Voltage Drop Calculator
Find the voltage lost along a run of wire and the smallest conductor that keeps it within the recommended 3%. Works for single-phase and three-phase, copper and aluminum.
Rule of thumb: keep branch-circuit voltage drop to 3% (3.6 V on 120 V, 7.2 V on 240 V) and feeder plus branch to 5%. On 120 V, 12 AWG copper at 16 A reaches 3% at about 57 feet.
Voltage drop calculator
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Voltage drop formula
Three-phase: VD = √3 × L × R × I ÷ 1000
% drop = VD ÷ source voltage × 100
L = one-way length in feet · R = resistance in Ω per 1000 ft · I = load current in amps.
Worked example. A 240 V, 40 A EV charger 150 feet from the panel on 8 AWG copper: 2 × 150 × 0.778 × 40 ÷ 1000 = 9.3 V, or 3.9%. Over the target. On 6 AWG: 2 × 150 × 0.491 × 40 ÷ 1000 = 5.9 V, or 2.5%.
The calculator uses DC resistance at 75°C. That is accurate for the sizes used on most branch circuits; for conductors larger than about 1/0 AWG carrying AC in steel conduit, reactance adds roughly 5–25%, so treat the result as a minimum and use effective-impedance values for critical feeders.
Maximum circuit length for 3% drop
One-way distance in feet, copper conductors. Blank cells exceed the conductor’s ampacity or breaker limit.
| Copper size | 5 A | 10 A | 12 A | 15 A | 16 A | 20 A | 24 A | 30 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 114 | 57 | 47 | 38 | — | — | — | — |
| 12 AWG | 181 | 90 | 75 | 60 | 56 | 45 | — | — |
| 10 AWG | 290 | 145 | 120 | 96 | 90 | 72 | 60 | 48 |
| 8 AWG | 462 | 231 | 192 | 154 | 144 | 115 | 96 | 77 |
| 6 AWG | 733 | 366 | 305 | 244 | 229 | 183 | 152 | 122 |
| 4 AWG | 1168 | 584 | 487 | 389 | 365 | 292 | 243 | 194 |
| 3 AWG | 1469 | 734 | 612 | 489 | 459 | 367 | 306 | 244 |
| 2 AWG | 1855 | 927 | 773 | 618 | 579 | 463 | 386 | 309 |
| 1 AWG | 2337 | 1168 | 974 | 779 | 730 | 584 | 487 | 389 |
| 1/0 AWG | 2950 | 1475 | 1229 | 983 | 922 | 737 | 614 | 491 |
| Copper size | 15 A | 20 A | 24 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|---|
| 14 AWG | 76 | — | — | — | — | — | — | — | — |
| 12 AWG | 121 | 90 | — | — | — | — | — | — | — |
| 10 AWG | 193 | 145 | 120 | 96 | — | — | — | — | — |
| 8 AWG | 308 | 231 | 192 | 154 | 115 | 92 | — | — | — |
| 6 AWG | 488 | 366 | 305 | 244 | 183 | 146 | 122 | — | — |
| 4 AWG | 779 | 584 | 487 | 389 | 292 | 233 | 194 | 146 | — |
| 3 AWG | 979 | 734 | 612 | 489 | 367 | 293 | 244 | 183 | 146 |
| 2 AWG | 1237 | 927 | 773 | 618 | 463 | 371 | 309 | 231 | 185 |
| 1 AWG | 1558 | 1168 | 974 | 779 | 584 | 467 | 389 | 292 | 233 |
| 1/0 AWG | 1967 | 1475 | 1229 | 983 | 737 | 590 | 491 | 368 | 295 |
| Copper size | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 176 | — | — | — | — | — | — | — |
| 12 AWG | 279 | 209 | — | — | — | — | — | — |
| 10 AWG | 446 | 335 | 223 | — | — | — | — | — |
| 8 AWG | 712 | 534 | 356 | 267 | 213 | — | — | — |
| 6 AWG | 1128 | 846 | 564 | 423 | 338 | 282 | — | — |
| 4 AWG | 1799 | 1349 | 899 | 674 | 539 | 449 | 337 | — |
| 3 AWG | 2262 | 1696 | 1131 | 848 | 678 | 565 | 424 | 339 |
| 2 AWG | 2856 | 2142 | 1428 | 1071 | 857 | 714 | 535 | 428 |
| 1 AWG | 3599 | 2699 | 1799 | 1349 | 1079 | 899 | 674 | 539 |
| 1/0 AWG | 4543 | 3407 | 2271 | 1703 | 1362 | 1135 | 851 | 681 |
Conductor resistance
| Size | Circular mils | Copper Ω / 1000 ft | Aluminum Ω / 1000 ft |
|---|---|---|---|
| 14 AWG | 4,110 | 3.14 | — |
| 12 AWG | 6,530 | 1.98 | 3.25 |
| 10 AWG | 10,380 | 1.24 | 2.04 |
| 8 AWG | 16,510 | 0.778 | 1.28 |
| 6 AWG | 26,240 | 0.491 | 0.808 |
| 4 AWG | 41,740 | 0.308 | 0.508 |
| 3 AWG | 52,620 | 0.245 | 0.403 |
| 2 AWG | 66,360 | 0.194 | 0.319 |
| 1 AWG | 83,690 | 0.154 | 0.253 |
| 1/0 AWG | 105,600 | 0.122 | 0.201 |
| 2/0 AWG | 133,100 | 0.0967 | 0.159 |
| 3/0 AWG | 167,800 | 0.0766 | 0.126 |
| 4/0 AWG | 211,600 | 0.0608 | 0.1 |
| 250 kcmil | 250,000 | 0.0515 | 0.0847 |
| 300 kcmil | 300,000 | 0.0429 | 0.0707 |
| 350 kcmil | 350,000 | 0.0367 | 0.0605 |
| 400 kcmil | 400,000 | 0.0321 | 0.0529 |
| 500 kcmil | 500,000 | 0.0258 | 0.0424 |
| 600 kcmil | 600,000 | 0.0214 | 0.0353 |
| 700 kcmil | 700,000 | 0.0184 | 0.0303 |
| 750 kcmil | 750,000 | 0.0171 | 0.0282 |
| 800 kcmil | 800,000 | 0.0161 | 0.0265 |
| 900 kcmil | 900,000 | 0.0143 | 0.0235 |
| 1000 kcmil | 1,000,000 | 0.0129 | 0.0212 |
Frequently asked questions
What is the maximum voltage drop allowed by the NEC?
For ordinary circuits the NEC does not set a mandatory limit. Informational notes recommend no more than 3% on a branch circuit or feeder and 5% total to the farthest outlet. Limits are mandatory in a few places, such as fire pumps and sensitive electronic equipment, and many energy codes make the 5% total enforceable.
What is the voltage drop formula?
Single-phase: VD = 2 × L × R × I ÷ 1000. Three-phase: VD = 1.732 × L × R × I ÷ 1000. L is the one-way length in feet, R the conductor resistance in ohms per 1000 feet and I the load current in amps. Using the K-factor form: VD = 2 × K × I × L ÷ circular mils, with K ≈ 12.9 for copper and 21.2 for aluminum.
How far can I run 12 gauge wire on a 20 amp circuit?
At a full 20 A load and 120 V, about 45 feet one way for a 3% drop; at a more typical 12 A load, about 75 feet. Beyond that, go up to 10 AWG.
Does upsizing the wire for voltage drop change anything else?
Yes: the equipment grounding conductor must be upsized proportionally, the larger wire must still fit the device terminals, and conduit and box fill need rechecking.
NECTables.com is an independent reference and is not affiliated with, endorsed by, or a substitute for the National Fire Protection Association (NFPA). NEC® and National Electrical Code® are registered trademarks of the NFPA. Values shown are commonly published engineering data and independent calculations based on the methods of NFPA 70; they are not a reproduction of the Code. Always confirm against the edition adopted in your jurisdiction, including local amendments, and consult a licensed electrician or engineer before doing electrical work.