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.
#1/0 AWG: 0.98 ohms/1000 ft → R = 0.29 ohms → VD = 5.8V (4.8%) – acceptable.
For three-phase:
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.
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. electrical cable size
Introduction Selecting the correct electrical cable size is one of the most critical decisions in any electrical installation—whether it’s a low-voltage automotive circuit, a residential feeder, or a high-voltage industrial distribution network. An undersized cable overheats, destroys insulation, causes fires, and creates excessive voltage drop. An oversized cable wastes material, increases cost, adds unnecessary weight, and may be difficult to terminate.
[ VD = 2 \times K \times I \times L / A ] Using approximate method: #12 AWG (6
#6 AWG: 2.4 ohms/1000 ft → R = 0.72 ohms → VD = 14.4V (12%) – still high.