SAFETY DISCLAIMER: This calculator is for estimation and education only. It is NOT a substitute for a licensed electrician or the National Electrical Code as adopted by your local authority having jurisdiction (AHJ). Voltage drop depends on factors this tool may not capture — conductor temperature, actual resistivity, conduit fill, and derating. Incorrect wire sizing can cause fire or electrocution. Always verify with a licensed electrician and obtain a permit.

Voltage Drop Calculator

Calculate how much voltage is lost over a wire run and find the minimum AWG to stay within your voltage-drop target. Uses the NEC standard formula (K × I × L / CM). Your inputs never leave your browser.

Inputs

Voltage Drop (V)
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Voltage Drop (%)
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Voltage at Load
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Min AWG to Meet Target
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SAFETY REMINDER: This tool provides estimates only. Always verify with a licensed electrician. Obtain a permit. Incorrect wire sizing can cause fire or electrocution.

Frequently Asked Questions

What formula does this voltage drop calculator use?

Vd = phaseFactor × K × I × L / CM — where phaseFactor is 2 for single-phase or √3 for three-phase, K is the resistivity constant (12.9 for copper, 21.2 for aluminum in ohm-cmil/ft), I is the design current in amps, L is the one-way run length in feet, and CM is the conductor cross-sectional area in circular mils. This formula is standard in NEC handbooks and Ugly's Electrical Reference.

Why is 3% the default voltage-drop target?

NEC 210.19(A) informational note recommends limiting branch-circuit voltage drop to 3%, and NEC 215.2(A) informational note recommends 3% on feeder circuits, for a combined maximum of 5% from service to outlet. 3% is not an NEC mandate, but it is the widely accepted engineering benchmark.

What is K (resistivity constant) for copper and aluminum?

K ≈ 12.9 ohm-cmil/ft for copper and ≈ 21.2 ohm-cmil/ft for aluminum at 75°C. These values are derived from the resistivity of each material and are widely published in NEC training materials, Ugly's Electrical Reference, and NECA/IBEW references.

Does run length mean one way or round trip?

Enter the one-way distance from the panel to the load — the formula accounts for the full conductor path. For single-phase, the factor of 2 represents the round trip (outgoing plus return conductor). For balanced three-phase, the factor of √3 (≈1.732) is the geometric line-to-line factor for a three-phase system, not a round-trip conductor count; in either case you still enter only the one-way run length.

Why does voltage drop matter more on longer runs?

Voltage drop is proportional to run length. On short circuits (under 50 ft at typical household loads), ampacity is usually the binding constraint. On long runs — outdoor lighting, subpanels, EV chargers at the far end of a property — voltage drop becomes the larger wire size driver.

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