Outlet, Device, Pull & Junction Boxes and Conduit Bodies

Article 314 sizes the humble box. Every conductor, device, clamp and equipment-grounding conductor claims a volume allowance, and the total can't exceed the box's cubic-inch rating. The point is heat: a stuffed box can't shed the warmth of its splices, and the crowding nicks insulation during makeup.

The article also demands that junction and pull boxes stay accessible — no burying a splice behind drywall — and sets how boxes must be supported and how far they can be set back from a finished surface. For large pull boxes it gives distance formulas so conductors aren't bent tighter than they can stand.

When it applies

Article 314 governs sizing and supporting your boxes and conduit bodies. Every conductor, device, clamp, and equipment ground inside counts toward box fill, and any splice has to stay accessible behind a cover you can reach later.

Common mistakes

• Cramming too many wires in a small box. Overfilled boxes overheat and make terminations fail.
• Burying a junction box in a finished wall with no accessible cover. Splices must stay reachable.
• Forgetting that clamps, devices, and grounds each add to the fill count, not just the conductors.

In the field

I opened a switch box in a remodel that was so packed I could barely fold the wires back in. Someone ignored the fill math and the connections were stressed and warm. I stepped up to a deeper box and everything relaxed. Do the fill calculation before you rough in, and never plaster over a junction. The next electrician, maybe you, has to get back into it.