NEC Table 314.16(B)(1) provides volume allowances to be used when calculating the number of 18 AWG through 6 AWG conductors permitted in a box. According to the table:

• A 14 AWG conductor has a volume of 2 cubic inches
• A 12 AWG conductor has a volume of 2.25 cubic inches
• A 10 AWG conductor has a volume of 2.5 cubic inches
• An 8 AWG conductor has a volume of 3 cubic inches

Section 314.16(B)(1) requires each conductor that originates outside the box and terminates or is spliced within the box to be counted once, and each conductor that passes through the box without splice or termination is also counted once.

Section 314.16(B)(2) is all about clamp fill. “Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 314.16(B)(1) shall be made based on the largest conductor present in the box”.

Section 314.16(B)(4) covers devices or equipment installed in the box. A typical duplex receptacle or light switch requires a double volume allowance to be made based on the largest wire terminating to the device.

Section 314.16(B)(5) covers equipment grounding conductors. “Where up to four equipment grounding conductors or equipment bonding jumpers enter a box, a single volume allowance in accordance with Table 314.16(B)(1) shall be made based on the largest equipment grounding conductor or equipment bonding jumper entering the box. A 1/4 volume allowance shall be made for each additional equipment grounding conductor or equipment bonding jumper that enters the box, based on the largest equipment grounding conductor or equipment bonding conductor entering the box.”

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to NFPA 70.