Voltage Drop Calculator
Pick the wire size and material, the system voltage and phase, the load current and the one-way run length. You get the drop in volts and percent against the NEC 3% and 5% recommendations, and the voltage that actually arrives at the load.
Panel to load. The calculator doubles it for the return conductor.
Voltage drop
5.79 V
Percent drop
4.82%
Over 3%: acceptable only if the feeder adds under 2%
Voltage at the load
114.2 V
12 copper ampacity
25 A
NEC 310.16, 75°C
Formula: 2 × 100 ft × 15 A × 1.9300 Ω/kft ÷ 1000 = 5.79 V. Resistance from NEC Chapter 9 Table 8 (DC, 75°C); AC reactance is negligible below 1/0 AWG.
Maximum one-way run for 3% drop, copper, 120 V
Feet of run at which each gauge reaches 3% drop at the current shown (single-phase). Double the distances for 240 V.
| AWG | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|---|
| 14 | 58 ft | 39 ft | 29 ft | — | — | — |
| 12 | 93 ft | 62 ft | 46 ft | — | — | — |
| 10 | 148 ft | 99 ft | 74 ft | 49 ft | — | — |
| 8 | 235 ft | 157 ft | 117 ft | 78 ft | 58 ft | 47 ft |
| 6 | 366 ft | 244 ft | 183 ft | 122 ft | 91 ft | 73 ft |
| 4 | 584 ft | 389 ft | 292 ft | 194 ft | 146 ft | 116 ft |
| 2 | 927 ft | 618 ft | 463 ft | 309 ft | 231 ft | 185 ft |
| 1/0 | 1475 ft | 983 ft | 737 ft | 491 ft | 368 ft | 295 ft |
Dashes: current exceeds the conductor's 75°C ampacity, so the question does not arise.
Percent drop at 20 A, 120 V, by gauge and run
| AWG (copper) | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft | 200 ft | 300 ft |
|---|---|---|---|---|---|---|---|
| 14 | 2.6% | 5.1% | 7.7% | 10.2% | 15.3% | 20.5% | 30.7% |
| 12 | 1.6% | 3.2% | 4.8% | 6.4% | 9.6% | 12.9% | 19.3% |
| 10 | 1.0% | 2.0% | 3.0% | 4.0% | 6.0% | 8.1% | 12.1% |
| 8 | 0.6% | 1.3% | 1.9% | 2.5% | 3.8% | 5.1% | 7.6% |
| 6 | 0.4% | 0.8% | 1.2% | 1.6% | 2.5% | 3.3% | 4.9% |
Amber: over 3%. Red: over 5%. 14 AWG is shown for the arithmetic; it is limited to a 15 A breaker by NEC 240.4(D).
Conductor resistance used (NEC Chapter 9, Table 8)
| Size | Copper Ω/kft | Aluminum Ω/kft | Cu ampacity 75°C | Al ampacity 75°C |
|---|---|---|---|---|
| 14 | 3.07 | 5.06 | 20 A | — |
| 12 | 1.93 | 3.18 | 25 A | 20 A |
| 10 | 1.21 | 2 | 35 A | 30 A |
| 8 | 0.764 | 1.26 | 50 A | 40 A |
| 6 | 0.491 | 0.808 | 65 A | 50 A |
| 4 | 0.308 | 0.508 | 85 A | 65 A |
| 3 | 0.245 | 0.403 | 100 A | 75 A |
| 2 | 0.194 | 0.319 | 115 A | 90 A |
| 1 | 0.154 | 0.253 | 130 A | 100 A |
| 1/0 | 0.122 | 0.201 | 150 A | 120 A |
| 2/0 | 0.0967 | 0.159 | 175 A | 135 A |
| 3/0 | 0.0766 | 0.126 | 200 A | 155 A |
| 4/0 | 0.0608 | 0.1 | 230 A | 180 A |
| 250 kcmil | 0.0515 | 0.0847 | 255 A | 205 A |
| 300 kcmil | 0.0429 | 0.0707 | 285 A | 230 A |
| 350 kcmil | 0.0367 | 0.0605 | 310 A | 250 A |
| 400 kcmil | 0.0321 | 0.0529 | 335 A | 270 A |
| 500 kcmil | 0.0258 | 0.0424 | 380 A | 310 A |
DC resistance at 75°C, uncoated conductors. For AC circuits above about 1/0 AWG, reactance adds to the impedance and the NEC Table 9 values should be used; below that the difference is within a few percent.
Frequently Asked Questions
What is the voltage drop formula?
For single-phase or DC: Vdrop = 2 × L × I × R ÷ 1000, where L is the one-way length in feet, I the current in amps and R the conductor resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8. The 2 accounts for the return conductor. For three-phase, replace the 2 with √3 (1.732). Percent drop is Vdrop ÷ system voltage × 100.
What is the maximum allowable voltage drop?
The NEC does not mandate a limit for most circuits; the informational note to 210.19(A) recommends no more than 3% on a branch circuit and 5% for the feeder and branch circuit combined. Some jurisdictions and energy codes make the 3% and 5% mandatory. Motors and electronics are the loads that suffer most: a motor at 90% voltage draws more current and runs hot.
How far can I run 12 gauge wire on a 20 amp circuit?
At a full 20 A on 120 V, 12 AWG copper stays under 3% out to about 46 feet one way. At 240 V the same wire and current reach about 93 feet, because the same volts dropped are a smaller share. Most 20 A circuits never carry 20 A continuously, so real runs are often longer; size for the load you actually expect.
Does voltage drop matter for a 12 V or 24 V system?
More than anywhere else. Three percent of 12 V is 0.36 V, which a few feet of thin wire at 10 A will consume. That is why solar, RV, boat and automotive wiring uses much heavier gauge than the current alone would suggest, and why the calculator lets you pick DC and low voltages.
Copper or aluminum?
Aluminum has about 1.6 times the resistance of copper for the same gauge, so it needs to be roughly two sizes larger to match. It is common and code-compliant for service entrances and large feeders where the cost saving is real; it needs the right terminations and anti-oxidant. For branch circuits under 8 AWG, copper is the norm.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.