atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Cabling Telephones Systems Wiring Back-to-back boxes in fire rated walls

We often run into situations in new construction where boxes are, by blueprint, placed directly back to back. Adjoining hotel rooms are often mirror images of each other.

I cannot track down exact requirements or code on placement of boxes directly back-to-back. NEC 300.21 puts horizontal placement of boxes with less than 24" separation on building codes, manufacturers specs, etc.

Every person I've talked to about this has different answers. "Don't do it", "it doesn't matter", "build a sheetrock box or partition around any back to back boxes", "use putty pads", "depends on the boxes" and "whatever the inspector wants" are just a few of them.

As far as I understand, the concerns are: sound transmission and maintaining a wall's fire rating, in that order.

I'm not too concerned about the sound transmission part, but I am concerned about doing my part in keeping a fire rated wall fire rated from ethical, liability, passing inspections, and cost stanpoints, respectively.

Is there any single definitive source of information that covers different box types' (metal, plastic, fiberglass) spacing and firestopping requirements?

I realize I can use adjustable brackets to keep a direct back-from-back situation from occuring, but 24" separation never happens - many places are very picky where they want their devices.

Staggered stud walls make this even trickier, with just 8" between studs and no fire break anywhere along an entire wall (no sealed stud cavities).

Any help or pointers toward the right direction would be greatly appreciated.

ETA: I probably should have posted this in the general category as it's not a cabling issue.

Jack

Well...yes, maybe.......never........oh, who cares? The local code enforcement people...

Placement of the boxes and wiring, everything, goes back to life, health, and safety issues first, the insurance companies next. NEC was started as Part 5 of the NYC fire code....now look where it is at.

The insurance companies like to receive money, not pay it out. So if the work is done properly the pay-outs are reduced. Then they are happy.

Now for opinion....there really isn't a good way to do back to back without conduit. Most places I have been will not allow back to backs, anyway.
I even had a hospital that would not allow outlets in the hallway walls of the patient rooms, all conduits had to be firestopped at the box, and conduit had to be physically connected to an open wire tray. :shrug:

So, to answer your question, well, maybe, it might, oh, heck, get the local code guy......

Jack you are correct in the 24" spacing unless you add fire rated blockage between the boxes. As for which section of code you find the answer I'm not sure.

The concern is maintaining the fire rating of the partition. All 300.21 says is that your penetration will not result in the spread of fire or products of combustion being substantially increased. So it is up to the local jurisdiction to interpret that and tell you what you can or cannot do. As you have found, that article means different things to different people so I would get it straight from the horses mouth.

-Hal

Thanks for the replies, gentlemen. Getting with the AHJ(s) would be so much easier if we did work primarily in one area.

I think I found what I was looking for:

https://www.constructionbook.com/ul-listing/

Jack

I don't think that will help. No way around it, you need to talk to the AHJ in each area. Otherwise play it safe and don't locate boxes in the same bay.

-Hal

Here in NYC I have installed metal boxes back to back in sheetrock walls. The last time I did this sort of work was about 5 years ago. It was for a New York State Agency in a Federal building. Installed as designed by the Architecht. They were extremely (anally) concerned about box placement.

We had no firestopping between the boxes (not that there was any room for it).

No questions from the inspectors.

However - like Ken said - Hospitals are going to be a different story anywhere. I did the infectious disease wing for Belleve Hospital and everything was firestopped AND sealed.

And like Hal said - every jurisdiction can and will make up their own rules. Get your answer first - and get it in writing.

Sam

The BICSI TDMM (11th Edition) Chapter 7 says:

"Electrical boxes that do not exceed 16 square inches in surface area but create membrane penetrations in a stud wall should be: Separated on opposite faces of walls by a horizontal distance of at least 2ft."

Even though NEC 300.21 doesn't give you a definite answer, the fine print note does give you a clue. As you alluded to in your original question, "Building codes also contain restrictions on membrane penetrations on opposite sides of a fire resistance-rated wall assembly. An example is the 24-in minimum horizontal separation that usually applies between boxes installed on opposite sides of the wall."

Nice! So, the NEC defers to the local building codes. As has been suggested by others, you'll have to check with the GC and the AHJ.

One question for you, why are you putting in boxes any way? My friend, that is sparky's job. Let him do it and you won't have to worry about any of this.

Every time I have to install an electrical box or a piece of conduit, I'm thinking that I've screwed up some where. "How did I fail to plan to get the EC to do this..." :shhh:

Larry makes an excellent point. Why are you installing boxes anyway? Let the sparkie do his job and take the heat if there is a problem.

-Hal

Actually I was working as a Sparkie on that job. My contractor had bid the whole job - Power, lighting, voice and data.

They got Power & Lighting only. So that's what I did. The closest I came to LV work on that job was installing 3/4" EMT stubs for V&D.

Sam