What is the NEC 3% voltage drop limit?

The NEC recommends (but does not mandate) a maximum 3% voltage drop on branch circuits and 5% total from service panel to outlet. Excessive voltage drop causes motors to run hot, lights to dim, and electronics to malfunction. For solar DC wire runs, even 1–2% drop is worth correcting by upsizing wire, since it directly reduces system output.

What AWG wire should I use for a 20A circuit?

12 AWG is the standard for 20A circuits in North American residential wiring (NEC 310.15). However, for long runs where voltage drop would exceed 3%, upsizing to 10 AWG reduces resistance and keeps drop within limits. Use this calculator to check your specific run length before committing to wire size.

How does temperature affect voltage drop?

Higher temperatures increase conductor resistance, raising voltage drop for a given load. Copper resistance increases roughly 0.4% per °C above 20°C. For rooftop solar runs where conduit temperature can reach 60–70°C in summer, the effective resistance can be 15–20% higher than at standard conditions.

Quick answer: A 40 A load on 100 feet of 8 AWG copper at 240 V drops about 2.0 V (0.83%) — well within the 3% NEC recommendation.

Quick Tools

Voltage Drop Calculator

Calculate voltage drop percentage for any wire run. Enter amps, distance, AWG gauge, and voltage — get a color-coded pass/fail against the NEC 3% limit.

Current (amps)

One-way distance (feet)

Wire gauge (AWG)

System voltage

Voltage drop

20.7%

✗ Exceeds 3% — consider larger wire gauge

Wire resistance

1.24 Ω/1000ft

Round-trip drop

2.48 V

How we calculate voltage drop

Voltage drop occurs as current flows through wire resistance. The formula accounts for both the hot and neutral conductors (factor of 2), wire resistance by AWG, run length, and load current.

V Drop = 2 × Length (ft) × Amps × Ω/ft

Percentage drop = (V Drop ÷ Supply Voltage) × 100. The NEC recommends keeping branch circuits below 3%.

Frequently Asked Questions

What is the NEC 3% voltage drop limit?: The NEC recommends (but does not mandate) a maximum 3% voltage drop on branch circuits and 5% total from service panel to outlet. Excessive voltage drop causes motors to run hot, lights to dim, and electronics to malfunction. For solar DC wire runs, even 1–2% drop is worth correcting by upsizing wire, since it directly reduces system output.
What AWG wire should I use for a 20A circuit?: 12 AWG is the standard for 20A circuits in North American residential wiring (NEC 310.15). However, for long runs where voltage drop would exceed 3%, upsizing to 10 AWG reduces resistance and keeps drop within limits. Use this calculator to check your specific run length before committing to wire size.
How does temperature affect voltage drop?: Higher temperatures increase conductor resistance, raising voltage drop for a given load. Copper resistance increases roughly 0.4% per °C above 20°C. For rooftop solar runs where conduit temperature can reach 60–70°C in summer, the effective resistance can be 15–20% higher than at standard conditions.

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Voltage Drop Calculator

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Reviewed April 2026

Methodology and source note

PowerSizing calculators use shared formulas, documented assumptions, and current planning inputs that are summarized on the methodology page. Use these tools for first-pass planning, comparison, and sanity checks, then confirm local code, pricing, utility tariff, and installer specifics before you buy equipment.