Calculate Cable Size Jun 2026

Use this as a starting point only—always verify with local codes.

Always size your cables based on the maximum current of the circuit and the length of the run to account for voltage drop. Undersizing a cable can lead to overheating and fire hazards. If you are unsure, consult a licensed electrician.

Calculating the correct cable size is a critical safety step to prevent overheating, fire hazards, and equipment damage. The goal is to select the smallest cable that satisfies three key requirements: current-carrying capacity (ampacity), acceptable voltage drop, and short-circuit performance. Core Calculation Process The general procedure involves these standardized steps: How to Calculate Cable Size: Complete Guide for Beginners calculate cable size

[ \textMin. Cross Section (mm²) = \frac2 \times \rho \times I \times LV_d ]

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Let’s break it down.

When in doubt, always go one size larger. It runs cooler, lasts longer, and allows for future upgrades to your electrical system.

The formula above assumes ideal conditions. If cables run through hot attics or are bundled together, use these derating factors (from NEC/CEC tables): If you are unsure, consult a licensed electrician

The millivolt drop per amp per meter (found in cable data sheets). L: Length in meters.

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