Wire Voltage Drop Calculator

Voltage drop is the reduction in electrical potential as current flows through a wire. Long wire runs, thin wires, and high currents all increase voltage drop. Too much voltage drop causes equipment to malfunction, motors to overheat, and lights to dim.

The formula:

Voltage Drop (V) = I × R_total

R_total = 2 × L × ρ / A

Where:

• I = current (Amperes)
• L = one-way wire length (meters or feet)
• ρ = resistivity of the conductor material (Ω·m)
• A = cross-sectional area of the wire (m²)
• The factor of 2 accounts for the return wire (complete circuit)

Resistivity of common conductor materials, at 20°C:

• Copper: 1.724 × 10⁻⁸ Ω·m (most common, lowest resistance)
• Aluminum: 2.82 × 10⁻⁸ Ω·m (the EC-grade alloy cable is actually drawn from; physics tables quote 2.65 for the pure metal, which nobody wires a house with)
• Silver: 1.59 × 10⁻⁸ Ω·m (best conductor, rarely used due to cost)

Why NEC tables give a bigger answer than this page does. Copper resistance climbs about 0.4% per degree, and NEC Chapter 9 Table 8 is written for a conductor at 75°C, the temperature a wire near its rating actually reaches. That puts the code figures about a quarter above the 20°C numbers used here: 14 AWG is 2.53 Ω per 1,000 ft cold and 3.14 Ω hot, which is 24% more. Use this page for the physics and a wire sized off Table 8 when an inspector is involved.

AWG (American Wire Gauge) cross-sectional areas:

Acceptable voltage drop limits:

Worth saying plainly: none of these are code requirements. NEC 210.19(A) and 215.2(A) put them in informational notes, which means an inspector cannot fail you for a 4% branch circuit. They are there because equipment misbehaves below them, not because the book forbids it.

Practical notes:

• Voltage drop matters most on long runs. A 50-foot (15 m) circuit rarely has issues; a 200-foot (61 m) run often does.
• If voltage drop exceeds 3%, use the next larger wire gauge.
• Voltage drop causes heat in the wire, which is both inefficient and a fire hazard.
• Use copper for residential wiring; aluminum is acceptable for large service entrance conductors.