Garage Construction Project

2026-06-03 - Octagon Box vs 20A Receptacle — Hole Mismatch

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Octagon Box vs. 20A Receptacle — Mounting Hole Mismatch

Southwire 4 inch octagon box with F-bracket above a 20A duplex receptacle, showing the device-yoke screws do not align with the box

Photo Details

  • Date: June 3, 2026
  • Time: 7:58 PM EDT
  • Phase: Phase 1 — 120V rough-in, materials staging
  • Subject: The on-hand Southwire 54151-F-UPC 4” octagon ceiling box (with F-bracket) laid above a 20A duplex receptacle, demonstrating that the receptacle’s yoke screws do not align with any hole in the box.

Description

Bench check during staging revealed the duplex receptacle cannot bolt directly to the octagon box — the holes don’t line up. This is by design, not a defective box:

  • Octagon boxes use 8-32 fixture-stud spacing for a light-fixture strap or fan, centered for luminaires.
  • A duplex receptacle uses 6-32 device-yoke spacing — a different pattern entirely.

A receptacle never mounts to the bare octagon box; it mounts to an intermediary single-device cover (e.g., RACO 731) or a 1-gang mud ring. The shopping list already anticipated this cover (see Shopping List — 120V, Cover Plates row), so the bare-box mismatch shown here is expected.

The real reason these boxes are being swapped out is box fill, not the hole pattern — see the Visible Elements note below and the correction in the shopping list.

Visible Elements

  • Box: Southwire 54151-F-UPC, 4” octagon, 1-1/2” deep, integral F-bracket
  • Device: 20A duplex receptacle (commercial/TR), 6-32 yoke screws
  • Mismatch: Device yoke holes fall on the box face between/beside the box’s fixture holes — no alignment
  • Fill problem (the deciding factor): A 1-1/2” octagon is 15.5 in³. A pass-through duplex box (12/2 in + out = 4× #12 @ 2.25 = 9.0, grounds 2.25, duplex device 4.5) needs 15.75 in³ → over the limit per NEC 314.16(B). The daisy-chained ceiling outlets are pass-through, so the octagons are non-compliant for those positions.

Resolution

Switch the eight ceiling duplex positions (3 GDO + 2 cord reel + 3 vehicle-bay lighting) to 4” square (1900) boxes, 2-1/8” deep (30.3 in³) + 1-gang 5/8”-raised mud ring (5/8” matches the Type X ceiling drywall). Keep one octagon for the stairwell light fixture, where an octagon is the correct box and a fixture mounts to it directly. Return the surplus octagons.

Graph View

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