Compute Conductor Equivalency for Devices and Box Fittings
Count the number of device mounting straps or yokes. Count each device strap as two conductors. A double-gang strap, one too wide to mount in a single-gang box counts as four conductors.
Count each fixture mounting stud or Hickey as one conductor. Count all internal cable clamps as one conductor. You must count these equivalent conductors as if they were real conductors when calculating conductor fill. The box volume that these equivalent conductors requires is based on the largest conductor in the box.
Multiply the total number of equivalent conductors by the volume required by the largest conductor in the box. The most common size conductors used are AWG 14, 12, 10, 8 and 6 which require 1.50, 1.75, 2.25, 2.75 and 4.50 cubic inches respectively.
Computing Volume Required for Actual Conductors
Count the number of conductors terminating in the box. Every conductor, except for ground conductors, entering but not passing through the box, count as one conductor each.
Count each wire, except for ground wires, passing through the box unbroken as one conductor each.
Count all the system ground wires, the bare copper or green insulated wires, as one conductor no matter the actual number.
Calculate the Required Box Size
Compute the total number of conductors to be installed in the box, both real and equivalent. If all the conductors are the same size, refer to NEC Table 314.16(A) for the box size needed. If the conductors are of different sizes go to the next step.
Multiply the volume required for each wire size by the total number of wires having that size. Multiply the total number of equivalent conductors by the volume required for the largest conductor.
Add up the subtotals and refer to the volume column of NEC Table 314.16(A) to ascertain the correct box size.