Voltage Drop CalculatorVoltage drop calculator
Wire size and voltage drop for AC and DC circuits. The answer updates as you type, with the working shown.
Result
Copper, 75 °C basis| Wire size | 12 AWG |
| Voltage drop | 2.96 V |
| Percent drop | 2.47 % |
| At the load | 117.04 V |
Within 3% target
12 AWG. Drop 2.96 volts, 2.47 percent. Within 3% target.
How this is worked out
V = 2 × K × I × L ÷ CM
2 × 12.9 × 15 A × 50 ft ÷ 6,530 CM = 2.96 V
2.96 V ÷ 120 V × 100 = 2.47 %. At the load: 120 V − 2.96 V = 117.04 V.
- L is the one-way length; the 2 counts the return conductor.
- K is 12.9 for copper and 21.2 for aluminum, in ohm·CM per foot on a 75 °C basis.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
How the number is made
For DC and single-phase circuits the drop is V = 2 × K × I × L ÷ CM. I is the load current in amps, L is the one-way length in feet, CM is the conductor area in circular mils, and K is 12.9 for copper or 21.2 for aluminum. The 2 counts the return conductor. Details and sources are on the method page.
Voltage drop and ampacity are two different checks
The drop check protects the equipment at the far end: the motor starts, the lights stay bright, the fridge runs. The ampacity check protects the wire against heating. Either one can set the wire size. The ampacity guide explains which one governs.
Frequently asked questions
What is voltage drop?
Voltage drop is the voltage lost along the wire because of its resistance. A longer run or a smaller wire loses more voltage before the load.
Is 3% a legal limit?
No. The 3% branch and 5% total figures are informational guidance in the National Electrical Code (NEC §210.19 informational note). A local authority may adopt them as requirements, so check yours.
Does the length include the return wire?
Enter the one-way distance from the source to the load. The formula counts the return conductor (the 2 in V = 2 × K × I × L ÷ CM).
Why does another calculator give a different answer?
Tools differ in target, length convention, and temperature basis. This tool shows its formula, constants and inputs under "How this is worked out" so you can compare.
Copper or aluminum: what changes?
Aluminum has a constant of 21.2 against 12.9 for copper, so the same drop needs about 1.6 times the conductor area. Switch the conductor material and the answer updates.
Does this check ampacity?
It checks ampacity only against a value you enter from your code book, after derating. The tool does not hold ampacity tables.
Is the result code compliant?
This tool does not check code compliance. Verify your design with the authority having jurisdiction and a licensed electrician.
Learn more: how to calculate voltage drop, ampacity vs voltage drop, how this calculator works.