Voltage Drop Calculator

How It Works

1. 1

   Pick wire gauge and material

   Select the AWG size installed or proposed, and choose copper or aluminum. Aluminum is not rated below 12 AWG in most codes.

2. 2

   Enter length, current, voltage, and phase

   Put in the one-way run length in feet, the load current in amps, the nominal circuit voltage, and whether the circuit is DC, single-phase, or three-phase.

3. 3

   Check the drop against the 3% guideline

   Read the voltage drop in volts and percent, plus the voltage arriving at the load. The calculator flags drops over 3% as outside the NEC recommendation.

Voltage Drop: Why Long Runs Burn Out Motors

Run 100 feet of 12 AWG copper to a 20-amp load and you lose 7.7 volts. On a 120 V circuit that's a 6.4% drop, past the 3% NEC recommends, and enough to shave roughly 19% off an incandescent bulb's brightness and rob a motor of starting torque. Every foot of wire between the panel and the load is a small resistor pulling voltage out of the circuit. For DC and single-phase AC, voltage drop follows V = 2 × I × R × L. The factor of 2 accounts for the round trip through the hot and neutral conductors. Three-phase balanced circuits use V = √3 × I × Z × L on a line-to-line basis. Resistivity for copper is 1.68 × 10⁻⁸ Ω·m at 20 °C; aluminum is 2.82 × 10⁻⁸ Ω·m, giving aluminum about 59.6% of copper's conductivity. Metals get more resistive as they heat up. A conductor operating near its insulation rating at 75 °C runs about 22% higher resistance than the same wire at 20 °C, which is why calculators that ignore temperature systematically undersize wire. NEC Article 210.19 Informational Note No. 4 recommends no more than 3% drop on a branch circuit and no more than 5% combined feeder-plus-branch. These are recommendations, not enforceable code in most sections, but motors, LED drivers, and electronic equipment all have real-world tolerances below those limits. The simplest fix is to go up one gauge. Each step drops resistance by roughly 21%.

Common pitfalls

• Using one-way distance on a single-phase circuit. The round-trip current flows through both the hot and the neutral, so the formula is V_drop = 2 x I x R_per_ft x L_one_way. Drop the 2 and a 100 ft run calculates at half its actual drop.

• Mixing up line-to-line and line-to-neutral on three-phase. Balanced three-phase uses V_drop = √3 x I x Z x L on a line-to-line basis (no factor of 2). Unbalanced or single-phase lines pulled from a three-phase panel still use the 2 x L formula.

• Ignoring conductor operating temperature. Copper resistance rises about 0.4% per °C. A conductor running at its 75 °C rating has 22% more resistance than the 20 °C table value, so a 3% calculated drop can become 3.7% at full load. Use the NEC Chapter 9 Table 8 resistances at the expected operating temperature, not the room-temperature values.

• Assuming the 3% branch-circuit guideline is enforceable. NEC 210.19 Informational Note No. 4 (branch) and 215.2 (feeder) recommend 3% and 5% combined, but informational notes are not mandatory code. Motors, LED drivers, and electronic loads often need tighter margins than the recommendation.

• Only sizing for ampacity. A 100 ft, 20 A run on 12 AWG meets ampacity but drops 6.4% at 120 V. Voltage drop is an independent check, and for long runs it almost always dictates wire size.

Frequently Asked Questions

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