Electrical Cable Size Direct

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.

[ VD = 2 \times K \times I \times L / A ] electrical cable size

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. Always consult the latest edition of your local

#1/0 AWG: 0.98 ohms/1000 ft → R = 0.29 ohms → VD = 5.8V (4.8%) – acceptable. Always involve a licensed electrical engineer for critical

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

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.

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