Voltage Drop Calculator
Volts lost, percentage drop and the longest run that stays under 3%, using NEC Chapter 9 conductor resistance.
Voltage drop = 2 × I × R × L ÷ 1000 on single-phase circuits, where R is ohms per 1000 ft. A 20 A load on 100 ft of 12 AWG copper at 120 V loses 7.72 V, which is 6.4%. Even 10 AWG drops 4.0%, so a full-load run that long needs 8 AWG to stay under 3%.
Voltage drop calculator
The formula, step by step
VD = k × I × R × L ÷ 1000
%VD = VD ÷ Vsource × 100
- k = 2 for single-phase and DC (out and back), 1.732 for three-phase.
- I = load current in amperes.
- R = conductor resistance in ohms per 1000 ft from NEC Chapter 9 Table 8 at 75 °C.
- L = one-way length in feet.
Example, three-phase feeder: 150 A on 200 ft of 4/0 aluminum at 480 V gives 1.732 × 150 × 0.100 × 200 ÷ 1000 = 5.20 V, or 1.08%. Comfortably inside 3%.
The K-factor shortcut
Older references write the same thing with circular mils: VD = 2 × K × I × L ÷ CM, with K = 12.9 Ω·cmil/ft for copper and 21.2 for aluminum. For the 12 AWG example that gives 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V, about -2% lower than the Table 8 figure because K is taken at a lower conductor temperature. This calculator uses Table 8, which errs slightly on the safe side.
Maximum distance for 3% drop
Read across to your current to find how far each copper size can run before it drops 3%. Anything longer, move down a row.
120 V single-phase
| Size (copper) | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|---|
| 14 AWG | 58 | 39 | — | — | — | — |
| 12 AWG | 93 | 62 | 46 | — | — | — |
| 10 AWG | 148 | 99 | 74 | 49 | — | — |
| 8 AWG | 231 | 154 | 115 | 77 | 57 | 46 |
| 6 AWG | 366 | 244 | 183 | 122 | 91 | 73 |
| 4 AWG | 584 | 389 | 292 | 194 | 146 | 116 |
240 V single-phase
| Size (copper) | 20 A | 30 A | 40 A | 50 A | 60 A | 80 A | 100 A |
|---|---|---|---|---|---|---|---|
| 12 AWG | 93 | — | — | — | — | — | — |
| 10 AWG | 148 | 99 | — | — | — | — | — |
| 8 AWG | 231 | 154 | 115 | 92 | — | — | — |
| 6 AWG | 366 | 244 | 183 | 146 | 122 | — | — |
| 4 AWG | 584 | 389 | 292 | 233 | 194 | 146 | — |
| 2 AWG | 927 | 618 | 463 | 371 | 309 | 231 | 185 |
| 1/0 AWG | 1475 | 983 | 737 | 590 | 491 | 368 | 295 |
The 240 V distances are exactly double the 120 V ones for the same current: the volts lost are the same, but they are a smaller share of a higher supply voltage.
Conductor resistance used (Table 8)
| Size | Area (cmil) | Copper Ω/1000 ft | Aluminum Ω/1000 ft | Cu Ω per 100 ft loop |
|---|---|---|---|---|
| 14 AWG | 4,110 | 3.07 | — | 0.614 |
| 12 AWG | 6,530 | 1.93 | 3.25 | 0.386 |
| 10 AWG | 10,380 | 1.21 | 2.04 | 0.242 |
| 8 AWG | 16,510 | 0.778 | 1.28 | 0.156 |
| 6 AWG | 26,240 | 0.491 | 0.808 | 0.098 |
| 4 AWG | 41,740 | 0.308 | 0.508 | 0.062 |
| 3 AWG | 52,620 | 0.245 | 0.403 | 0.049 |
| 2 AWG | 66,360 | 0.194 | 0.319 | 0.039 |
| 1 AWG | 83,690 | 0.154 | 0.253 | 0.031 |
| 1/0 AWG | 105,600 | 0.122 | 0.201 | 0.024 |
| 2/0 AWG | 133,100 | 0.0967 | 0.159 | 0.019 |
| 3/0 AWG | 167,800 | 0.0766 | 0.126 | 0.015 |
| 4/0 AWG | 211,600 | 0.0608 | 0.1 | 0.012 |
When 3% matters and when it does not
- Motors and compressors (well pumps, air conditioners, shop tools) draw several times their running current at start. A circuit that drops 5% running may dip 15–20% at start, causing slow starts, nuisance trips and short motor life.
- LED drivers and electronics tolerate a wide input range, so a 4% drop on a lighting circuit is rarely noticed.
- Resistive heaters simply produce less heat: power falls with the square of voltage, so a 5% drop costs about 10% of output.
- EV chargers are continuous loads. Keep the drop at or below 3% at full charging current so the charger does not derate itself.
Limits of this calculation
The calculation assumes unity power factor and DC resistance. For AC feeders of 250 kcmil and larger, or inductive loads with a low power factor, use the effective impedance from NEC Chapter 9 Table 9; it adds a few percent to the result. It also assumes the conductors are at 75 °C, which is the right assumption for a fully loaded circuit and slightly pessimistic for a lightly loaded one.
Related calculators and charts
- Wire size calculatorAmpacity plus voltage drop in one answer.
- AWG wire gauge chartEvery gauge with diameter, mm² and resistance.
- 50 amp wire sizeMinimum size and distance table for 50 A circuits.
Questions people ask
What is an acceptable voltage drop?
The NEC informational notes in 210.19 and 215.2 suggest no more than 3% on a branch circuit and 5% total for feeder plus branch. They are recommendations, not enforceable rules, but most designers, utilities and equipment makers treat them as the target.
Do I enter the one-way or the total wire length?
One-way, from the source to the load. On a single-phase or DC circuit the current goes out on one conductor and back on the other, so the formula multiplies by 2. Three-phase uses √3 (1.732) instead.
Why does my result differ from other calculators?
Calculators use either the conductor resistance from NEC Chapter 9 Table 8 (this page), the K-factor shortcut (12.9 for copper, 21.2 for aluminum) or AC impedance from Table 9. For 4/0 and smaller at unity power factor they agree within a few percent. For large conductors on AC, Table 9 gives a slightly higher drop because of reactance.
How do I reduce voltage drop?
Use a larger conductor, shorten the run, move the load to 240 V instead of 120 V (same power, half the current, a quarter of the percentage drop), or split the load across two circuits. Swapping aluminum for copper of the same size also cuts the drop by about 38%.
Does voltage drop waste electricity?
Yes. The drop times the current is power turned into heat in the wire. In the 12 AWG example above, 7.7 V × 20 A ≈ 154 W is lost for as long as the load runs.