What it calculates
Estimate voltage drop and percentage loss over a circuit run.
Voltage Drop Calculator
Estimate circuit voltage drop from conductor size, load, voltage, distance and conductor material.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
Electrical planning
How to Use This Calculator
Use the field units shown below, then compare your result with the worked example. This tool reports estimated drop; it does not check conductor ampacity or approve a wire size.
How the calculation works
Estimated drop uses conductor resistance behavior based on material, circular-mil area, load and round-trip circuit length.
What the result means
The percentage result shows how much of the source voltage is estimated to be lost before the load.
Check one-way distance in a simplified drop model
For 20 amps, a 100-foot one-way run, copper and the stored 12 AWG area of 6,530 circular mils, this model calculates 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 volts. At 120 volts that is about 6.58%.
Contractor tip: This tool reports estimated drop; it does not check conductor ampacity or approve a wire size. Compare the calculation assumptions with the actual system before using its result.
Verify the final quantity
The factor of 2 already represents the out-and-back circuit path. Enter one-way distance, not a doubled route. This is a simplified resistance calculation and does not model every AC, temperature or phase condition.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.