How to calculate voltage drop
Voltage drop = 2 × current × resistivity × one-way length ÷ wire cross-section for a single-phase circuit (use √3 instead of 2 for three-phase). The result is the voltage your wire loses as heat before it reaches the load. The everyday target is to keep it under 3% on a branch circuit, and under 5% for the feeder and branch combined.
Rather not do the algebra? Use the voltage drop calculator → Enter the current, the run length and the wire size and it returns the drop in volts and percent, in AWG and feet or mm² and metres, for copper or aluminium.
1. What voltage drop actually is
Every conductor has resistance. When current flows through that resistance it gives up a small amount of voltage as heat, all along the wire, so the voltage that arrives at the far end is a little lower than the voltage that left the panel. That loss is voltage drop. It is not a fault or a wiring mistake — it is physics, and it happens on every circuit. The question is never whether you have voltage drop, only whether you have too much of it.
Too much drop has real consequences. Incandescent and some LED lights dim and flicker. Motors, compressors and pumps draw extra current to make up the lost voltage, which makes them run hotter and shortens their life. Heaters and kettles take longer to reach temperature. And the energy lost in the wire is wasted as heat in your walls, which you pay for and never use. On a long run feeding a well pump, an air conditioner or a shed, voltage drop is usually the reason the wire ends up a size or two bigger than the breaker alone would suggest.
2. The formula, piece by piece
The single-phase formula is VD = 2 × I × ρ × L ÷ A:
- I is the current in amps — usually the load, or the breaker rating if you are sizing for the worst case.
- L is the one-way length from the panel to the load. The 2 out front accounts for the current travelling out and back, so you measure the distance once and the formula doubles it. (Three-phase uses √3 instead of 2 because of how the phases combine.)
- A is the conductor's cross-section — its area. Bigger wire, less resistance, less drop. This is what you change when the drop is too high.
- ρ (rho) is the metal's resistivity. Copper is about 0.0214 Ω·mm²/m at normal operating temperature; aluminium is about 0.0352 — roughly 60% more, which is why aluminium runs need to be a size larger for the same drop.
North America sizes wire by AWG gauge and measures runs in feet, while most of the world uses mm² and metres. The physics is identical — only the units differ. The same calculation in imperial units is often written with a constant K (about 12.9 for copper, 21.2 for aluminium): VD = 2 × K × I × L ÷ CM, where CM is the wire's area in circular mils. It is the same equation wearing different clothes, and the calculator gives the same answer whichever units you pick.
3. A worked example
Say you are running a 20-amp, 120-volt circuit 80 feet out to a workshop, in copper 12 AWG. Twelve-gauge copper is about 6,530 circular mils (3.31 mm²). Using the imperial form: VD = 2 × 12.9 × 20 × 80 ÷ 6,530 = 41,280 ÷ 6,530 ≈ 6.3 volts. As a percentage that is 6.3 ÷ 120 = 5.3% — over both the 3% and the 5% guideline. The fix is to step up to 10 AWG, which has more copper and brings the same run back down to about 4 volts, or 3.3%. Move to 8 AWG and you are comfortably under 3%.
Notice what changed and what did not. The breaker is still 20 amps and 12 AWG is still legal for a 20-amp circuit on the basis of ampacity — the upsize is purely to control voltage drop over distance. That is the distinction behind the most common wiring question of all.
4. How far will a wire carry the current?
"How far will 12-gauge wire carry 20 amps?" really means "how long can the run be before voltage drop crosses the 3% line?" The table below answers it for copper at a 20-amp load, single-phase, at both common voltages. Distances are the one-way run length where the drop reaches 3%; double the voltage and you roughly double the reach, because the same volts lost are a smaller share of a bigger number.
| Copper wire | Area | Reach at 120 V | Reach at 240 V |
|---|---|---|---|
| 12 AWG | 3.3 mm² | ~45 ft | ~90 ft |
| 10 AWG | 5.3 mm² | ~72 ft | ~145 ft |
| 8 AWG | 8.4 mm² | ~115 ft | ~230 ft |
| 6 AWG | 13.3 mm² | ~183 ft | ~366 ft |
So 12-gauge copper carries 20 amps about 45 feet at 120 volts before the drop reaches 3%, and 10-gauge about 72 feet. In metric terms, a 2.5 mm² copper conductor carrying 16 amps at 230 volts reaches roughly 25 metres on the same 3% basis. These are planning figures from the formula above, not a substitute for your local wiring rules — but they are exactly the numbers that explain why a long circuit gets upsized.
5. Voltage drop is not the same as ampacity
The question "can I run 14-gauge wire on a 20-amp circuit?" trips people up because it mixes two different limits. Ampacity is how much current a wire can carry before it overheats — 14 AWG copper is rated as a 15-amp conductor, so it belongs on a 15-amp breaker and may not be used on a 20-amp one, full stop. Voltage drop is a separate, distance-based concern: a wire that is perfectly fine for the current may still lose too much voltage over a long run. You always satisfy ampacity first — that is the safety limit set by your wiring rules — and then check voltage drop, upsizing further if the run is long. A short circuit can ignore drop entirely; a long one is often governed by it.
For three-phase circuits the only change is the multiplier: √3 (about 1.732) replaces the 2, which is why a three-phase feeder of the same length and current drops less than a single-phase one. Aluminium, on the other hand, drops more — about 60% more for the same size — so aluminium feeders are typically run a gauge or two larger than the copper equivalent. The calculator handles all four combinations; the rest of the trade's arithmetic lives on the electrical tools hub.
6. Where the 3% / 5% guideline comes from
The familiar 3%-and-5% targets are recommendations, not always hard law. In the United States they appear as informational notes in the National Electrical Code: one suggesting branch circuits be designed for no more than 3% drop, and another that the feeder and branch together stay under 5%. Informational notes are advisory — your local authority decides what is actually enforced — but the figures are so widely adopted that most of the industry treats them as the working standard, and other countries publish closely similar guidance. They exist because below roughly these levels, equipment runs as intended and the wasted energy stays small; above them, the problems in section 1 start to show up. Calculate to plan and to compare wire sizes, then confirm the design against the wiring rules in force where you are, and use a licensed electrician where your jurisdiction requires one.
Common questions
- How far will 12-gauge wire carry 20 amps?
- About 45 feet one-way on a 120-volt circuit before copper 12 AWG reaches the 3% voltage-drop guideline at 20 amps, and roughly 90 feet at 240 volts. Past that, move up to 10 AWG or shorten the run.
- How far will 10-gauge wire carry 20 amps?
- About 72 feet one-way at 120 volts and roughly 145 feet at 240 volts before copper 10 AWG hits 3% at 20 amps. The extra copper over 12 AWG buys roughly 60% more distance for the same drop.
- Can I run 14-gauge wire on a 20-amp circuit?
- No. 14 AWG copper is a 15-amp conductor and belongs on a 15-amp breaker — that is an ampacity (heat) limit, which is separate from voltage drop. Voltage drop answers how far a wire reaches before the voltage sags; ampacity answers how much current it can carry at all.
- What is an acceptable voltage drop?
- No more than 3% on a branch circuit and no more than 5% feeder-plus-branch combined is the widely-used guideline. In the US those figures come from informational notes in the wiring code — recommendations rather than hard rules — and most countries publish similar guidance.
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.