Electrical Cable: Size

Always consult the latest edition of your local electrical code (NEC, CEC, IEC, BS 7671) as legal requirements supersede engineering approximations. Last reviewed: April 2026. This guide is for informational purposes. Always involve a licensed electrical engineer for critical installations.

#6 AWG: 2.4 ohms/1000 ft → R = 0.72 ohms → VD = 14.4V (12%) – still high. electrical cable size

#2 AWG: 1.54 ohms/1000 ft → R = 0.46 ohms → VD = 9.2V (7.7%) – getting close. Always consult the latest edition of your local

Conclusion: For long runs, voltage drop (not ampacity) dictates cable size. Here, 150 ft at 20A needs 1/0 AWG copper despite #12 AWG being fine for 20A at short distances. For feeders and services, ensure cable can survive fault current. Most NEC installations skip this for small branch circuits because upstream breakers trip quickly, but for large feeders (e.g., 1000A service with 50kA fault current), verify using adiabatic equation. Always involve a licensed electrical engineer for critical

[ VD = 2 \times K \times I \times L / A ]

[ A = \frac\sqrtI_sc^2 \times tk ]

Example: 90°C wire in 50°C ambient → factor 0.71. NEC 310.15(C)(1): More than 3 current-carrying conductors in a raceway or cable.