Voltage Drop Calculator
Calculate Voltage Drop
Result
| Voltage Drop | — |
|---|---|
| Voltage Drop Percentage | — |
| Voltage at End | — |
How to Use This Voltage Drop Calculator
Undersized wire causes dim lights, tripped breakers, and motors that never reach full speed. Voltage drop across a long run depends on wire material, gauge, current, and distance—and guessing wrong means rework or a fire hazard. This calculator estimates drop in volts, percent of supply voltage, and voltage remaining at the load.
- Select Wire Material. Choose Copper or Aluminum.
- Pick Wire Size (AWG). Select 10 through 20 AWG from the dropdown.
- Enter Voltage (V). Type system voltage (e.g., 120 or 240).
- Select Phase. Choose DC / Single Phase AC or 3-Phase AC.
- Enter Current (Amps) and Distance. Type load current and one-way wire length in feet or meters, then press Calculate. Review Voltage Drop, Voltage Drop Percentage, and Voltage at End.
Compare percent drop against NEC recommendations—typically 3% for branch circuits and 5% total feeder plus branch. For broader circuit math, pair with the Ohm's Law Calculator or Electricity Calculator for running-cost estimates.
Voltage Drop Calculator Formulas and Practical Applications
Wire resistance scales with length and inversely with cross-section (AWG). Copper has lower resistance than aluminum at the same AWG—about 60% of aluminum's resistance—so copper runs drop less voltage for the same current and length.
R = (Resistance per 1000 ft ÷ 1000) × One-Way Distance (ft)
V_drop = 2 × R × I (DC / single-phase)
V_drop = √3 × R × I (three-phase)
Percent Drop = (V_drop ÷ Supply Voltage) × 100
Practical Drop Limits
- Branch circuits: aim ≤ 3% drop
- Feeder + branch combined: aim ≤ 5% total
- Motors and electronics: may need tighter limits per nameplate
Frequently Asked Questions
Why multiply by 2 for single-phase?
Current travels out on the hot conductor and returns on the neutral (or second hot in some configurations). The round-trip distance doubles the effective resistance compared to one-way length alone.
What voltage drop is acceptable?
NEC informational notes suggest 3% max on branch circuits and 5% combined from service to outlet. Sensitive equipment (motors, electronics) may need tighter limits. Local codes and manufacturer specs may be stricter.
Does this include temperature or conduit derating?
No. Resistance tables assume standard conditions. High ambient temperature, bundled conductors, and long conduit runs increase effective resistance beyond this estimate.
Which AWG sizes are supported?
10, 12, 14, 16, 18, and 20 AWG for copper or aluminum wire. Select material and gauge before entering voltage, phase, current, and one-way distance.
What does Distance (one-way) mean?
Enter the length from source to load on a single conductor path. The calculator applies the round-trip multiplier (×2 for DC/single-phase, √3 factor for three-phase) internally.