How to use the voltage drop calculator
- Choose the conductor material: copper or aluminum.
- Choose AWG / kcmil (US sizes) or mm² (metric sizes), then pick the wire size.
- Enter the one-way length from the panel or source to the load, in feet or meters.
- Enter the load current in amps and the source voltage.
- Pick single-phase or DC, or three-phase. For three-phase, enter the line-to-line voltage, such as 208 V or 480 V.
- Read the drop in volts and percent, the voltage left at the load, and the table comparing every size.
How voltage drop is calculated
The calculator uses the K-factor method common in US electrical practice:
K is the conductor's resistivity in ohm-circular-mils per foot. These values reproduce the DC resistances for stranded conductors at 75 °C in NEC Chapter 9, Table 8; for example, 12.9 ÷ 6,530 cmil gives 1.98 Ω per 1,000 ft for 12 AWG copper. The 2 accounts for current flowing out and back on two wires. In a balanced three-phase circuit the factor is √3 (1.732) instead. Metric sizes are converted at 1 mm² = 1,973.5 circular mils.
Worked example
The defaults are a 12 AWG copper circuit, 50 ft one way, carrying 16 A at 120 V, single-phase:
- Resistance: 12.9 ÷ 6,530 × 1,000 = 1.975 Ω per 1,000 ft
- Voltage drop: 2 × 12.9 × 16 × 50 ÷ 6,530 = 20,640 ÷ 6,530 = 3.16 V
- Percent drop: 3.16 ÷ 120 × 100 = 2.63%, inside the 3% guideline
- Voltage at the load: 120 − 3.16 = 116.84 V
- Power lost as heat in the wires: 3.16 V × 16 A = 50.6 W
The same run in 14 AWG copper loses 5.02 V (4.18%), and in 10 AWG only 1.99 V (1.66%). In 12 AWG aluminum it would lose 5.19 V (4.33%).
The 3% and 5% guideline
The National Electrical Code has informational notes (in 210.19 for branch circuits and 215.2 for feeders) suggesting no more than 3% drop on a branch circuit and no more than 5% in total from the service to the farthest outlet. These notes are advice, not enforceable rules, so an inspector can't fail a job for exceeding them. A few specific NEC articles, such as those for fire pumps and sensitive electronic equipment, do set mandatory limits. Too much drop makes motors run hot, lights dim and electronics misbehave.
| Wire size | Copper, Ω per 1,000 ft | Aluminum, Ω per 1,000 ft |
|---|---|---|
| 14 AWG | 3.14 | 5.16 |
| 12 AWG | 1.98 | 3.25 |
| 10 AWG | 1.24 | 2.04 |
| 8 AWG | 0.781 | 1.28 |
| 6 AWG | 0.492 | 0.808 |
| 4 AWG | 0.309 | 0.508 |
| 2 AWG | 0.194 | 0.320 |
| 1/0 AWG | 0.122 | 0.201 |
Limits of this method
The calculation uses resistance only. For conductors up to about 1/0 AWG, or loads near unity power factor, that is close enough. Large cables in steel conduit feeding motors also have reactance, so their drop can be a little higher; NEC Chapter 9, Table 9 gives AC impedance values for that case. Voltage drop is also only one check. A wire must have enough ampacity for the current (NEC 310.16) and the right overcurrent protection, and the smallest size that passes the drop test may still be too small for the breaker. Use the Ohm's law calculator for other resistance and power questions.
Frequently asked questions
Is voltage drop a code requirement?
Mostly no. The 3% and 5% figures sit in NEC informational notes, which are recommendations. Local codes, energy codes and equipment makers may still require them, so check before you rely on a larger drop.
Should I enter the one-way or round-trip length?
Enter the one-way distance from the source to the load. The formula already doubles it for the return conductor in single-phase circuits. Entering the round trip would double the result.
Why does aluminum wire have more voltage drop?
Aluminum conducts about 61% as well as copper, so the same size has roughly 1.6 times the resistance. Installers usually go up one or two sizes when they use aluminum.
How do I reduce voltage drop?
Use a larger wire, shorten the run, or raise the voltage. Moving a 16 A load from 120 V to 240 V halves the current for the same power and cuts the percentage drop to a quarter.
Related calculators
- Ohm's Law Calculator — Enter any two of voltage, current, resistance and power to get the other two, with the working shown.
- Electricity Cost Calculator — Energy use in kWh and running cost per day, month and year for any appliance, with typical wattages.
- Resistor Color Code Calculator — Read 4-, 5- and 6-band resistor color codes, or find the color bands for any resistance value.
Results are calculated in your browser. Double-check anything important before you rely on it. See our disclaimer.