Voltage Drop Quick Table (Max Cable Length for 3% Drop)
Find the longest cable run that stays within 3% voltage drop for 12 V DC and 230 V AC, by wire size and current.
Voltage Drop Quick Table: Maximum Cable Length for 3% Drop
Voltage drop is the voltage lost along a cable because of its resistance. Too much drop makes lights dim, motors run hot and electronics reset. A common design target is 3% for a branch circuit and 5% in total from source to load. These tables give the longest run (one way, in metres) that stays within 3%, so you can size a cable without a calculator.
L = one-way length (m), I = current (A), R = conductor resistance (Ω/m = Ω/km ÷ 1000)
Table 1: 12 V DC, maximum one-way length (m) for 3% drop (0.36 V)
| Wire size | 5 A | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|---|---|---|
| 16 AWG | 2.7 | 1.4 | 0.9 | 0.7 | 0.5 | 0.3 | 0.3 |
| 14 AWG | 4.3 | 2.2 | 1.4 | 1.1 | 0.7 | 0.5 | 0.4 |
| 12 AWG | 6.9 | 3.5 | 2.3 | 1.7 | 1.2 | 0.9 | 0.7 |
| 10 AWG | 11 | 5.5 | 3.7 | 2.7 | 1.8 | 1.4 | 1.1 |
| 8 AWG | 17 | 8.7 | 5.8 | 4.4 | 2.9 | 2.2 | 1.7 |
| 6 AWG | 28 | 14 | 9.3 | 6.9 | 4.6 | 3.5 | 2.8 |
| 4 AWG | 44 | 22 | 15 | 11 | 7.4 | 5.5 | 4.4 |
| 2 AWG | 70 | 35 | 23 | 18 | 12 | 8.8 | 7.0 |
| 1/0 AWG | 112 | 56 | 37 | 28 | 19 | 14 | 11 |
| 2/0 AWG | 141 | 70 | 47 | 35 | 23 | 18 | 14 |
| 4/0 AWG | 224 | 112 | 75 | 56 | 37 | 28 | 22 |
Other voltages and limits: for 24 V multiply the length by 2, for 48 V by 4. For a 5% limit multiply by 1.67, for 2% by 0.67. Lengths are for copper at 20°C; a hot cable (about 70°C) has roughly 20% more resistance, so multiply by 0.8 for hot installations. Currents are in amps across the top row.
Table 2: 230 V single-phase AC, maximum one-way length (m) for 3% drop (6.9 V)
| Copper size | 6 A | 10 A | 16 A | 20 A | 25 A | 32 A |
|---|---|---|---|---|---|---|
| 1.5 mm² | 48 | 29 | 18 | 14 | 11 | 8.9 |
| 2.5 mm² | 78 | 47 | 29 | 23 | 19 | 15 |
| 4 mm² | 125 | 75 | 47 | 37 | 30 | 23 |
| 6 mm² | 187 | 112 | 70 | 56 | 45 | 35 |
| 10 mm² | 314 | 189 | 118 | 94 | 75 | 59 |
| 16 mm² | 500 | 300 | 188 | 150 | 120 | 94 |
| 25 mm² | 791 | 475 | 297 | 237 | 190 | 148 |
| 35 mm² | 1097 | 658 | 411 | 329 | 263 | 206 |
Resistance values are IEC 60228 maximum DC resistance at 20°C. Reactance is ignored, which is acceptable for these small sizes; it becomes noticeable for larger cables on long AC runs. Use 400 V three-phase by replacing the factor 2 with √3 (1.73) and the allowed drop with 3% of 400 V.
Table 3: Voltage drop per amp per metre
Multiply by the current and the one-way length (m) to get the drop in millivolts.
| Wire size | R (Ω/km) | DC / single-phase (mV/A/m) | Three-phase (mV/A/m) |
|---|---|---|---|
| 16 AWG | 13.17 | 26.3 | 22.8 |
| 14 AWG | 8.28 | 16.6 | 14.3 |
| 12 AWG | 5.21 | 10.4 | 9.02 |
| 10 AWG | 3.28 | 6.55 | 5.68 |
| 8 AWG | 2.06 | 4.12 | 3.57 |
| 6 AWG | 1.30 | 2.59 | 2.24 |
| 4 AWG | 0.82 | 1.63 | 1.41 |
| 2 AWG | 0.51 | 1.03 | 0.888 |
| 1/0 AWG | 0.32 | 0.645 | 0.558 |
| 2/0 AWG | 0.26 | 0.511 | 0.443 |
| 4/0 AWG | 0.16 | 0.322 | 0.278 |
1. Try 8 AWG: R = 2.061 Ω/km, so Vdrop = 2 × 6 × 20 × 0.002061 = 0.49 V (4.1%). Too much.
2. Try 6 AWG: R = 1.296 Ω/km, so Vdrop = 2 × 6 × 20 × 0.001296 = 0.31 V (2.6%). Within 3%.
3. Table 1 agrees: 6 AWG at 20 A allows 6.9 m one way, 8 AWG only 4.4 m.
4. Also check the ampacity: 6 AWG at 20 A is well inside the wire gauge table limits.
- Using the total round-trip length as the length. These tables already include the return conductor; enter the one-way distance from source to load.
- Checking only ampacity. Low-voltage DC systems (12 V, 24 V solar and battery wiring) usually fail on voltage drop long before they fail on current.
- Ignoring starting current. Motors and compressors draw several times their running current for a moment, which can pull a long cable below the voltage the load needs.
- Applying the 3% limit to every part. The recommended totals are about 3% for branch circuits and 5% for feeder plus branch together. Check your local code.
Frequently asked questions
How much voltage drop is acceptable? Many codes and design guides recommend no more than 3% on a branch circuit and 5% overall. Sensitive electronics and low-voltage DC systems often need less.
How do I reduce voltage drop? Use a thicker conductor, shorten the run, raise the system voltage (for example 24 V instead of 12 V), or reduce the current.
Why is the drop worse on 12 V than on 230 V? The same cable loses the same number of volts, but 0.36 V is 3% of 12 V while it is only 0.16% of 230 V.
Safety note. These figures are reference values and not a substitute for your local electrical code or a qualified electrician.