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Voltage Drop Calculator - Wire Sizing Tool

Calculate voltage drop across electrical wire runs. Enter current, wire length, gauge, and material to ensure your wiring meets NEC voltage drop standards.

Formula

Single-phase: Vd = 2 ร— I ร— (R/1000) ร— L | Three-phase: Vd = โˆš3 ร— I ร— (R/1000) ร— L

I = current (A), R = resistance per 1,000 ft from NEC Table 8 (ฮฉ), L = one-way distance (ft).

Worked Examples

Example 1: 20A, 12 AWG, 100 ft

Problem:120V single-phase, 20A load, 12 AWG copper, 100 ft run

Solution:Vd = 2 ร— 20 ร— (1.98/1000) ร— 100

Result:7.92 V drop (6.6%) โ€” oversized; use 10 AWG

Example 2: 20A, 10 AWG, 100 ft

Problem:120V single-phase, 20A load, 10 AWG copper, 100 ft run

Solution:Vd = 2 ร— 20 ร— (1.24/1000) ร— 100

Result:4.96 V drop (4.1%) โ€” acceptable

Frequently Asked Questions

What is voltage drop?

The loss of voltage as current travels through a wire due to resistance. Longer wires and higher currents produce more drop.

How do I reduce voltage drop?

Use a larger wire gauge (lower AWG number), shorten the run, reduce current load, or increase supply voltage.