What is voltage drop?

Resistance in copper wire dissipates voltage as heat over distance. A long run drops voltage between source and load — too much drop means fixtures don't receive enough voltage to run properly (LED dims, motors slow, amplifiers underperform).

What's the NEC limit?

NEC informational note: voltage drop should not exceed 3% on branch circuits, 5% total (feeder + branch). The calculator flags non-compliance.

Copper has ~38% less resistance per unit area, so it can carry more current with less voltage drop in the same gauge. Aluminum is cheaper but needs to be one or two AWG sizes larger to match copper performance.

Why doubled distance?

Current has to flow OUT to the load AND back to source through the neutral or return conductor. Resistance is in both legs of the loop. The calculator handles this automatically — just enter ONE-WAY distance.

For balanced 3-phase loads, voltage drop is calculated using √3 instead of 2 for the loop. Calculator handles this when 3-phase is selected.

Cable Voltage Drop Calculator (NEC 3% Rule)

Quick answer: Enter wire AWG, material, load amps, distance, and voltage. Get voltage drop %, voltage at load, and NEC compliance check.

Calculate voltage drop across long power runs — common in event production for FOH, upstage racks, festival distros, and outdoor venues. Compares against NEC 2023 recommendation of ≤3% on branch circuits and ≤5% total. Critical for ensuring fixtures and amplifiers receive proper voltage at the load.

Last reviewed: April 2026Report an error

Wire AWG

Material

Load (amps)

One-way distance (ft)

Voltage

3-phase

Voltage Drop

9.69 V (4.66%)

Drop: 9.69 V (4.66%). Voltage at load: 198.31 V. Above 3% but under 5% — acceptable for total but tight. Upsize one AWG if possible.

Drop %

4.66%

Voltage at Load

198.3 V

NEC Compliant

Marginal

The Formula

V_drop = (factor × K × I × L) / CMA

factor = 2 for single-phase, √3 for 3-phase
K = Resistance constant: 12.9 (copper), 21.2 (aluminum)
I = Load current in amps
L = One-way distance in feet
CMA = Circular mil area of conductor (AWG-specific)

Source: NEC 2023 Informational Note 210.19(A) ↗

How we verify our formulas →

How to Use This Voltage Drop Calculator

1Pick conductor AWG (14, 12, 10, 8, 6, 4, 2, 1/0, 2/0, 4/0).
2Pick conductor material (copper standard, aluminum cheaper).
3Enter the load in amps.
4Enter ONE-WAY distance from source to load (calculator doubles for return).
5Enter service voltage and phase configuration.
6Read voltage drop %, voltage at load, and NEC compliance.

Frequently Asked Questions

: Resistance in copper wire dissipates voltage as heat over distance. A long run drops voltage between source and load — too much drop means fixtures don't receive enough voltage to run properly (LED dims, motors slow, amplifiers underperform).
: NEC informational note: voltage drop should not exceed 3% on branch circuits, 5% total (feeder + branch). The calculator flags non-compliance.
: Copper has ~38% less resistance per unit area, so it can carry more current with less voltage drop in the same gauge. Aluminum is cheaper but needs to be one or two AWG sizes larger to match copper performance.
: Current has to flow OUT to the load AND back to source through the neutral or return conductor. Resistance is in both legs of the loop. The calculator handles this automatically — just enter ONE-WAY distance.
: For balanced 3-phase loads, voltage drop is calculated using √3 instead of 2 for the loop. Calculator handles this when 3-phase is selected.

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