Voltage drop calculator
Voltage drop, load voltage and wire loss for a run, and the smallest wire size that keeps the drop within your target.
- Drop as share of supply
- 8.58 %
- Voltage at the load
- 21.94 V
- Power lost in the wire
- 20.6 W
- Smallest gauge within 3 % drop
- 8 AWG
- Resistance per conductor
- 10.3 Ω/km
| Size | Ω/km | Drop (V) | Drop (%) | Within 3 % |
|---|---|---|---|---|
| 18 AWG | 26.1 | 5.22 | 21.8 | No |
| 16 AWG | 16.4 | 3.28 | 13.7 | No |
| 14 AWG | 10.3 | 2.06 | 8.58 | No |
| 12 AWG | 6.5 | 1.3 | 5.42 | No |
| 10 AWG | 4.07 | 0.814 | 3.39 | No |
| 8 AWG | 2.551 | 0.51 | 2.13 | Yes |
| 6 AWG | 1.608 | 0.322 | 1.34 | Yes |
| 4 AWG | 1.01 | 0.202 | 0.842 | Yes |
| 3 AWG | 0.802 | 0.16 | 0.668 | Yes |
| 2 AWG | 0.634 | 0.127 | 0.528 | Yes |
| 1 AWG | 0.505 | 0.101 | 0.421 | Yes |
| 1/0 AWG | 0.399 | 0.0798 | 0.332 | Yes |
| 2/0 AWG | 0.317 | 0.0634 | 0.264 | Yes |
| 3/0 AWG | 0.2512 | 0.0502 | 0.209 | Yes |
| 4/0 AWG | 0.1996 | 0.0399 | 0.166 | Yes |
| 250 kcmil | 0.1687 | 0.0337 | 0.141 | Yes |
| 300 kcmil | 0.1409 | 0.0282 | 0.117 | Yes |
| 350 kcmil | 0.1205 | 0.0241 | 0.1 | Yes |
| 400 kcmil | 0.1053 | 0.0211 | 0.0877 | Yes |
| 500 kcmil | 0.0845 | 0.0169 | 0.0704 | Yes |
| 600 kcmil | 0.0704 | 0.0141 | 0.0587 | Yes |
| 700 kcmil | 0.0603 | 0.0121 | 0.0503 | Yes |
| 750 kcmil | 0.0563 | 0.0113 | 0.0469 | Yes |
| 800 kcmil | 0.0528 | 0.0106 | 0.044 | Yes |
| 900 kcmil | 0.047 | 0.0094 | 0.0392 | Yes |
| 1000 kcmil | 0.0423 | 0.00846 | 0.0352 | Yes |
| 1250 kcmil | 0.0338 | 0.00676 | 0.0282 | Yes |
| 1500 kcmil | 0.02814 | 0.00563 | 0.0235 | Yes |
| 1750 kcmil | 0.0241 | 0.00482 | 0.0201 | Yes |
| 2000 kcmil | 0.02109 | 0.00422 | 0.0176 | Yes |
How this is calculated
R(T) = R75 × [1 + α (T − 75)] α = 0.00323 Cu, 0.00330 Al DC, AC 1-phase: Vd = 2 × I × R × L AC 3-phase: Vd = √3 × I × R × L Loss: P = 2 I² R L (DC, 1-phase), 3 I² R L (3-phase) L = one-way length, R per meter of one conductor
- Resistance
- NEC Chapter 9 Table 8 DC resistance at 75 °C for stranded uncoated copper and aluminum, 18 AWG to 2000 kcmil, corrected to the conductor temperature with the equation in Table 8, note 2.
- AC
- AC results use resistance only, as if the power factor were 1. Reactance and skin effect (NEC Chapter 9 Table 9) are not included because those values were not verified for this page; the error grows with conductor size, mostly from 1/0 AWG up.
- Target
- The target percentage is yours to set. For general circuits the NEC gives 3 % (branch) and 5 % (feeder plus branch) only in an informational note to 210.19(A), not as a requirement.
- Not included
- Sizes smaller than 18 AWG (not in Table 8), connector and contact resistance, and coated or compact stranding.
Questions
It depends on what the load tolerates. For building wiring, an NEC informational note to 210.19(A) cites 3 % on a branch circuit and 5 % for feeder and branch together as reasonable efficiency. On a 12 V DC system 3 % is only 0.36 V, so low-voltage runs usually need a much larger wire than ampacity alone requires.
Drop is set by current and resistance, not voltage, so the same 0.5 V loss is 4 % at 12 V and about 0.4 % at 120 V. At 10 A over 10 m one way, 14 AWG copper at 75 °C drops 2.06 V: 17 % of 12 V. Keeping that under 3 % needs 6 AWG.
Enter the one-way length from source to load. For DC and single-phase AC the calculator doubles it for the return conductor; for three-phase it uses √3 times the one-way length, which gives line-to-line drop for a balanced load.
Copper resistance changes by 0.323 % per °C relative to 75 °C (Table 8, note 2). 14 AWG copper is 10.3 Ω/km at 75 °C and 8.47 Ω/km at 20 °C, so a cold run drops about 18 % less than the 75 °C figure.