Voltage Drop Calculator
Find the voltage drop and percent drop in a wire run, and the smallest wire size that keeps the drop under your limit. Works for DC (12 V, 24 V, 48 V), single-phase and three-phase AC, copper or aluminium, AWG/kcmil or mm².
Voltage drop formulas
L is the one-way length of the run (source to load), R is the conductor resistance per unit length, and I is the load current. The factor 2 covers the outgoing and return conductor; in a balanced three-phase circuit the line-to-line drop is √3 times the drop in one conductor. The calculator uses the DC resistance at 75 °C from NEC Chapter 9, Table 8 (stranded conductors) for AWG/kcmil sizes. For mm² sizes it uses the IEC 60228 class 2 maximum resistance at 20 °C, corrected to 75 °C (copper α = 0.00393, aluminium α = 0.00403) so both systems are compared on the same basis.
For AC circuits up to roughly 1/0 AWG (50 mm²) the DC resistance gives an accurate answer. For larger conductors, inductive reactance and skin effect make the true drop somewhat higher, especially at low power factor; NEC Chapter 9, Table 9 effective-impedance values are the more precise method there.
How to use the result (wire size)
The smallest size for your max % drop is the first conductor in the table whose voltage drop does not exceed your limit. The NEC informational notes in 210.19(A) and 215.2(A) suggest no more than 3% on a branch circuit or feeder and 5% overall; IEC 60364-5-52 suggests 3% for lighting and 5% for other uses. Many 12 V automotive and marine designers use 3% for critical loads and 10% for non-critical ones. Voltage drop is only one check — the conductor must also have enough ampacity for the current.
Worked examples
12 V DC: a 10 A load 20 ft from the battery on 10 AWG copper (1.24 Ω/1000 ft). VD = 2 × 20 × 1.24/1000 × 10 = 0.496 V, which is 0.496/12 = 4.13%. That exceeds 3%, so the calculator recommends 8 AWG (0.778 Ω/kft → 0.311 V, 2.59%).
Three-phase: a 480 V feeder carrying 100 A over 300 ft of 1/0 AWG copper (0.122 Ω/kft). VD = 1.732 × 300 × 0.122/1000 × 100 = 6.34 V = 1.32%, comfortably below 3%.
Single-phase 240 V: 30 A over 150 ft on 10 AWG copper: VD = 2 × 150 × 0.00124 × 30 = 11.16 V = 4.65%. Upsizing to 6 AWG gives 4.42 V = 1.84%.
12V voltage drop chart
Maximum one-way cable length in feet for a 3% drop (0.36 V) on a 12 V DC system, copper conductors. For 10% (1.2 V) multiply the lengths by 3.33; for 24 V systems double them.
| Current | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG | 1/0 AWG | 2/0 AWG | 4/0 AWG |
|---|---|---|---|---|---|---|---|---|---|---|
| 5 A | 11 | 18 | 29 | 46 | 73 | 117 | 186 | 295 | 372 | 592 |
| 10 A | 6 | 9 | 15 | 23 | 37 | 58 | 93 | 148 | 186 | 296 |
| 15 A | 4 | 6 | 10 | 15 | 24 | 39 | 62 | 98 | 124 | 197 |
| 20 A | 3 | 5 | 7 | 12 | 18 | 29 | 46 | 74 | 93 | 148 |
| 30 A | 2 | 3 | 5 | 8 | 12 | 19 | 31 | 49 | 62 | 99 |
| 40 A | 1 | 2 | 4 | 6 | 9 | 15 | 23 | 37 | 47 | 74 |
| 50 A | 1 | 2 | 3 | 5 | 7 | 12 | 19 | 30 | 37 | 59 |
| 75 A | – | 1 | 2 | 3 | 5 | 8 | 12 | 20 | 25 | 39 |
| 100 A | – | – | 1 | 2 | 4 | 6 | 9 | 15 | 19 | 30 |
| 150 A | – | – | – | 2 | 2 | 4 | 6 | 10 | 12 | 20 |
| 200 A | – | – | – | 1 | 2 | 3 | 5 | 7 | 9 | 15 |
Conductor resistance table (NEC Chapter 9, Table 8)
Direct-current resistance at 75 °C, stranded, uncoated copper and aluminium.
| Size | Copper Ω/1000 ft | Aluminium Ω/1000 ft | Copper Ω/km |
|---|---|---|---|
| 18 AWG | 7.95 | 13.1 | 26.1 |
| 16 AWG | 4.99 | 8.21 | 16.4 |
| 14 AWG | 3.14 | 5.17 | 10.3 |
| 12 AWG | 1.98 | 3.25 | 6.5 |
| 10 AWG | 1.24 | 2.04 | 4.07 |
| 8 AWG | 0.778 | 1.28 | 2.55 |
| 6 AWG | 0.491 | 0.808 | 1.61 |
| 4 AWG | 0.308 | 0.508 | 1.01 |
| 3 AWG | 0.245 | 0.403 | 0.804 |
| 2 AWG | 0.194 | 0.319 | 0.636 |
| 1 AWG | 0.154 | 0.253 | 0.505 |
| 1/0 AWG | 0.122 | 0.201 | 0.4 |
| 2/0 AWG | 0.0967 | 0.159 | 0.317 |
| 3/0 AWG | 0.0766 | 0.126 | 0.251 |
| 4/0 AWG | 0.0608 | 0.1 | 0.199 |
| 250 kcmil | 0.0515 | 0.0847 | 0.169 |
| 300 kcmil | 0.0429 | 0.0707 | 0.141 |
| 350 kcmil | 0.0367 | 0.0605 | 0.12 |
| 400 kcmil | 0.0321 | 0.0529 | 0.105 |
| 500 kcmil | 0.0258 | 0.0424 | 0.0846 |
| 600 kcmil | 0.0214 | 0.0353 | 0.0702 |
| 750 kcmil | 0.0171 | 0.0282 | 0.0561 |
Ways to reduce voltage drop
- Use a larger conductor — each 3 AWG sizes up roughly halves the resistance.
- Shorten the run or move the source (panel, battery, inverter) closer to the load.
- Use a higher system voltage: the same power at 24 V instead of 12 V halves the current and quarters the percentage drop.
- Use copper instead of aluminium; aluminium has about 1.64× the resistance of copper of the same size.
Frequently asked questions
How do I calculate voltage drop?
Multiply the conductor resistance per unit length by the one-way length, the current, and 2 (for DC or single-phase) or √3 (for three-phase). Divide by the source voltage and multiply by 100 for the percentage.
What is an acceptable voltage drop?
A common target is 3% for a branch circuit or feeder and 5% total from service to load (NEC informational notes, IEC 60364 guidance). For 12 V DC systems, 3% is typical for electronics and navigation, and up to 10% for lighting or non-critical loads.
Should I enter one-way or round-trip length?
Enter the one-way distance from source to load. The formula already doubles it for the return conductor in DC and single-phase circuits.
Why is 12V voltage drop such a problem?
Voltage drop in volts depends only on current and wire resistance, so the same 0.5 V drop is 0.4% of 120 V but 4% of 12 V. Low-voltage systems also carry more current for the same power, so they need much thicker wire.
How do I size wire for voltage drop?
Enter the voltage, current, length and maximum percent drop. The calculator tries every size from smallest to largest and reports the first one that stays within your limit. Then confirm the wire also meets ampacity requirements.
Does the calculator work for 3 phase voltage drop?
Yes. Choose AC three-phase and enter the line-to-line voltage (for example 208, 400 or 480 V). The drop is calculated with the √3 factor for a balanced load.
What resistance values are used?
AWG and kcmil values are NEC Chapter 9 Table 8 DC resistance at 75 °C for stranded conductors. mm² values are IEC 60228 class 2 maximums at 20 °C corrected to 75 °C.