In a 3-gang masonry box stamped at 47.4 in3 that will contain two 4-way switches and one 3-way switch, with eleven 12 AWG THHN/THWN conductors terminating on the switches and no EGCs installed, how many additional 12 AWG conductors may be pulled through this box unspliced and unlooped?

• A. 2
• B. 3
• C. 4
• D. 5

Answer: (C)

Box fill is determined by NEC 314.16(B): every conductor that originates outside the box and terminates or is spliced inside counts as one fill unit, every conductor that passes through unspliced counts as a half unit, all equipment grounding conductors (EGCs) together count as one unit, and each device yoke adds one unit. For 12 AWG conductors, one fill unit equals 2.25 in³.

In this 3-gang masonry box, eleven 12 AWG conductors terminate on the switches, so they contribute 11 units. The box has three devices (two 4-ways and one 3-way), so add 3 more units for the device yokes. Total fill units so far: 11 + 3 = 14. Volume used by these 14 units: 14 × 2.25 = 31.5 in³. Box volume is 47.4 in³, so remaining volume is 47.4 − 31.5 = 15.9 in³.

Through conductors count as one-half of a unit each, which is 1.125 in³ per 12 AWG conductor. Maximum additional through conductors = floor(15.9 ÷ 1.125) = floor(14.13) = 14. Therefore, up to 14 more 12 AWG conductors could be pulled through unspliced and unlooped in this box.