Voltage Drop Calculator
Voltage drop occurs when electrical current travels through a wire with resistance — the longer the run or the higher the current, the more voltage is lost before reaching the load. This calculator helps electricians, engineers, and DIYers size wire correctly for branch circuits, feeder runs, solar installations, and low-voltage lighting by computing the expected voltage drop and whether it falls within the NEC-recommended 3% guideline.
Calculator
Formula
Formula
V_drop = 2 × L × I × R_per_ft
V_drop is the total voltage drop in volts. L is the one-way wire length in feet (multiplied by 2 for the round trip). I is the current in amperes. R_per_ft is the resistance of the conductor per foot in ohms/ft, derived from the AWG gauge and conductor material (copper or aluminum) using published resistance tables (NEC Chapter 9, Table 9).
Worked example
A 20 A circuit runs 75 ft of 12 AWG copper wire at 120 V. Calculate the voltage drop.
- 1.12 AWG copper resistance ≈ 0.001588 Ω/ft (from NEC Table 9)
- 2.V_drop = 2 × 75 ft × 20 A × 0.001588 Ω/ft
- 3.V_drop = 2 × 75 × 20 × 0.001588 = 4.764 V
- 4.Drop % = (4.764 / 120) × 100 = 3.97%
- 5.Voltage at load = 120 − 4.764 = 115.24 V
Voltage drop is 4.76 V (3.97%). This exceeds the NEC-recommended 3% guideline (3.6 V). Consider upgrading to 10 AWG copper for this run.
Assumptions and limitations
- Uses NEC Chapter 9, Table 9 resistance values at 75°C conductor temperature.
- Calculates round-trip (2-wire) voltage drop using 2 × L × I × R_per_ft.
- For 3-phase circuits, the multiplier is √3 instead of 2 — this calculator is for single-phase and DC only.
- NEC recommends ≤ 3% drop for branch circuits and ≤ 5% total including feeder.
- Resistance values assume stranded copper or solid aluminum at listed temperature.
Frequently asked questions
Related tools and guides
Solve for V, I, R, or P given any two values.
Convert feet to meters, inches to cm, and more.
Convert watts, kilowatts, and horsepower.
Why wire sizing uses AWG (imperial) while resistance is in SI ohms.