I have a single-gang switch box (12.5 cu) that has 3 14/2 wires inside of it. I would like to re-wire the switch to be a 3-way switch.

As far as I can tell, the 3 wires are power to the box, power from the switch to the lights and then a 3rd power line to additional outlets (I have yet to 100% verify this).

I was hoping to run 14/3 from the existing switch to the new one and leave the old wiring to the lights alone. That being said, by my count the 3 wires alone should be in a 20.0 cu box so adding in a 4th would be a bad idea.

What alternatives do I have? Do I need to wire the 3-way switch through the lights and to the new switch?

Any guidance is appreciated.



Actually your count is off.

The correct total is 24 cu. in

After adding a 14/3 you'd have 12 conductors...

  • Tapped current carrying conductors = 9
  • All the grounds = 1
  • The switch = 2
  • 12* 2 = 24 cu. in

Update, some of the math was off. Now corrected.

  • 1
    Isn't 12 * 2 = 24 cu ins? – ArchonOSX Dec 29 '15 at 17:08
  • You were right the first time. Yoke mounted devices (switches and receptacles) count as a double volume allowance. From the 2011 NEC 314.16(B)(4) (4) Device or Equipment Fill. For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table 314.16(B) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap. – ArchonOSX Dec 29 '15 at 18:06
  • I owe you one ArchonX – Kris Dec 29 '15 at 18:18

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