Voltage Drop CalculatorVoltage drop chart
How far you can run each wire size before the drop passes 3% (or 5%), in AWG and feet or mm² and metres. Every number comes from the same formula as the calculator.
Longest one-way run in feet for a 3% drop: copper, 120 V, single-phase or DC.
| Wire size | 5 A | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|---|---|
| 14 AWG | 115 | 57 | 38 | 29 | 19 | 14 | 11 |
| 12 AWG | 182 | 91 | 61 | 46 | 30 | 23 | 18 |
| 10 AWG | 290 | 145 | 97 | 72 | 48 | 36 | 29 |
| 8 AWG | 461 | 230 | 154 | 115 | 77 | 58 | 46 |
| 6 AWG | 733 | 366 | 244 | 183 | 122 | 92 | 73 |
| 4 AWG | 1,165 | 582 | 388 | 291 | 194 | 146 | 116 |
| 3 AWG | 1,469 | 734 | 490 | 367 | 245 | 184 | 147 |
| 2 AWG | 1,852 | 926 | 617 | 463 | 309 | 232 | 185 |
| 1 AWG | 2,336 | 1,168 | 779 | 584 | 389 | 292 | 234 |
| 1/0 AWG | 2,945 | 1,473 | 982 | 736 | 491 | 368 | 295 |
| 2/0 AWG | 3,714 | 1,857 | 1,238 | 928 | 619 | 464 | 371 |
| 3/0 AWG | 4,683 | 2,341 | 1,561 | 1,171 | 780 | 585 | 468 |
| 4/0 AWG | 5,905 | 2,953 | 1,968 | 1,476 | 984 | 738 | 591 |
Voltage drop only: a small wire may not be allowed to carry the larger currents at all, so check its ampacity too (see the ampacity guide). On a balanced three-phase circuit, multiply by 1.155 (2 ÷ √3) at the same line-to-line voltage.
How far can you run 12 gauge wire?
On a 20 A circuit at 120 V, carrying the full 20 A, 12 AWG copper reaches a 3% drop at about 46 ft one way. A lighter load goes further: at 15 A it is about 61 ft. The usual US branch circuits at 3%:
| Circuit | At 120 V | At 240 V |
|---|---|---|
| 14 AWG, 15 A | 38 ft | 76 ft |
| 12 AWG, 20 A | 46 ft | 91 ft |
| 10 AWG, 30 A | 48 ft | 97 ft |
The NEC small-conductor rule (240.4(D)) normally limits the breaker for copper 14, 12 and 10 AWG to 15, 20 and 30 A, which is why these three pairs are the usual ones. For a longer run, go up a size and check it in the calculator.
Voltage drop per 100 feet
Multiply the figure by the current and by the one-way length in hundreds of feet. The return wire is already counted. Example: 12 AWG copper, 20 A, 100 ft one way: 0.395 × 20 × 1 = 7.90 V.
| Wire size | Copper | Aluminum |
|---|---|---|
| 14 AWG | 0.628 | 1.03 |
| 12 AWG | 0.395 | 0.649 |
| 10 AWG | 0.248 | 0.408 |
| 8 AWG | 0.156 | 0.257 |
| 6 AWG | 0.0983 | 0.162 |
| 4 AWG | 0.0618 | 0.102 |
| 3 AWG | 0.0490 | 0.0806 |
| 2 AWG | 0.0389 | 0.0639 |
| 1 AWG | 0.0308 | 0.0507 |
| 1/0 AWG | 0.0244 | 0.0402 |
| 2/0 AWG | 0.0194 | 0.0319 |
| 3/0 AWG | 0.0154 | 0.0253 |
| 4/0 AWG | 0.0122 | 0.0200 |
| 250 kcmil | 0.0103 | 0.0170 |
| 300 kcmil | 0.00860 | 0.0141 |
| 350 kcmil | 0.00737 | 0.0121 |
| 400 kcmil | 0.00645 | 0.0106 |
| 500 kcmil | 0.00516 | 0.00848 |
The 3% rule
The 3% figure is for one branch circuit or feeder; 5% is the total from where the supply comes in to the load. What it means depends on where you are:
- United States: informational notes in the 2023 NEC, the note to 210.19 for branch circuits and Informational Note No. 2 to 215.2(A)(2) for feeders. They describe reasonable efficiency, not a requirement, unless your authority adopts them.
- Canada: a rule. CEC Rule 8-102 limits the drop to 3% in any one feeder or branch circuit and 5% in total. See the Canadian calculator.
- United Kingdom: BS 7671 deems a design to comply when the drop from the origin of the installation is within 3% for lighting and 5% for other uses. See the SWA cable calculator.
- Australia and New Zealand: AS/NZS 3000 sets its own limit. Check the current edition.
- 12 V and 24 V systems: 3% is a sensible starting target for vans and RVs. Boats follow ABYC E-11: 3% for critical circuits, 10% for the rest. See the 12V calculator and the boat wire size calculator.
How the chart is worked out
L = (target ÷ 100 × V × CM) ÷ (2 × K × I)
This is the voltage drop formula solved for the one-way length L, in feet. K is 12.9 for copper and 21.2 for aluminum on a 75 °C basis; in metric, ρ is 0.02145 and 0.03524 ohm·mm² per metre with the area in mm². The chart is for DC and single-phase circuits. Reactance, which matters for large conductors on AC, is left out. Details are on the method page.
Frequently asked questions
What is the 3% voltage drop rule?
Keep the drop in any one branch circuit or feeder to 3% of the supply voltage, and the total from the service to the load to 5%. In the US these figures are informational notes in the NEC, not requirements unless your authority adopts them. In Canada, Rule 8-102 of the CEC makes them a rule. In the UK, BS 7671 treats 3% for lighting and 5% for other uses as deemed to satisfy.
How far can you run 12 gauge wire on a 20 amp circuit?
About 46 ft one way at 120 V, carrying the full 20 A, before the drop passes 3%; about 91 ft at 240 V. At 15 A it is about 61 ft at 120 V. Past that, use 10 AWG or check the run in the calculator.
How far can you run 14 gauge wire?
On a 15 A circuit at 120 V, about 38 ft one way at full load before the drop passes 3%, or 76 ft at 240 V.
How far can you run 10 gauge wire at 30 amps?
About 48 ft one way at 120 V and 97 ft at 240 V, at a 3% drop.
What is the voltage drop per 100 feet of 12 gauge wire?
About 0.395 V per amp for 100 ft one way in copper, counting the return wire. At 20 A that is 7.9 V, or 6.6% of 120 V.
Does a 5% drop let me run further?
Yes, 5/3 as far: 5% is two-thirds more than 3%. But in the US and Canada 5% is the total for feeder and branch circuit together, so a branch circuit on its own usually gets 3% or less.
Related: voltage drop calculator, wire gauge chart (AWG to mm²), how to calculate voltage drop, ampacity vs voltage drop, how the calculator works.