Metal box & NM cable

dnbob

Senior Member
Location
Rochester, MN
I saw an install today that they used all metal boxes with NM cable. Other than fire rating, which does not apply, and better protection from drywall roto zips, is there an advantage to use these? It was a commercial building. Most were shallow, so box fill didn't seem to play in either.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I saw an install today that they used all metal boxes with NM cable. Other than fire rating, which does not apply, and better protection from drywall roto zips, is there an advantage to use these? It was a commercial building. Most were shallow, so box fill didn't seem to play in either.
If redundant grounding is desired or required, the metal box allows the yoke of a "self-grounding" device to serve as an alternate EGC path. Belt and suspenders. Not as significant in labor saving as when you are are also using metallic raceways though.
Possibly just leftover specifications?
Some commercial buildings are not allowed to use NM, so there may have been a note on the specs intended to go with metallic raceway or cable?
 

renosteinke

Senior Member
Location
NE Arkansas
I use metal boxes almost exclusively.

They're stiffer, and mount better than the plastic ones. I very much prefer the clamping arrangement. Since I use 4-squares with mud rings, I always have plenty of space. I can readily transition to other wiring methods as needed.

Of course, the real reason is that I learned the trade in a place that did not allow NM, and my practice has focused on commercial and industrial work. Plastic boxes and NM just don't seem 'right' to me!
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
Now here you never see NM in a commercial building. So I really don't know why they would use metal boxes and NM cable. Might just be like Reno said.
 

PetrosA

Senior Member
I know guys who only use metal with NM, but that's resi and I don't understand them.

In commercial work I would only use metal boxes unless it were clearly a building where plastic were allowed and I was retrofitting something (you can't get old work metal boxes to fit into the hole left by a plastic box). Many times just the fact that commercial buildings have metal stud walls is enough of a reason to use metal, since it's so flimsy to begin with that you need the extra support that metal box designs can offer. The kind of wire I'd use would also be up to what was allowed. Many commercial buildings here are required to be in MC, but some are allowed to be in NM for various reasons. One such example would be a very old building being used commercially where the framing would make pulling MC or AC next to impossible without exposing studs to avoid damaging the cable (and many owners would rather preserve horsehair-on-lath plaster than redo the walls...), but even there I'd use metal boxes since plastic old work boxes are not designed properly for that kind of plaster wall.
 

RLyons

Senior Member
I thought I have heard that you can use NM commercially if the building is under 3 stories???
I like the 4x4 w/muddring idea but not to fond of the gangable boxes when attached to metal studs as the always seem to make outlets mushy...unless there is a trick I am missing.
Are you guys actually using plastic? or fiberglass boxes? We always use fiberglass.
 

PetrosA

Senior Member
I thought I have heard that you can use NM commercially if the building is under 3 stories???
I like the 4x4 w/muddring idea but not to fond of the gangable boxes when attached to metal studs as the always seem to make outlets mushy...unless there is a trick I am missing.
Are you guys actually using plastic? or fiberglass boxes? We always use fiberglass.
I don't know what they're called, but they used to make simple rails for old work wallcases for mounting on studs in plastered homes which was just a hunk of sheet metal bent over on itself the depth of the ears on an old work box. You see them in this area in homes built in the 20s and 30s. Set the ears back, nail one rail up, prop the box in, nail the other rail on and you're set. Very secure. Maybe they still make something like that?

Here in SE PA I can't even think of one supply house where I've seen fiberglass boxes in stock. Most of the ones I've seen were in houses built back in the 70s and 80s. I think guys saw the issues with the screw holes stripping out and shied away after that.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
I thought I have heard that you can use NM commercially if the building is under 3 stories???
I like the 4x4 w/muddring idea but not to fond of the gangable boxes when attached to metal studs as the always seem to make outlets mushy...unless there is a trick I am missing.
Are you guys actually using plastic? or fiberglass boxes? We always use fiberglass.
NEC removed the three story restriction several cycles ago but local fire codes may come into play, also the NEC does not allow NM in a dropped ceiling in other than dwelling units.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
when you buy stuff by the skid load the cost of things gets skewed. it is often less expensive to just use the same part for multiple uses if you can rather than buying different parts because you only stock one set of parts instead of more than one.

the workers are generally faster if they use the same parts more often as well, so maybe there is a small amount of labor savings as well. keep in mind that at $60 an hour, a minute of labor costs $1.

if you are doing metal studs the metal boxes are probably your best bet anyway.
 

liquidtite

Senior Member
Location
Ny
S
NEC removed the three story restriction several cycles ago but local fire codes may come into play, also the NEC does not allow NM in a dropped ceiling in other than dwelling units.
I thought nm was allowed in dropped ceiling in commercial buildings as long as the drop ceiling is not a plenum, and if it's under 3, stories
 

jumper

Senior Member
S
I thought nm was allowed in dropped ceiling in commercial buildings as long as the drop ceiling is not a plenum, and if it's under 3, stories
Nope.

334.12 Uses Not Permitted.
(A) Types NM, NMC, and NMS. Types NM, NMC, and
NMS cables shall not be permitted as follows:

(2) Exposed in dropped or suspended ceilings in other
than one- and two-family and multifamily dwellings
 

RLyons

Senior Member
I don't know what they're called, but they used to make simple rails for old work wallcases for mounting on studs in plastered homes which was just a hunk of sheet metal bent over on itself the depth of the ears on an old work box. You see them in this area in homes built in the 20s and 30s. Set the ears back, nail one rail up, prop the box in, nail the other rail on and you're set. Very secure. Maybe they still make something like that?

Here in SE PA I can't even think of one supply house where I've seen fiberglass boxes in stock. Most of the ones I've seen were in houses built back in the 70s and 80s. I think guys saw the issues with the screw holes stripping out and shied away after that.

I have seen the box with arms in older houses and boy they are a pain to remove without destroying the surrounding plaster.

The only time I have stripped out a fiberglass box is when the screw is to short to make a good grab, and if that happens I switch to a fine thread sheetrock screw.
The plastic boxes I've seen always get ruined by the rotozip not to mention the havoc spackle creates in the screw holes.

I've seen a lot of nm above drop ceilings what year was that considered legal?
 
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