Installing a ground bar in a NEMA 3R box

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Mr. Serious

Mr. Serious

Senior Member
Location
Oklahoma, USA
Occupation
Electrical Contractor
I haven't had my contractor license for very long, so I keep reading things to make sure I'm doing everything properly.

I've read a lot of forum threads on properly connecting raceways to outdoor enclosures, and I understand most of it.

But, what I haven't seen addressed at all is this: how would you mount a ground bar inside a NEMA 3R enclosure? I intend to use a 16" x 16" enclosure for a pull box, with about 6 conduits coming into it. The current-carrying conductors would pass through without splices or connections, but I want to connect all the equipment grounding conductors together in the box, and one of the conduits will have only a grounding electrode conductor in it.

Indoors, I have installed ground bars by tapping a couple of holes for 10-32 screws in the back of the box, and mounting the ground bar with machine screws. I considered doing it that way for an enclosure on the outside of the house. If I make threaded holes with machine screws in them to mount a ground bar, would that enclosure still be watertight? Good enough, at least? NEMA 3R enclosures already have holes in the back for mounting, and I think water would be just as likely to get inside there as through extra mounting holes in the back of the box. Is there any code prohibition against tapping holes to mount a ground bar in a 3R enclosure in a wet location? If there are no rules, can anyone offer an argument about whether or not it's a good idea?

I know some enclosures come with one stud on the inside for the purpose of bonding, but I was thinking of using a Milbank 16164-SC3R-NK, and the information doesn't show that it has one. So it seems like you would have to tap a hole in it to properly ground it anyway, or rely on lock nuts with metal conduit.
 
You can order 3R enclosures with mounting studs for this purpose.

Roger
 
There is no issue with threaded holes in a 3R enclosure. There may be an issue with the thickness of the enclosure and 250.8(A)(5).
 
don_resqcapt19 said:
There is no issue with threaded holes in a 3R enclosure. There may be an issue with the thickness of the enclosure and 250.8(A)(5).
Click to expand...

Thank you. So the metal would need to be 1/16 inch thick to engage two threads if I use screws with 32 threads per inch. The Milbank document says all those enclosures are 12, 14, or 16 gauge steel. Going by the listed weight, estimating the sheet metal area, and looking up sheet steel specifications, I find that the box I was interested in must be 16-gauge, just a little too small to catch two threads. If it was one of the bigger boxes made from 14 or 12 gauge steel, the metal would be thick enough, but in this case with the 16-gauge steel, I need to use a nut on the back to secure it.
 
A 3-R enclosure need not be water-tight, as water must be allowed to drain.

You just don't want any unsealed penetrations at or above energized parts.
 
Mr. Serious said:
Thank you. So the metal would need to be 1/16 inch thick to engage two threads if I use screws with 32 threads per inch. The Milbank document says all those enclosures are 12, 14, or 16 gauge steel. Going by the listed weight, estimating the sheet metal area, and looking up sheet steel specifications, I find that the box I was interested in must be 16-gauge, just a little too small to catch two threads. If it was one of the bigger boxes made from 14 or 12 gauge steel, the metal would be thick enough, but in this case with the 16-gauge steel, I need to use a nut on the back to secure it.
Click to expand...
Yes, many enclosures are not thick enough to engage two threads of a 32 thread per inch screw. That includes many of the cabinets used for panelboards.
 
Plastic 3R box.

-or-

Through bolt and nut. Dab of silicone if it makes you feel better.

—and-

Split bolt for the grounds instead of running all the EGCs to the bar. Just install a lug and run one conductor from the lug to the split bolt. 👍
 
I always try to mount them to the back of the box, rather than the bottom. I've seen too many ground bars that were mounted on the bottom, corroded and covered with dirt.
 
If u are not making any splices and just pulling the current carrying conductors straight through, I think u don't need to ground the pull can but if the CCC and ground are all black conductors u do need to mark the ground.
 
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