Quick Hits

EPISODE 8 — Construction Specs: Material, Spacing & Doors — Article 312 FINALE (312.100, 312.101, 312.102)

Slide 1 — Hook "Final episode of Article 312! New Part — Construction Specifications. How these boxes are actually built, and the internal clearances they need."

Slide 2 — 312.100 Material

• Metal cabinets/cutout boxes need corrosion protection inside AND out.

• Strength: enclosures need enough strength/rigidity for the job — sheet steel construction can't be thinner than 0.053" uncoated.

Slide 3 — 312.101 Spacing (the internal clearance rules)

• Base: at least 1/16" airspace between a device's base and the metal cabinet wall (except at actual support points).

• Doors: at least 1" airspace between any live metal part (including enclosed fuses) and the door — reduced to ½" if the door is lined with approved insulating material or made of metal at least 0.093" thick uncoated.

• Live parts: at least ½" airspace between cabinet walls/back/metal gutter partitions/door and the nearest exposed current-carrying part, for voltages up to 250V — bumped to 1" for voltages 251-1000V AC/1500V DC (with a narrower ½" allowance for 251-600V if the same lined/thick-metal-door condition from above is met).

• Switch clearance: cabinets/cutout boxes must be deep enough that doors can still close with 30A branch-circuit panelboard switches, combination cutout switches, or other single-throw switches in ANY open position.

• Wiring space: enclosures connecting to MORE than 8 conductors (branch circuits, meter loops, feeders, power circuits, etc. — not counting the supply circuit) need back-wiring space or side gutters/compartments — and those side spaces generally need to be tight enclosures via covers/barriers/partitions (exception for side spaces holding only conductors that enter directly opposite their terminating device).

Slide 4 — Quiz + CTA Q: "For voltages 251-1000V AC (1500V DC), the minimum airspace between a cabinet's walls/back/door and the nearest exposed current-carrying part is:" A) ½ in. B) 1 in. C) 1½ in. D) 2 in.

✅ Answer: B — 312.101(A)(3): 1" (25.4 mm) minimum for that voltage range (versus ½" for 250V or less).

CTA: 🎉 That's a wrap on ALL of Article 312 — Cabinets, Cutout Boxes, and Meter Socket Enclosures! Follow for more daily quick-hit study notes as we move into Article 314 (Outlet, Device, Pull, and Junction Boxes) next.

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