I am evaluating switch rated plugs and receptacles for machinery. A junction box needs to be ordered for the receptacle. I have no idea what "NPT" means in relation to how I want to order the junction box. I understand that NPT has something to do with tapered thread count, but that is the limit of my knowledge. Does the NPT have to match the trade size of conduit used leading to the junction box?
Junction Box NPT
- Thread starter fifty60
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A junction box at the termination of an EMT conduit (rrrrrr... tubing) should have KO's (knockouts). If the box is to contain a receptacle, it would be a device box, though the cover may be the distinguishing factor between a junction and device box (e.g. raised cover on a 4" sq. box). Depending on environmental conditions, sometimes a bell box with threaded holes and hole plugs is used to keep the interior debris and finger free.
Sierrasparky
Senior Member
- Location
- USA
- Occupation
- Electrician ,contractor
Those are all ordered with internal pipe threads. So 1/2 conduit uses 1/2 npt
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If you're using EMT with an EMT connector into the threaded box then NPT or straight thread (NPSM) wouldn't matter since the EMT connector has a straight thread. Code wise there may be an issue.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Both RMC and IMC are required to have tapered threads.
Darr247
Member
- Location
- Michigan USA
It's always been my understanding that NPT stands for National Pipe Taper (otherwise "NPT thread" is redundant).
I'm not going to buy a copy of ANSI B1.20.1 and post it here to prove that, though.
Either i am missing something or i am just a simlpeton. You use the size conduit you need for your wires and that is the size pipe thread you order on the box. nothing more nothing less. it is simply a method to terminating the conduit to the box. EMT, IMC, Rigid, PVC, Flex etc. any conduit with threaded end, connectors or adapters. All conduit (raceway) has them. 1/2" conduit 1/2" thread 3/4" conduit 3/4" thread so on and so on.
The something you are missing is that [typically] boxes with threaded entries are only listed for use with threaded conduits (IMC/RMC)... and not with conduit, tubing, or cable connectors.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Basic principle:
Connectors have running (non-tapered) threads so that you can go through a variable thickness wall and still tighten the nut exactly the same way on the other side.
Pipe fittings which are many threads deep can use tapered threads because you will always seat the pipe to approximately the same depth in them.
A tapered thread can produce a water/gas tight seal. A running thread cannot, without use of a gasket, etc.
I never looked into the listing of threaded-entry bell boxes, only having a few threads at most and usually supplied with a few screw-in hole plugs. Don't have time to research with current work schedule. Just wondering whether they are listed for threaded conduit only, or because they have only a minimal number of threads, are okay for use with connectors...???
Then i guess i am missing something, as i have never seen this before. So a weatherproof UF connector in a bell box is illegal? a Sealtite connector is illegal? Never had an inspector pick this up.
I will do some more research
I'm uncertain about threaded-entry bell boxes (see my next most recent post in this thread). Almost all other threaded-entry boxes that I'm aware of are listed for use with threaded conduit.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Take a look at this article:
http://www.ecmag.com/section/codes-standards/fittings-hubs
For what it's worth 99% of all residential services around here utilize an SE connector into a meter hub.
http://www.ecmag.com/section/codes-standards/fittings-hubs
For what it's worth 99% of all residential services around here utilize an SE connector into a meter hub.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
And therefore are probably not 100% watertight (not a problem if the meter box can drain
) and may have poor grounding performance, but are not being relied on for grounding in this situation (The connector is not being used to bond the SE to the EGC system, that function is coming from the bonding jumper firmly attached to the firmly attached neutral.) In this particular application, the poor performance is not necessarily a serious concern. If you do the same thing in a horizontal run of EMT in a wet area or where the EMT is being used as the EGC, you can have real problems, according to the results cited in the article.
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