Bushing for a Threaded Hole in an Enclosure

[Jagnnm]

All,

I have an Explosion Proof enclosure with threaded holes. The inside of the enclosure has rough edges and I want to install a bushing to protect the wiring. The only thing I can think of that would work would be a chase nipple. Has anyone seen a better option.

Regards,

 

[Coppersmith]

Normally, boxes that have integral hubs have smooth surfaces where they enter the box. It's odd that yours do not. I see one of the conductor's is a #3, so a bushing would be required. I can't think of a better way (that is listed) than what you have suggested. A chase nipple topped by a plastic bushing. Obviously, you would have to de-terminate or cut the conductors to do this. Another way that isn't listed but would provide protection is to squirt some silicone into the hub and hold the conductors away from the sides while it hardens.

300.4(G) Insulated Fittings. Where raceways contain 4 AWG
or larger insulated circuit conductors, and these conductors
enter a cabinet, a box, an enclosure, or a raceway, the conductors
shall be protected by an identified fitting providing
a smoothly rounded insulating surface, unless the conductors
are separated from the fitting or raceway by identified
insulating material that is securely fastened in place.

Exception: Where threaded hubs or bosses that are an
integral part of a cabinet, box, enclosure, or raceway provide
a smoothly rounded or flared entry for conductors.

 

[Jagnnm]

Thank you for your reply.

 

[wwhitney]

How about a T&B Insuliner? Here's the 1/2", just increment the part number for larger sizes:

http://www.tnb.com/ps/fulltilt/index.cgi?part=422&co=US

Not sure how they are held in place.

Cheers, Wayne

 

[synchro]

I assume these threaded holes are NPT which would have a narrower diameter on the inside of the enclosure. If so then it's likely that you won't be able to screw in an NPS straight threaded chase nipple from the inside of the enclosure. But you can try it.

 

[Forqnc]

I assume these threaded holes are NPT which would have a narrower diameter on the inside of the enclosure. If so then it's likely that you won't be able to screw in an NPS straight threaded chase nipple from the inside of the enclosure. But you can try it.

When we design our Nema 7 boxes, we have to specify thread type, so it could NPT or NPS depending on the designer.

 

[Jagnnm]

The conduit hubs appear to be NPT and the optional device holes are NPS. So I would most likely not be able to use the chase nipple to correct the issue. Thanks for bringing this up. We will most likely have to use an insert like wwhitney suggested.

Thanks for all you support.

Regards,

 

[don_resqcapt19]

When we design our Nema 7 boxes, we have to specify thread type, so it could NPT or NPS depending on the designer.

It is required to be NPT if you want to comply with the NEC, assuming it is not metric.

500.8(E) Threading. The supply connection entry thread form shall be NPT or metric. ...

 

[don_resqcapt19]

Normally, boxes that have integral hubs have smooth surfaces where they enter the box. ...

I have never seen a tapped explosion proof box that had a smoothly rounded surface. If the hubs were cast into the box, then yes, but in this type of box where the holes can be tapped at many locations, all that I have seen look like the picture in this thread.

 

[retirede]

It is required to be NPT if you want to comply with the NEC, assuming it is not metric.

What exactly is a “supply connection”?

We always specified NPS threads for NEMA 7 boxes used for control systems.

 

[don_resqcapt19]

What exactly is a “supply connection”?

We always specified NPS threads for NEMA 7 boxes used for control systems.

I assume that the "supply connection" is the entry into the enclosure. it is my opinion that all threads used for any fittings in a Classified area must be NPT.

I looked at the spec sheets for 3 different brands of NEMA 7 enclosures and they all specify that the tapped holes be NPT.

One of them specified that for Groups B&C that the seal fitting be with in 1.5" of the enclosure. I don't think I have ever seen that spec before. That is much more restrictive than the code rule of within 18".