I want to connect a wire to get power for for a ceiling fan to an existing junction box in the attic that is used right now only for a smoke detector. Box currently has a 12-3 wire coming in, and the smoke detector wires (connected to a quick disconnect). Plastic box says it can take 8 12-gauge wires. How do I count the three smoke detector wires (white, black, red, smaller than 12-guage)? What's my total count so far, and can I add another 12-2 wire?

I believe the smoke detector wires are 14. Attaching picture.enter image description here

  • 1
    When 120v interconnected smoke alarms first appeared many local jurisdictions passed local amendments requiring those detectors to be on their own dedicated circuit. Over the years the requirement has been largely dropped. (Load is almost zero, they aren’t susceptible to interference, and a shared circuit is more likely to be discovered tripped, not to mention NFPA never addressed it) however, there are still some state and localities that still have the dedicated circuit requirement.
    – Tyson
    Commented Apr 22, 2019 at 18:21
  • I take it the smoke detector comes with its own wire pigtails? Can you tell us what gauge they are? Commented Apr 23, 2019 at 0:40

1 Answer 1


For box fill calclations, you start with cubic inches...different wires take different amounts of space.

  • 18 AWG wires take 1.5 cubic inches
  • 16 AWG wires take 1.75
  • 14 AWG takes 2.00
  • 12 AWG takes 2.25
  • 10 AWG takes 2.50
  • 8 AWG takes 3.00
  • 6 AWG takes 5.00 cubic inches

Larger wires require special engineering.

all grounds are broken out and count only as 1 of rhe largest ground wire.

All cable clamps together count as 1 of the largest wire clamped.

If there is a yoke (mounted piece of equipment in the box), that counts as 2 of the largest wire going to it.

You have to total up the cubic inches and make sure the box is big

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.