[ R = \rho \fracLA ]
[ VD = 2 \times K \times I \times L / A ]
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. 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.
#1/0 AWG: 0.98 ohms/1000 ft → R = 0.29 ohms → VD = 5.8V (4.8%) – acceptable. [ R = \rho \fracLA ] [ VD
[ VD = \sqrt3 \times K \times I \times L / A ]
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. Apply all that apply: 3
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.