 1 Div 1 you can use couplings, along with GUA fittings, unions, etc. between the seal and enclosure as long as they don't exceed the trade size of the conduit. 501.15(A)(1)
> 
> 
> 
> ...


I see that you are correct that the code does permit a coupling between the enclosure and the seal.


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## backstay (Feb 3, 2011)

John Valdes said:


> Maybe I was doing it wrong all those years, but when I used the threading machine, I used the same dies as everyone else used.
> I could be running conduit one day and have a pipe fitter come right behind me and use the same die.
> So, either my conduit ends were tapered, or the pipe fitters (mechanics) had to use extra tape. :laughing:


All pipe threading dies are tapered. But you can adjust the die to "take" more or less material. If a pipe fitter used my machine, I don't know if he would get a good seal. Never let a pipe fitter near you stuff, next thing you know, your 555 bender shoe will be bent because they were "trying" to bend steam pipe!


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## John Valdes (May 17, 2007)

don_resqcapt19 said:


> I said our conduit uses straight thread *couplings*, not that the conduit itself has straight threads.


Thanks Don. Got it.



backstay said:


> All pipe threading dies are tapered. But you can adjust the die to "take" more or less material. If a pipe fitter used my machine, I don't know if he would get a good seal. Never let a pipe fitter near you stuff, next thing you know, your 555 bender shoe will be bent because they were "trying" to bend steam pipe!


It was never my machine. It always belonged to the contractor or facility I worked in.
But thanks for the clarification.

Im out of thanks, so thanks to both of you.


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## bevans5172chevymonte (6 mo ago)

I out some small offsets in my black gas pipe at my old house so I didn't have to do the double 90 thing to get around a beam.


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