Electrical Cable Size — Latest

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.

#10 AWG: 3.9 ohms/1000 ft → R = 2×150/1000×3.9 = 1.17 ohms → VD = 23.4V (19.5%) still too high.

For three-phase:

This comprehensive piece explains the engineering principles behind cable sizing: current-carrying capacity (ampacity), voltage drop, short-circuit temperature rise, and correction factors. It also walks through practical step-by-step calculations based on international standards (NEC, IEC, BS 7671). 1.1 Ohm’s Law and the Resistance of a Conductor The resistance of a copper or aluminum conductor determines its two main limitations: heating and voltage drop.

: 120V single-phase, 20A load, 150 ft one-way, copper, target VD <3% (3.6V). 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.

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

#1/0 AWG: 0.98 ohms/1000 ft → R = 0.29 ohms → VD = 5.8V (4.8%) – acceptable.