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

[ VD = \sqrt3 \times K \times I \times L / A ]

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

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.

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.

For three-phase:

#2 AWG: 1.54 ohms/1000 ft → R = 0.46 ohms → VD = 9.2V (7.7%) – getting close.