Back-to-Back Electrical Box Installation: Simplify Multifamily Projects with Allied Moulded
Mirrored floor plans are common in apartments, condos, and duplexes. But placing electrical boxes on opposite sides of a shared wall can add installation steps and material costs. Allied Moulded’s back-to-back-rated fiberglassBOX™ products offer a simpler option for qualifying wall assemblies.
According to Allied Moulded’s Back-to-Back FAQ flyer, eligible boxes can be installed directly behind one another—or on the same stud facing opposite walls—without putty pads when the listed installation requirements are met.
What Does Back-to-Back Mean?
The flyer distinguishes between a standard rating requiring at least 3 inches of horizontal separation within the same cavity and the back-to-back rating, which permits less than 3 inches of spacing or mounting on the same stud facing opposite walls.
For contractors working with mirrored kitchens, living rooms, or utility rooms, this flexibility can simplify box placement and streamline electrical wiring.
Less Material, Fewer Installation Steps
For eligible installations, eliminating putty pads reduces material needs and the labor required to apply them. Across repeated layouts in a multifamily project, those savings can add up. Actual savings depend on the number of qualifying boxes, material prices, and installation labor.
Which Wall Assemblies Qualify?
The flyer identifies these key requirements:
- Eligible Allied Moulded fiberglassBOX models covered by the back-to-back rating.
- A 2-hour fire-resistance-rated wall assembly.
- Wood and/or non-load-bearing steel studs with gypsum construction.
- A minimum wall cavity depth of 5½ inches, as in the minimum single 2×6 wall construction described in the flyer.
- Non-metallic sheathed cable.
- Total box openings no greater than 100 square inches per 100 square feet of wall.
A single 2×4 stud wall does not provide the required 5½-inch cavity depth. Product eligibility and the complete wall assembly requirements matter; the back-to-back rating does not apply to every electrical box or wall configuration.
Common Questions About Back-to-Back Installation
Can five-gang boxes be installed back-to-back?
Yes. Allied Moulded reports that two eligible five-gang fiberglassBOX outlet boxes were tested directly behind one another in the same stud cavity and met the requirements for the described 2-hour-rated wall construction with a minimum 5½-inch cavity depth.
Is caulk required where the cable enters the box?
The flyer states that no caulk is necessary at knockouts penetrated by non-metallic sheathed cable in the tested, back-to-back-listed products.
What about unused open knockouts?
Unused open knockouts must be closed. The flyer identifies Allied Moulded’s Speed “K” Klamp™ as an accepted closure method. Open, unused knockouts without these clamps require an approved closure method.
Does every fiberglassBOX model qualify?
No. The flyer lists specific eligible series and mounting variants. It states that a box whose base model is not listed is not classified for back-to-back use without an acceptable means of fireblocking. Check the exact model against Allied Moulded’s listing before selecting it for the application.
Plan Your Next Multifamily Project
For projects with repeated or mirrored layouts, Allied Moulded’s back-to-back rating offers an opportunity to simplify installation and reduce putty-pad material and labor costs where the rating applies.
To confirm eligible models and installation requirements, visit Allied Moulded or call 1-800-722-2679.
