How to Size a Wire: Voltage Drop and Ampacity
Undersized wire causes voltage drop, heat buildup, and energy waste. Oversized wire wastes money. This tool checks both the voltage drop requirement and the current-carrying capacity (ampacity) and recommends the larger of the two standard sizes.
The Voltage Drop Formula
Required cross-section (mm²) = (2 × one-way length × current) ÷ (conductivity × allowable voltage drop in V). Copper conductivity = 56 S·m/mm²; aluminum = 35 S·m/mm². The formula accounts for the full round-trip (2× the one-way length). The calculated minimum is then rounded up to the nearest standard IEC gauge.
Standard IEC Wire Sizes
IEC standard cross-sections: 1.5 / 2.5 / 4 / 6 / 10 / 16 / 25 / 35 / 50 / 70 / 95 / 120 mm². Household outlet circuits typically use 2.5 mm². Dedicated appliance circuits (dryers, water heaters) need 4–6 mm². Sub-feeders use 10 mm² and above.
NEC Voltage Drop Recommendation
The NEC recommends keeping voltage drop to 3% or less on branch circuits, and no more than 5% total across the feeder and branch. For motors and sensitive electronics, 2% is a better target to protect equipment and reduce heat loss.
Frequently Asked Questions
Enter the one-way distance from the panel to the load. The calculator multiplies by 2 to account for the current path through both the hot and neutral conductors. For example, if the panel is 15 m away from the outlet, enter 15.
This calculator is for single-phase 2-wire circuits. For 3-phase circuits the formula uses √3 (≈ 1.732) instead of 2. For 3-phase wiring, consult an electrician or use a dedicated 3-phase voltage drop calculator.
The breaker must be rated at or below the wire's ampacity. Using a 30 A breaker on a 20 A wire removes the overcurrent protection. A common rule of thumb is to size the breaker at no more than 80% of the wire's rated ampacity for continuous loads.