⚡ Voltage Drop Calculator

The NEC (National Electrical Code) recommends (not mandates) a maximum 3% voltage drop on branch circuits and 5% total from source to the load (feeder + branch). Higher voltage drops cause equipment to run hot, motors to draw more current, and lights to dim. Always design to stay within 3% for sensitive loads.

Voltage Drop = 2 × L × I × ρ / A, where L is one-way cable length (m), I is current (A), ρ is resistivity (Ω·m), and A is cross-section (m²). The factor of 2 accounts for both the live and neutral (return) conductors. Percentage drop = (V_drop / V_supply) × 100.

Copper has lower resistivity (better conductivity) and is preferred for most residential and commercial wiring. Aluminum is lighter and cheaper per metre but requires larger cross-sections for equivalent current-carrying capacity. Aluminum is commonly used for large feeders and service entrance cables. Never mix copper and aluminum connections without proper anti-oxidant compound and listed connectors.

Excessive voltage drop causes: reduced motor torque (motors may stall), overheating of motors and other loads, reduced lighting output, unreliable operation of electronic equipment, and increased energy waste as heat in the cable. For long runs, upsize the cable to reduce resistance.