Voltage drop calculator
Enter the load current, the one-way run length and the wire size. You'll get the voltage drop in volts and as a percentage, checked against the 3% / 5% guideline — instantly, in AWG or mm², copper or aluminium, single- or three-phase.
Run length is the one-way distance from the panel to the load — not there-and-back. The calculator accounts for the return conductor.
✓ Within the common 3% branch-circuit guideline.
Single-phase: drop = 2 × I × ρ × L ÷ A. Uses copper resistivity at ~75 °C operating temperature (the SI form of the K = 12.9 Ω·cmil/ft hand method). It is a planning estimate, not a full AC impedance calculation, and the 3% / 5% figures are a widely-used guideline — your local wiring rules set what you may actually install.
Common questions
- What is voltage drop?
- Voltage drop is the voltage lost in a wire between the panel and the load, because every conductor has resistance and current flowing through it gives up some voltage as heat. It grows with more current, a longer run, or a smaller wire. A motor or heater fed at a sagging voltage runs hot and inefficiently, and lights dim — which is why long runs get upsized.
- How far will 12-gauge wire carry 20 amps?
- On a 120-volt circuit, about 45 feet one-way before copper 12 AWG reaches the common 3% voltage-drop guideline at a full 20-amp load; roughly double that — about 90 feet — at 240 volts, because the same drop in volts is a smaller percentage of a higher voltage. Beyond that, step up to 10 AWG or shorten the run. Enter your exact numbers above to see the percentage.
- How far will 10-gauge wire carry 20 amps?
- About 72 feet one-way on 120 volts before copper 10 AWG hits the 3% guideline at 20 amps, and roughly 145 feet at 240 volts. 10 AWG has about 60% more copper than 12 AWG, so it carries the same current further before the drop matters — the trade-off is cost and stiffness.
- Can I run 14-gauge wire on a 20-amp circuit?
- No — that is an ampacity limit, separate from voltage drop. 14 AWG copper is treated as a 15-amp conductor and belongs on a 15-amp breaker; putting it on a 20-amp breaker lets it carry more current than it can safely shed as heat. This calculator answers the distance question (how far before the voltage sags); the wire-for-the-breaker question is set by your wiring rules.
- What is an acceptable voltage drop?
- The widely-used guideline is no more than 3% on a branch circuit and no more than 5% for the feeder and branch circuit combined. In the US these come from informational notes in the NEC — recommendations, not hard requirements — and most other countries publish similar guidance. The calculator flags green up to 3%, amber from 3–5%, and over 5% it tells you to upsize or shorten the run.
- How do I calculate voltage drop?
- For a single-phase circuit, drop = 2 × current × resistivity × one-way length ÷ conductor area (the 2 covers the out-and-back conductors). Three-phase uses √3 instead of 2. We use copper or aluminium resistivity at about 75 °C operating temperature — the SI form of the standard K = 12.9 (copper) / 21.2 (aluminium) Ω·cmil/ft hand method — so the AWG and mm² answers match.
Want the why behind the numbers? Read the voltage drop guide → or browse the electrical tools hub →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.