Electrical Pipe threads

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Hi,
Can somebody give me the complete lowdown on electrical pipe threads?
If you buy a stick of rigid from the parts store, does it not have a bit of a tapper? And what is the standard for this tapper? And If i go thread a nipple on a threading machine, does it not need a tappered thread? I think there is something about it in the code. So is the NPT, national pipe thread the electrical standard? Or is NPT not even a standard?
Thanks fellas!
 
Re: Electrical Pipe threads

Article 344 covers requirements for RMC. The standard is 3/4" taper per foot. 344.42(B) states that running threads shall not be used on conduit for connection at couplings. For more specific detail, see ANSI/ASME B.1.20.1-1983 for standard pipe thread information.
 
Re: Electrical Pipe threads

Yes, conduit purchased from a supply house has tapered threads. The taper standards/specs are, I believe, published/set by ANSI. NPT is the standard for conduit thread taper. Most pipe machine/die threads are tapered. Straight run threads are allowed for certain nipples and some special applications.
 
Re: Electrical Pipe threads

There are at least two manufacturers that market threading machines for 1/2 thru 2" conduit that cannot cut tapered threads. I have asked them about this at trade shows and they don't seem to care that their machines cannot provide threads in compliance with the code. I asked about different dies, but they told me that the their motor does not have enough horsepower to cut tapered threads. I saw both machines at a NECA show so there is no doubt that they are marketing to electrical contractors. They were both small attractively priced machines, but they can't meet the rules in the NEC. Be careful what you buy. Just think of the cost if the inspector came in near the end of the job and discovered that the conduit threads were not code compliant.
Don
 
Re: Electrical Pipe threads

That is pretty interesting I imagine they bury that info pretty well in their advertising, if they mention it at all.

I never thought about a threader that could not cut tapered threads, you know they are not selling these to pipe fitters, that would be a lot of leaks. :)

That would really be a profit killer at the end of a job.

[ March 17, 2003, 02:08 PM: Message edited by: iwire ]
 
Re: Electrical Pipe threads

I recall once years ago before I became an electrican I used an electrical coupling for a water pipe. It leaked. There was no taper in the coupling and it never tightned up.
 
Re: Electrical Pipe threads

Don,
As iwire says, "very interesting"...
I have been spoiled all these years by Rigid and Greenlee equipment... Good thing I guess

I know they make straight thread die sets for these machines but I have never had occasssion to use them

[ March 17, 2003, 06:44 PM: Message edited by: Len_B ]
 
Re: Electrical Pipe threads

Electrical pipe threads are generally tapered the same as plumbing threads. For external threads, that is; "running thread" is the exception.
Electrical fittings with female threads are generally not tapered; thus, electrical couplings will leak.
Electrical rigid pipe differs from plumbing Schedule 40 only in that the ASTM spec has a specification for the interior smoothness, and that specification is absent from the plumbing spec. Electrical pipe is, of course, always galvanised.
 
Re: Electrical Pipe threads

Here is a trivia test about pipe threads.

What constitutes a running thread?

What is the internal difference and dimensions of a coupling for conduit and water pipe?

How can one tell if a conduit coupling is not tight?
 
Re: Electrical Pipe threads

1. running thread does not have a taper
2. electrical conduit coupling does not have a taper and plumbing does. for 1/2"
NPT:
11/16 with taper
23/32 without taper
23/32 streight

3. you can see the threads

[ March 18, 2003, 09:39 PM: Message edited by: hurk27 ]
 
Re: Electrical Pipe threads

1. Running thread is when the standard thread length is exceeded.

2. Water pipe couplings can be straight or tapered. The manufacturing process determines the final shape.

3. More than 2 1/2 threads exposed, constitute a running thread. A running thread is a loose connection.
 
Re: Electrical Pipe threads

Bennie, are the answers to #1 and #2 from a "standard" or are they just one of the things you should know?

As I read 344.28 I noted that the taper is only required for "in the field" threading. Does this mean manufacturers can furnish other threading styles?

Straight threads, or what are sometimes called conduit threads are actually known as "NPSM", or National Pipe Straight Mechanical. Same threads per inch as NPT..., but only minimal taper at the end where the thread stops, (or looked at the other way, the first couple of threads of the die are all that have taper).

I have an old set of "Toledo" brand threading equipment. Does anyone know if there is a company that supports this product? They apparently went out of business. And, no, Ridgid die teeth don't fit.

Wayne
 
Re: Electrical Pipe threads

While we are on the subject of pipe threads, how many use compound on the threads and if so do you use a conductive product or just plumbers pipe dope?
Brian

[ March 24, 2003, 11:19 PM: Message edited by: monkey ]
 
Re: Electrical Pipe threads

Originally posted by monkey:
While we are on the subject of pipe threads, how many use compound on the threads and if so do you use a conductive product or just plumbers pipe dope?
Brian
Click to expand...
I use teflon paste or tape when the fittings are to be exposed to wetting. I have never had them fail ground loop testing even though the thread tape is certainly not conductive. If the fittings are made up wrench tight there is plenty of metal to metal contact.
--
Tom
 
Re: Electrical Pipe threads

Look at the last part of 300.6(A): " ... Where corrosion protection is necessary and the conduit is threaded in the field, the threads shall be coated with an approved electrically conductive, corrosion-resistant compound."
T&B make a product suitable for this application. It is called "KOPR-SHIELD".
There is no way to make our conduit connections water tight because of the straight thread couplings. The code requirement is only to provide corrosion protection.
Don
 
Pipe thread count

Pipe thread count

I understand that the code shows that in the field cutting of conduit we are to use tapered threads. I understand that the reason for this is so that when we tighten down the threads the pipe doesn't bottom out and flare at it opening inside of the pipe. I also understand that most electrical threaded fittings are made with NPS threads.

This doesn't matter. At industrial facilities (i.e. meat packing facilities, vegetable packing facilites, non-food related facilities [like clothing or semiconductors], and so on), they higher anyone for the electrical/electronic and other maintenance departments. These employees tend to gorilla tight the tapered threads whether it be electrical or mechanical pipes and pipe fittings. They flare.

I been told by journeyman electrican that there is a minimum amount of working threads that are allowed. I see this as the problem. They cut too many threads. I believe that this overage causes them to overtighted before the fittings bottom out. There should be a maximum amount as well as a minimum amount. So when the employee overtightens and bottoms out the fitting the threads act as some sort of shear point and the pipe breaks forcing the employee to redo his/her work and be more careful next time.

At these industrial plants they contract low budget electrical and millwright contractors and they are all licensed and certified. These contractors do the same as the actual employees and overtighten the fittings.

The electrical employees and electrical contractors have also been known to use water pipe fittings for electrical fittings when they don't have enough electrical fittings.
 
Widget.

Conduit threads are NPT (National Pipe Thread) which are tapered threads

Most conduit fittings (excluding conduit couplings and reducing bushings) are also tapered threads.

The conduits is not supposed to 'bottom out' it is supposed to be tightened till you get an interference fit between the tapers.

Conduit couplings are straight cut, this allows the conduits to thread into the couplings until the end pipe thread meets the start of the coupling assuming the conduit is threaded normally.

I really do not understand the issue you ar seeing.
 
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