I'm putting two 12-2 NM cables and one 12-3 NM cable into a box with a single duplex receptacle and internal clamps.

Use NEC 314.16(B) for box fill calculations. Count each current-carrying conductor (ungrounded/hot and neutral) entering the box as one unit. Grounds count as one unit total.

Your conductors:

• 12-2 NM: 2 conductors × 2 cables = 4 units
• 12-3 NM: 3 conductors × 1 cable = 3 units
• Grounds: 1 unit (all grounds combined)
• Total = 8 units

For #12 AWG wire, each unit requires 2.25 sq in (NEC Table 314.16(C)):

8 units × 2.25 = 18 sq in minimum box volume

A standard 4" × 4" × 1-1/2" box = 21 cu in — sufficient.

Important notes:

• The internal clamps count toward fill if they're inside the box (they're part of the cable assembly here, so no extra count needed).
• The duplex receptacle does not count as a conductor, but if it has internal clamps that take up space, ensure the box is deep enough.
• If using a metal box, ensure proper grounding per 250.118.

Recommendation: Use a 4" × 4" × 2-1/8" box (27.5 cu in) for extra working room and to comply with 314.16(A) fill requirements comfortably.

Box Fill Calculation (NEC 314.16):

1. Conductors: Two 12-2 cables = 4 current-carrying conductors (2 hots, 2 neutrals). One 12-3 cable = 3 current-carrying conductors (2 hots, 1 neutral). Total Conductors = 7.*

1. Grounding Conductors (314.16(B)(5)): All equipment grounding conductors count as 1 volume allowance (largest size present = 12 AWG). Grounds = 1.

1. Device (Receptacle) (314.16(B)(4)): One duplex receptacle (one yoke) = 2 volume allowances (based on 12 AWG). Device = 2.

1. Internal Clamps (314.16(B)(2)): Internal cable clamps (molded in box) count as 1 volume allowance (largest conductor = 12 AWG). Clamps = 1.

Total Volume Allowances = 7 + 1 + 2 + 1 = 11.

Required Volume (Table 314.16(B)):

• 12 AWG = 2.25 cu. in. per allowance.
• 11 × 2.25 cu. in. = 24.75 cu. in.

Selection: You need a box marked ≥ 25.0 cu. in. (Standard 4" square × 2-1/8" deep = 30.3 cu. in. works; a standard 3"×2"×3.5" device box at 18 cu. in. is too small).

Grounding Note: Pigtail the grounds to the receptacle green screw and the metal box (if used) per 250.148.