NEC WIRE SIZING (310.16 & VD)

The NEC (NFPA 70) Table 310.16 lists allowable ampacity for insulated copper and aluminum conductors by AWG/kcmil and insulation rating (60°C, 75°C, 90°C columns). Ampacity is derated for ambient temperature (Table 310.15(B)(1)) and conductor count in conduit (Table 310.15(C)(1)). Voltage drop is recommended <3% on a single feeder or branch, <5% combined; one-way drop V_drop = 2·I·L·R / 1000 for single phase. Conduit fill is limited per Table 1 of Chapter 9 (40% for >2 conductors).

• I_design = I_continuous · 1.25 + I_noncontinuous
• VD = 2·I·L·(R·cosφ + X·sinφ) / 1000 (1φ)
• VD = sqrt(3)·I·L·... (3φ)
• %VD = VD / V_source · 100
• I_allow = I_table · k_temp · k_count

References & StandardsNFPA 70 (NEC), Tables 310.16, 310.15(B)(1), Chapter 9 Table 1

Use for branch-circuit and feeder sizing in residential, commercial, and industrial installations. Determine load current first (continuous loads at 125% per NEC 210.19(A)/215.2(A)), pick conductor for ampacity at the lowest applicable termination temperature (often 75°C), then verify voltage drop. Increase conductor size if voltage drop exceeds limits even though ampacity is satisfied. Check conduit fill before specifying raceway.