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Electrical box facing to exterior wall for outdoor receptacle (C) Daniel Friedman InspectAPedia.comElectrical Receptacle Box Fire Separation

Offset required for receptacle boxes in the same stud bay

Is it permissible to install an indoor-facing and outdoor facing pair of receptacle boxes in the same wall stud bay?

What offset spacing is required between two such electrical boxes in the same framing cavity, and why?

This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or "plugs" or "sockets") are simple devices that are easy to install, but there are details to get right if you want to be safe.

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Electrical Outlet Box Fire Separation Between Inside Wall & Exterior Wall Receptacles

Plastic electrical boxes in new electrical wiring installation (C) Daniel Friedman InspectApedia.comAn electrical outlet must be properly located on the wall, according to local electrical codes and the National Electrical Code.

Examples of proper electrical outlet locations are shown in our page top sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, eductation, and report writing tool firm.

But what happens when the electrician wants to install an electrical receptacle on both the interior wall (facing the occupied space) and on the exterior wall (facing outside) from within the same stud bay?

Watch out: do not install a pair of interior and exterior electrical receptacle boxes in the same stud bay back-to back. The result of such an installation can violate the fire resistance of the wall. Instead, as you will read below, the boxes need to be offset from one another.

A typical receptacle box offset amount is equal to the thickness of the stud bay, such as 3 1/2" or 5 1/2", but greater offsets are required in fire-rated walls, ceilings, and floors.

Often, for convenience the outdoor electrical receptacle will simply be extended off of an existing circuit of indoor receptacles, but even if the two receptacles and their boxes are on different circuits, they still should not go back-to-back in the same wall cavity.

Question: can there be an indoor (into kitchen) and outdoor outlet in the same bay?

2017/09/01 Sharon said:

In new construction, can there be an indoor (into kitchen) and outdoor outlet (on back porch) in the same bay?

Reply: 3 1/2" or 5 1/2" separation requirements for indoor & outdoor receptacles in the same stud bay

Electrical box facing to exterior wall for outdoor receptacle (C) Daniel Friedman InspectAPedia.comSharon,

You ask an important fire safety question that I would phrase as "What is the required offset or separation between an inside electrical receptacle box and an outdoor receptacle box mounted in the same wall cavity".

The concern is that if the boxes were mounted back-to-back the fire-rating of the wall would be compromised.

Typically in an insulated wall we want to see the electrical receptacle boxes offset from one another by a distance equal to no less than the depth of the wall cavity.

Example: in a 2x4 framed stud wall the boxes would be offset from one another horizontally (assuming they're along the same horizontal line) by no less than 3 1/2" (that's the actual depth of a 2x4).

If the wall were framed with 2x6 studs then you'd want a minimum of 5 1/2" horizontal separation between the indoor and outdoor receptacle boxes in the same stud bay.

In new construction when the wall is still opened and not yet insulated there are two easy solutions to receptacle box separation.

  1. In a wall framed with studs 16" o.c. we have 15 1/2" between the facing sides of a pair of studs in the same stud bay. Using 4" boxes or smaller (in box width) we'd have about 7"
    of separation (15 1/2 - 8") if we simply fastened the indoor electrical box and outdoor electrical box to opposite stud faces.

    In 24" o.c. spaced 2x6 wall framing we'd have even more separation.
  2. In a wall framed with either 2x4 or 2x6 studs, the electrician might simply fasten the indoor and outdoor electrical boxes on opposing sides of the same stud, one box facing the building interior, the other facing the building exterior.

    You can see this installation in the UL document we provide below describing use of special fire-rated electrical boxes.

More Separation Required Between Electrical Boxes in Fire Rated Ceiling & Wall Assemblies

In more-stringent fire-rated wall construction such as walls separating rooms in a hotel or motel, that fire separation distance between electrial boxes on two sides of the wall may need to be as much as 24" as you'll see in this excerpt from the IBC model code:

IBC 711.3.2:

1. Steel boxes that do not exceed 16 square inches (4” x 4”) may be installed provided the total area of openings does not exceed 100 square inches for any 100 square feet of wall area.

2. Boxes on opposite sides of a wall shall be separated as follows:

a. By a horizontal distance of not less than 24”.

b. By a horizontal distance not less than the depth of the wall cavity where the wall cavity is filled with cellulose loose-fill or mineral fiber insulation. (Not standard fiberglass insulation)

c. By solid fire-blocking complying with section 716.2.1. (Not practical for back to back boxes)

d. By other listed materials and methods. (This would include fire stop pads inside boxes or outside that covered all 5 exposed sides)

3. Listed box assemblies that have been tested for use in fire rated assemblies can be used in accordance with their listing and instructions.

Watch out: in fire-rated wall construction where metal stud framing is used fire-resistant putty may be required to seal wiring openings, depending on the electrical box size and circuit. See the 3M Fire Barrier Putty document given below.

References for indoor/outdoor receptacle box separation in walls & in fire-rated walls, ceilings, floors, or fire rated assemblies


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