Free Voltage Drop Calculator

Find the voltage drop across a wire run based on gauge, length, and current.

Voltage drop matters for long wire runs — excessive drop wastes energy as heat and can cause equipment to malfunction. The National Electrical Code recommends keeping voltage drop under 3% for branch circuits and under 5% total (feeder + branch combined) for most applications.

Worked Example

Example: 12 AWG wire (1.588 Ω/1000ft), 50ft one-way run, 15A load. Voltage drop = 15 × 1.588 × (50×2/1000) = 2.38V — on a 120V circuit, that's about 2%.

Formula / Method

Voltage Drop = Current (A) × Resistance (Ω/1000ft) × (2 × Length (ft) / 1000) — the ×2 accounts for the round-trip circuit path.

Worked Example

Example: 12 AWG wire (1.588 Ω/1000ft), 50ft one-way run, 15A load. Voltage drop = 15 × 1.588 × (50×2/1000) = 2.38V — on a 120V circuit, that's about 2%.

Formula / Method

Voltage Drop = Current (A) × Resistance (Ω/1000ft) × (2 × Length (ft) / 1000) — the ×2 accounts for the round-trip circuit path.