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.

Using approximate method: #12 AWG (6.5 ohms/1000 ft) → R total = 2×150/1000×6.5 = 1.95 ohms. VD = 20×1.95 = 39V (32%!) → completely unacceptable. electrical cable size

Example: 50A load, 75°C terminals, 3 conductors in conduit, ambient 30°C. Table says #8 AWG (50A at 75°C) is borderline; #6 AWG (65A) provides margin. Never use raw table values directly. Apply all that apply: 3.1 Ambient Temperature Correction (NEC Table 310.15(B)(2)(a)) If ambient ≠ 30°C (86°F), multiply ampacity by factor. Always consult the latest edition of your local

[ VD = 2 \times K \times I \times L / A ] Always involve a licensed electrical engineer for critical

For three-phase:

[ R = \rho \fracLA ]

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.