The formula
VD = 2 × K × I × L ÷ CM
K = 12.9 copper · 21.2 aluminium (75 °C)
K = 12.9 copper · 21.2 aluminium (75 °C)
L is the one-way distance — the 2 already accounts for the return path. CM is the conductor area in circular mils; larger wire means more copper and proportionally less drop.
| AWG | Circular mils | AWG | Circular mils |
|---|---|---|---|
| 14 | 4,110 | 6 | 26,240 |
| 12 | 6,530 | 4 | 41,740 |
| 10 | 10,380 | 2 | 66,360 |
| 8 | 16,510 | 1/0 | 105,600 |
Example: 120 V, 15 A, 100 ft
A typical outbuilding or garage circuit. Ampacity tables happily allow 14 AWG for 15 amps. Voltage drop disagrees:
| Wire | Drop | % of 120 V | Verdict |
|---|---|---|---|
| 14 AWG | 9.42 V | 7.85% | Fails |
| 12 AWG | 5.93 V | 4.94% | Fails |
| 10 AWG | 3.73 V | 3.11% | Marginal |
| 8 AWG | 2.34 V | 1.95% | Passes |
| 6 AWG | 1.47 V | 1.23% | Passes |
This is the trap: a circuit can be perfectly legal on ampacity and still deliver poor voltage at the far end. Ampacity protects the wire from overheating. Voltage drop protects the equipment.
Check any run against the 3% guideline.Voltage drop calculator →
Why 12 V systems suffer
Take a solar, RV or boat circuit: 12 V, 20 A, 30 ft one-way. A short run by household standards.
12 V · 20 A · 30 ft — drop as % of supply
12 AWG19.8%
10 AWG12.4%
8 AWG7.8%
6 AWG4.9%
4 AWG3.1%
2 AWG1.9%
You need 2 AWG to comfortably pass — for a 30-foot run carrying 240 watts. The identical 1.49 V lost on 10 AWG would be a rounding error at 120 V.
Three ways to fix a failing run
- Upsize the conductor. Each step down in AWG number cuts drop by roughly 37%. Simple, and often expensive over long distances.
- Raise the system voltage. The most powerful fix. That same 100 ft, 15 A run on 12 AWG drops 5.93 V either way — but at 240 V that is 2.47% and passes, where at 120 V it was 4.94% and failed. Doubling voltage halves the percentage drop.
- Shorten the run. Drop is directly proportional to length. Moving a sub-panel closer to the load is sometimes cheaper than the copper.
Why higher voltage wins twice: for the same power, doubling the voltage halves the current. Since drop is proportional to current and the percentage is measured against a larger number, the improvement compounds. This is why long solar array runs use high-voltage strings rather than thick low-voltage cable.
What excessive drop actually does
- Motors draw more current at reduced voltage, running hotter and shortening their life.
- Lighting dims noticeably, and LED drivers may flicker or drop out.
- Heaters lose output with the square of voltage — a 10% drop is roughly 19% less heat.
- Everything wastes the lost voltage as heat in the wall.
The underlying mechanism is nothing more than Ohm's law applied to the conductor: the wire has resistance, current flows through it, and V = I × R does the rest.
Run your own numbers
FAQ
What is the voltage drop formula?
VD = 2 × K × I × L ÷ CM for DC and single-phase circuits, where K is 12.9 for copper and 21.2 for aluminium at 75 °C, I is amps, L is the one-way length in feet, and CM is the conductor's area in circular mils. The 2 accounts for current travelling out and back; three-phase uses 1.732 instead.
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% total across feeder and branch combined. These are recommendations in informational notes rather than enforceable requirements, but equipment performance and efficiency both suffer beyond them.
What wire do I need for a 100 ft, 15 A circuit at 120 V?
8 AWG copper to stay under 3%. Standard 14 AWG drops 7.85% and 12 AWG drops 4.94% — both fail. 10 AWG comes in at 3.11%, marginally over. Ampacity alone would have allowed 14 AWG.
Why is voltage drop worse on 12 V systems?
Because the drop is a percentage of the source voltage. Losing 1.5 volts on a 120 V circuit is 1.25% and harmless; losing the same 1.5 volts on a 12 V circuit is 12.5% and will dim lights and stall motors. Low-voltage runs need dramatically heavier cable.
Does aluminium wire drop more voltage than copper?
Yes, roughly 64% more for the same size, because its K value is 21.2 versus copper's 12.9. Aluminium conductors are normally specified one to two sizes larger than the copper equivalent to compensate.
Sources
Primary references used for the figures and rules on this page.
- NFPA 70: National Electrical Code — NFPA
- Electrical Standards — OSHA