Electrical – Octagonal Electrical Box Wire Capacity

electricaljunction-boxwire

The wiring for a bathroom light and a receptacle are in a 3.5 x 1-1/2 in. octagon electrical box. Is this the correct size for this number of wires? There are 3 blacks, 3 whites, 1 red, and 3 grounds. It's a 15 amp circuit. I'm not sure whether the gauge is 14 or 12. The wires in the bushing on the side of the box are Romex. The other wire is 50 yrs old. The house is in the USA. I've read some fill charts on-line, but I don't understand them as there are various increases/decreases in the conductor count for certain situations.

Correction: The box is 3.5 in., not 4 in.

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Best Answer

A random check of a manufacturer shows 3-1/2 x 1-1/2 octagon boxes are 11.8 cubic inches. It may also be stamped on your box. You need:

  • 1 wire unit for all the grounds
  • 4 wire units for your hots
  • 3 wire units for your neutrals
  • 1 wire unit for the internal clamp
  • 2 wire units if a receptacle is inset into that box. If the item is surface mount, 0 wire units.

Total 8 wire units. A wire unit takes 2.0 cubic inches for 14AWG and 2.25 cubic inches for 12 AWG. So that's 18 or 20.25 cubic inches. Your box has 11.8?

You have a few options.

  • Add a box extension, a 1-1/2" extension would solve your space problem. Nobody cares which holes the wires come in, I never use knockouts in extensions because then it's hard to remove.
  • Put a second nearby box to offload some of the wires.
  • Convert to a 4" square box (21 cu. in.) with a mud ring (? cu. in.)giving a face similar to a 3-1/2" octagon box. This may also help you address your depth problem mentioned in the other post. It's ridiculously easy to change boxes since all your wires are flexible and terminate in the box. (by "ridiculously" I'm contrasting it with thru wires which must be pulled, steel conduit, substituting a wider box so you have to cut the conduit, and doing it 21 feet in the air in a boom lift!)
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