Type V construction is the most common construction type for residential dwellings including single family homes and multifamily dwellings such as apartments.

IBC Section 602.5 states that Type V construction is that type of construction in which the structural elements, exterior walls and interior walls are of any materials permitted by this code.

In Type V construction it is quite common to have fire separation requirements including rated walls, floors or ceilings that the electrician will need to be aware of. Common walls separating sleeping units in a multifamily dwelling are a good example.

In the International Residential Code (IRC) Section R302.4.2 addresses membrane penetrations of electrical boxes installed in fire rated walls.

A membrane penetration is a breach in one side of a floor-ceiling, roof-ceiling or wall assembly to accommodate an item installed into or passing through the breach. When an electrician makes a membrane penetration in a fire-resistance-rated wall, such as installing an electrical box, the exceptions to the rules in IRC R302.4.2 must be followed.

The general rule in R302.4.2 states “Where walls are required to have a fire-resistance rating, recessed fixtures shall be installed so that the required fire-resistance rating will not be reduced.”. IRC Section R302.4.2 provides exceptions to this rule and allows an electrical box to make a membrane penetration and breaks it down into four basic categories:

1. Rules for steel electrical boxes that do not exceed 16 square inches in area
2. Rules for listed electrical boxes of any material
3. Rules for steel electrical boxes of any size where protected by listed putty pads or other listed materials and methods, and installed in accordance with the listing
4. Ceiling membrane penetrations by listed luminaires

In Section R302.4.2, the exceptions permitting membrane penetrations of electrical boxes are as follows:

1. Membrane penetrations of maximum 2-hour fire-resistance-rated walls and partitions by steel electrical boxes that do not exceed 16 square inches in area, provided that the aggregate area of the openings through the membrane does not exceed 100 square inches in any 100 square feet of wall area. The annular space between the wall membrane and the box shall not exceed 1/8 inch. 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 where the wall or partition is constructed with individual noncommunicating 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, rockwool or slag mineral wool insulation.

1.3. By solid fireblocking in accordance with Section R302.11.

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

1.5. By other listed materials and methods.

2. Membrane penetrations by listed electrical boxes of any material, provided that 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 R302.11.

2.3. By protecting both boxes with listed putty pads.

2.4. By other listed materials and methods.

4. Ceiling membrane penetrations by listed luminaires or by luminaires protected with listed materials that have been tested for use in fire resistance-rated assemblies and are installed in accordance with the instructions included in the listing.

Not many steel electrical boxes are listed. Most are not. If installing steel boxes (recessed) closer than 24 inches horizontally on opposite sides of fire rated walls, exception 1 above provides only a few options for the electrician. Most of the other options in exception 1 above apply to the wall framer or wall insulator.

Many electrical companies will bid electrical work without ever considering the additional cost of putty pads or the labor to install them when a plan set shows hundreds of steel electrical boxes installed closer than 24 inches to each other on opposite sides of fire rated walls. Most electrical companies are not familiar with what type of insulation (if any) will be used in the stud cavities and if the insulation type will be installed in such a way that steel electrical boxes can be installed closer than 24 inches horizontally. When bidding the electrical work, or laying out the steel box installation, always look for the wall designs to see if the rated walls have staggered stud configurations, where each stud bay communicates to the adjacent one (see image), or if the walls are built with “noncommunicating stud cavities”. All of these things must be considered by the electrician even though the requirements mentioned above are not outlined in the NEC^®.