Receptacle in a knotty pine wall

The answer

The box is recessed behind the face of the wood paneling; in a combustible surface the box, ring, or extender must be flush with the surface or project from it.

Correct choice: 1. The box is recessed behind the face of the wood paneling

314.20 lets a box sit back up to 1/4 in. in noncombustible finishes such as drywall, plaster, or tile. In walls of wood or other combustible material, the box front edge, plaster ring, extension ring, or listed extender must be flush with the finished surface or project from it. Here the paneling went up over boxes set for the original wall, so the box edge is set back by the thickness of the boards, leaving the raw wood of the cutout exposed to any heat or arcing at the device terminals. The receptacle ears resting on the pine do not cure it: 406.5(A) covers holding the yoke at the surface only for boxes set back as 314.20 permits, and in a combustible surface that setback is zero. This is common in Minnesota cabins and basements where tongue-and-groove pine went up over existing wiring.

How to fix it

Code references

• NEC 2023 314.20 — Flush-Mounted Installations

  In noncombustible surfaces such as concrete, tile, gypsum, or plaster, the front edge of the box, plaster ring, extension ring, or listed extender shall not be set back more than 1/4 in. from the finished surface. In walls or ceilings of wood or other combustible surface material, the box, ring, or listed extender shall be flush with the finished surface or project from it.

  Drills citing 314.20

• NEC 2023 406.5(A) — Boxes That Are Set Back

  Receptacles mounted in boxes set back of the finished surface as permitted by 314.20 shall be installed so the mounting yoke or strap is held rigidly at the finished surface.

  Drills citing 406.5(A)

2026 NEC:same rule and number. No 2026 change to 314.20 turned up in Article 314 change summaries; the 1/4-in. setback for noncombustible surfaces and flush-or-project rule for combustible surfaces appear unchanged. Not confirmed against the 2026 text. Confidence: low

Minnesota

Why the other choices are wrong

• The gap around the box is over 1/8 in. and must be patched. The 1/8-in. gap repair rule in 314.21 is written for noncombustible surfaces such as drywall and plaster; the problem in wood is where the box edge sits.
• Only metal boxes are permitted in wood-paneled walls. Nonmetallic boxes are fine in combustible walls; what matters is that the box front edge, ring, or extender is flush with or projects from the wood.
• Nothing. This installation is compliant. The 1/4-in. setback allowance in 314.20 applies only to noncombustible surfaces. In wood or other combustible finishes the box must be flush or project.