boxes in stud space

[trojans4]

Can someone tell me the code reference about back to back boxes in a stud space? I am trying to find out if you can have two switch boxes back to back in a single stud space.

 

[Smart $]

Can someone tell me the code reference about back to back boxes in a stud space? I am trying to find out if you can have two switch boxes back to back in a single stud space.

I don't believe there is any restriction in the NEC. However, as I recall, there is in building codes for fire rated walls.

 

[electricalist]

Fire rated walls are a no no. Plus it seems like back to back with 5/8 mud ring it may be a little too wide for 3.5 in studs. As I remember 4sq deep being too big. Or it could have been lack of strapping space on the stud to keep distance from edge of stud code compliant.. thats if your using mc. And me assuming 2 switch legs 1 hot in 1 hot out. . If its for plugs you might spend more time trying to strap a mc . Jumper if only 1 box is yours and its more damage or do it. Do it. Although the other stud will be easier.

 

[Nom Deplume]

Fire rated walls are a no no. Plus it seems like back to back with 5/8 mud ring it may be a little too wide for 3.5 in studs. As I remember 4sq deep being too big. Or it could have been lack of strapping space on the stud to keep distance from edge of stud code compliant.. thats if your using mc. And me assuming 2 switch legs 1 hot in 1 hot out. . If its for plugs you might spend more time trying to strap a mc . Jumper if only 1 box is yours and its more damage or do it. Do it. Although the other stud will be easier.

Two 1-1/2" deep 4" sq boxes are 3" total and will fit back to back in a 3-1/2" stud wall.

 

[trojans4]

Thanks for the replies. I was thinking there was some rule about back to back boxes in a stud space but couldn't find it in the NEC code book. So localities could prohibit it as far as fire codes it appears.

 

[jumper]

We have a very knowledgable member here from Iowa also. Hopefully he see this thread and chime in on local codes that could/would apply.

 

[arcsnsparks98]

I wired a sprint store last year (knoxville, tn) and the inspector had me go to the tenants space on the other side of the firewall between them and locate any boxes in the wall. That way I could place my boxes in a different cavity. Also, any boxes over 16 in. sq. had to have a fire rated mastic placed inside them to seal any place that air could move through. Building code, not NEC.

 

[iwire]

Thanks for the replies. I was thinking there was some rule about back to back boxes in a stud space but couldn't find it in the NEC code book. So localities could prohibit it as far as fire codes it appears.

Yes, it is spelled out in the UL orange book.

Also when dealing with a fire wall typical old work boxes are not allowed. The boxes must be secured to a framing member.

 

[north star]

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trojans4,

The Building Code would govern if your application is a fire rated wall assembly.
From the `12 IBC, Section 714.3.2 Membrane penetrations:
"Membrane penetrations shall comply with Section 714.3.1.........Where walls or
partitions are required to have a fire-resistance rating, recessed fixtures shall
be installed such that the required fire-resistance will not be reduced.
Exceptions:
1. Membrane penetrations of maximum 2-hour fire-resistance-rated walls and
partitions by steel electrical boxes that do not exceed 16 square inches
(0.0 103 m²) in area, provided the aggregate area of the openings through the
membrane does not exceed 100 square inches (0.0645 m²) in any 100 square
feet (9.29 m²) of wall area.........The annular space between the wall membrane
and the box shall not exceed ¹/₈ inch (3.1 mm).........Such boxes on opposite
sides of the wall or partition shall be separated by one of the following:

1.1. By a horizontal distance of not less than 24 inches (610 mm) where the
wall or partition is constructed with individual non-communicating stud cavities;

1.2. By a horizontal distance of not less than the depth of the wall cavity
where the wall cavity is filled with cellulose loose-fill, "rock wool" or slag
mineral wool insulation;

1.3. By solid fire blocking in accordance with Section 718.2.1;

1.4. By protecting both outlet boxes with listed putty pads; or

1.5. By other listed materials and methods.

2. Membrane penetrations by listed electrical boxes of any
material, provided such boxes have been tested for use in
fire-resistance-rated assemblies and are installed in accordance with
the instructions included in the listing........The annular space between
the wall membrane and the box shall not exceed ¹/₈ inch (3.1 mm)
unless listed otherwise..........Such boxes on opposite sides of the wall
or partition shall be separated by one of the following:

2.1. By the horizontal distance specified in the listing of the electrical
boxes;

2.2. By solid fireblocking in accordance with Section 718.2.1;

2.3. By protecting both boxes with listed putty pads; or

2.4. By other listed materials and methods.

3. Membrane penetrations by electrical boxes of any size or type,
which have been listed as part of a wall opening protective material
system for use in fire-resistance-rated assemblies and are installed
in accordance with the instructions included in the listing.

4. Membrane penetrations by boxes other than electrical boxes,
provided such penetrating items and the annular space between the
wall membrane and the box, are protected by an approved membrane
penetration firestop system installed as tested in accordance with
ASTM E 814 or UL 1479, with a minimum positive pressure differential
of 0.01 inch (2.49 Pa) of water, and shall have an F and T rating of not
less than the required fire-resistance rating of the wall penetrated
and be installed in accordance with their listing.

5. The annular space created by the penetration of an automatic
sprinkler, provided it is covered by a metal escutcheon plate.



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[chris kennedy]

If you have an interest in the correct terminology when you have boxes in stud spaces you are most likely dealing with a fire partition not a fire wall.

Also from the 2012 IBC

FIRE PARTITION. A vertical assembly of materials designed to restrict the spread of fire in which openings are protected.

FIRE WALL. A fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.

 

[jxofaltrds]

RCO -IRC based

302.4.2 Membrane penetrations.

2. Membrane penetrations by listed electrical boxes of any materials
provided the boxes have been tested for use in fire-resistance-rated
assemblies and are installed in accordance with the instructions
included in the listing. The annular space between the wall
membrane and the box shall not exceed 1/8 inch (3.1 mm) unless
listed otherwise. Such boxes on opposite sides of the wall shall be
separated by one of the following:
2.1.By the horizontal distance specified in the listing of the
electrical boxes;
2.2. By solid fireblocking in accordance with Section 302.11;
2.3. By protecting both boxes with listed putty pads; or
2.4. By other listed materials and methods.

Most of us use:

302.11 Fireblocking. In combustible construction, fireblocking shall be provided
to cut off all concealed draft openings (both vertical and horizontal) and to form
an effective fire barrier between stories, and between a top story and the roof
space.
302.11.1 Fireblocking materials.
1. Two-inch (51 mm) nominal lumber.

 

[JDB3]

Sure sounds like some high paid attorney wrote some of the requirements!!!!!!!! :dunce:

 

[north star]

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"Sure sounds like some high paid attorney wrote some of the requirements!!!!!!!! :dunce: "

Actually, the various codes & standards are composed, ...voted on
by the various Code Officials across this great land [ i.e. - Code
Committees, Code Making Panels, etc. ].

A lot of times, we are our own worst enemy !


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