4x4x2-1/8 Square Deep Metal Box Conductor & Device Capacity
Determine maximum conductor capacity and device allowance for a 4x4x2-1/8 inch deep square metal box (30.3 cu in) under NEC 314.16.
Calculated strictly in accordance with NEC Table 314.16(A) & 314.16(B).
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Step-by-Step Solution
Formal engineering derivation citing applicable National Electrical Code articles.
Per NEC Table 314.16(A), a 4 x 4 x 2-1/8 inch square metal box provides 30.3 cubic inches of interior volume.
Volume allowance per 12 AWG conductor is 2.25 cubic inches.
Compute maximum pure conductor capacity: 30.3 cu. in. / 2.25 cu. in. = 13 conductors (29.25 cu. in.).
When installing a receptacle (deduct 2 allowances = 4.5 cu in) and ground wire (deduct 1 allowance = 2.25 cu in), maximum remaining 12 AWG conductors is (30.3 - 6.75) / 2.25 = 10 conductors.