Classic tool
Voltage Drop Calculator
Estimate voltage drop for DC, single-phase, and three-phase circuits using length and cable resistance.
Use this voltage drop calculator to estimate how many volts are lost along a circuit before power reaches the load. It works for DC, single-phase, and balanced three-phase circuits as long as you enter supply voltage, current, one-way length, and conductor resistance per unit of length.
This is useful when reviewing long runs, comparing conductor options, checking a manufacturer cable table, or spotting early when the load-end voltage may fall more than expected. In a few seconds you get equivalent path resistance, voltage drop in volts, drop percentage, and estimated voltage at the load.
You can work in meters or feet. The key is to keep length and resistance in the same unit, such as Ω/m with meters or Ω/ft with feet. That keeps the estimate consistent even when your cable data comes from a different supplier or table.
Use clear inputs to get a more useful result.
How the calculation works
For DC and single-phase circuits, the calculator uses a factor of 2 because current travels out and back. For balanced three-phase circuits, it uses a factor of √3. Equivalent path resistance is factor × one-way length × resistance per unit length. Voltage drop is then calculated as current × equivalent path resistance.
How to enter resistance
Use the conductor resistance in the same unit as the length you entered. If length is in meters, resistance should be in ohms per meter. If length is in feet, use ohms per foot. You can usually find that value in a cable data sheet, manufacturer table, or design note.
Reading the result
- Voltage drop in volts shows how much supply voltage is lost across the run.
- The percentage value helps compare the result with the design limit or local code target you follow.
- If remaining load voltage is too low, review conductor size, material, distance, or circuit current.