Voltage drop formula and how to calculate it
Every conductor has resistance, so some voltage is lost along the run. The calculator uses the standard circular-mil formula:
Single phase: VD = 2 × K × I × L ÷ CM
Three phase: VD = 1.732 × K × I × L ÷ CM
K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I is the current in amps, L is the one-way length in feet, and CM is the conductor area in circular mils from NEC Chapter 9, Table 8.
Example
A 20 A load on 12 AWG copper, 100 ft from a 120 V panel: 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V. That is a 6.6% drop, well over the 3% guideline, so you would step up to 10 AWG or 8 AWG. The wire size calculator finds the smallest size that passes automatically.
DC and 12 V voltage drop
DC uses the same single-phase formula, because current flows out and back on two conductors. There is no power factor and no reactance, so the result is exact for the resistance values used. Pick "DC or single phase AC" and enter your battery or supply voltage: 12, 24 or 48.
Low voltages make small drops matter. At 12 V, 3% is only 0.36 V, so a run that is fine at 120 V can fail badly. Example: a 10 A load on 10 AWG copper, 15 ft one-way from a 12 V battery: 2 × 12.9 × 10 × 15 ÷ 10,380 = 0.37 V, which is a 3.1% drop. Moving the same load to 8 AWG cuts it to about 2%. For 12 V solar, RV, boat and trailer wiring, see the 12V wire gauge chart.
Circular mil area by wire size
| Size | Circular mils | Size | Circular mils |
|---|---|---|---|
| 14 AWG | 4,110 | 2 AWG | 66,360 |
| 12 AWG | 6,530 | 1 AWG | 83,690 |
| 10 AWG | 10,380 | 1/0 | 105,600 |
| 8 AWG | 16,510 | 2/0 | 133,100 |
| 6 AWG | 26,240 | 3/0 | 167,800 |
| 4 AWG | 41,740 | 4/0 | 211,600 |
How much voltage drop is there per 100 feet?
On 12 AWG copper carrying 20 A, about 7.9 V per 100 ft of one-way run, which is 6.6% at 120 V. On 10 AWG it is about 5.0 V and on 8 AWG about 3.1 V. Voltage drop rises in direct proportion to current and distance.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% combined for feeder plus branch. Sensitive electronics and motors benefit from staying under 3%.
Do I use one-way or round-trip length?
Enter the one-way distance from the source to the load. The single-phase formula already doubles it to account for the return conductor.
Does voltage drop matter on 240 V circuits?
Yes, but the same volts lost is a smaller percentage of 240 V, so a 240 V circuit can usually run farther on the same wire than a 120 V circuit.
How do I calculate voltage drop for 12 volt DC?
Use the same formula: VD = 2 × K × I × L ÷ CM, with K = 12.9 for copper, and divide by 12 for the percentage. A 20 A load on 10 AWG copper over 10 ft one-way drops 0.50 V, which is 4.1% of 12 V. Choose "DC or single phase AC" and enter 12 as the voltage.
What voltage drop is acceptable on a 12V system?
The NEC's 3% recommendation is a common target, which is 0.36 V on 12 V. Many designers allow more for loads that tolerate it, but check the lowest voltage your equipment accepts, because a 12 V battery sags under load and the wire drop comes on top of that.
Why is aluminum drop higher?
Aluminum has about 61% of copper's conductivity, so for the same size and current it loses more voltage. Aluminum is usually upsized by about two AWG sizes to match copper.
Sources
Reference charts
- Wire Gauge Chart
- AWG to mm Chart
- 12V Wire Gauge Chart
- Amps per Wire Size
- Breaker to Wire Size Chart
- Wire Size for 200 Amp Service
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