What is the maximum recommended voltage drop?

The NEC recommends no more than 3% voltage drop on a branch circuit and no more than 5% combined (feeder + branch). Above 3% you may notice dimmer lights or reduced motor torque.

Does the return conductor also contribute?

Yes — current flows through both the hot and neutral (or ground-return) conductors, so the effective resistance is twice the one-way value.

Voltage Drop Calculator

Calculate voltage drop through copper wire. Enter AWG gauge, one-way wire length in metres and current in amps to find drop, total resistance and power loss.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter wire gauge (awg), one-way wire length and current in the fields above.
Results update instantly as you type — or click Calculate.
Read your voltage drop and the full breakdown beneath it.

Voltage drop in a conductor is V = I × R. For a round-trip circuit the total resistance is R = ρ × 2L / A, where ρ = 1.724 × 10⁻⁸ Ω·m for copper, L is the one-way length (m) and A is the cross-sectional area (m²) of the wire. The NEC recommends keeping voltage drop below 3% of supply voltage.

Formula

Round-trip resistance: R_total = ρ × (2L) / A

Voltage drop: V_drop = I × R_total

Power loss: P_drop = I² × R_total

where ρ = 1.724 × 10⁻⁸ Ω·m, A (m²) = area_mil × 5.067 × 10⁻¹⁰, and L is the one-way wire length in metres.

How it works

The calculator determines copper resistivity-based DC resistance for the chosen AWG gauge, doubles the one-way wire length to account for the complete circuit (out and return conductors), then applies Ohm's law to find voltage drop and Joule's law (P = I²R) to find heat dissipated in the wire.

Results are accurate for DC and low-frequency AC on a single conductor pair of uniform cross-section; temperature rise, stranding factors, and AC skin effect are not modelled.

Worked example

Worked example — AWG 14, 10 m, 5 A

AWG 14 has 4110 cmil; cross-section A = 4110 × 5.067 × 10⁻¹⁰ = 2.0825 × 10⁻⁶ m².
Resistance per metre = ρ / A = 1.724 × 10⁻⁸ / 2.0825 × 10⁻⁶ ≈ 8.278 × 10⁻³ Ω/m.
Round-trip resistance R_total = 8.278 × 10⁻³ × 2 × 10 m ≈ 0.1656 Ω.
Voltage drop V_drop = 5 A × 0.1656 Ω = 0.8278 V.
Power loss P = 5² × 0.1656 = 4.139 W.

Voltage drop = 0.8278 V; total resistance = 0.1656 Ω.

Key terms

Voltage drop: The reduction in electrical potential along a conductor caused by its resistance; excessive voltage drop reduces the voltage available to the load.
Round-trip resistance: The total resistance of both the supply and return conductors in a circuit, equal to twice the one-way resistance for identical wires.
Resistivity (ρ): A material property expressing how strongly it opposes current flow; copper's value is approximately 1.724 × 10⁻⁸ Ω·m at 20 °C.
Joule heating: Heat generated in a conductor by current flow, equal to I²R watts; this represents power lost as heat rather than delivered to the load.
AWG (American Wire Gauge): A standardised wire-sizing scale; lower AWG numbers mean larger, lower-resistance conductors.

Frequently asked questions

What is the maximum recommended voltage drop?: The NEC recommends no more than 3% voltage drop on a branch circuit and no more than 5% combined (feeder + branch). Above 3% you may notice dimmer lights or reduced motor torque.
Does the return conductor also contribute?: Yes — current flows through both the hot and neutral (or ground-return) conductors, so the effective resistance is twice the one-way value.