Are metal junction boxes permitted inside panel chases or are plastic work boxes required by code?

Both metal junction boxes and non-metallic plastic boxes are permitted by electrical code inside continuous insulation panel assemblies, provided standard non-metallic boxes pair with NM cable and metal boxes are properly grounded.

Running branch wiring through continuous insulation panels eliminates thermal bridging from stud cavities, but installers face distinct National Electrical Code (NEC) rules when mounting outlet and switch boxes. Across residential construction in the United States and Canada (NEC 2023 and CEC Section 12), cavity depth constraints in 2.0-inch to 2.5-inch foam panels dictate box selection, volume ratings, and attachment methods.

If you only do one thing: Choose standard non-metallic single-gang or old-work remodel boxes rated for at least 14 to 18 cubic inches when running 14/2 or 12/2 NM-B wire.

• Code compliance (NEC Article 314): Non-metallic plastic boxes are approved for non-metallic sheathed cable (Type NM-B), while metal boxes are code-compliant for both armored cable (Type MC or AC) and NM-B as long as the metal box is bonded directly to the equipment grounding conductor per NEC 250.148.
• Box depth specifications: Standard InSoFast panels feature 2.0-inch or 2.5-inch EPS thickness, which accommodates 1.25-inch to 1.5-inch shallow electrical boxes or 2.0-inch old-work plastic boxes without requiring chisel work into concrete backing or container steel.
• Box volume ratings: Ensure any selected plastic or metal box satisfies NEC 314.16 box fill calculations, allowing a minimum volume of 2.00 cubic inches per 14 AWG conductor and 2.25 cubic inches per 12 AWG conductor, plus required allowances for devices, internal clamps, and grounding leads.
• Mechanical fastening: Secure boxes either directly to the integrated polypropylene stud flanges using standard drywall screws or seat them solidly into the cutout foam opening using expanding low-pressure polyurethane foam adhesive.
• Raceway alignment: Cut openings directly in line with the panel's built-in horizontal and vertical electrical chases to allow NM-B wire to enter through rear or side knockouts without sharp 90-degree bends.

• Watch out for: Using deep 2.5-inch or 3.5-inch metallic masonry boxes on standard 2.0-inch foam panels, which requires labor-intensive chipping into the structural masonry or steel substrate behind the panel.
• Watch out for: Omitting the grounding pigtail to a metal junction box when pulling non-metallic cable, which creates an electrical shock hazard and violates local building code.
• Watch out for: Over-tightening mounting screws into plastic box tabs, which can crack non-metallic casings and loosen device retention once drywall is hung.

Calculate your circuit amperage and wire count to select between an 18-cubic-inch plastic box or a shallow metal box with a drywall mud ring, then verify specific local amendments with your municipal electrical inspector.