Voltage Drop Calculator
Calculate voltage drop in electrical wiring. Enter voltage, current, distance, and wire gauge to determine if wire size is adequate per NEC standards.
Load current in amperes
One-way length from source to load
What is voltage drop and why is it important?
Voltage drop is the reduction in voltage as electricity flows through wires due to resistance. Example: 120V at source, 115V at device = 5V drop. Why it matters: Appliances need rated voltage to work properly, excessive drop causes dim lights, overheating motors, equipment malfunction, wasted energy as heat. NEC limits: 3% for branch circuits, 5% total (feeder + branch). Example: 120V circuit, max 3.6V drop allowed. Causes: Long wire runs, small wire gauge (higher resistance), high current draw. Solutions: Use larger wire gauge, shorten distance, reduce load, use higher voltage (less loss).
How do you calculate voltage drop in a wire?
Formula: Voltage Drop = (2 × K × I × L) / CM. K = Resistance constant (12.9 copper, 21.2 aluminum at 75°C). I = Current in amps. L = One-way length in feet. CM = Wire size in circular mils (AWG to CM conversion). 2× accounts for round trip (hot and neutral). Example: 20A, 100ft run, 12 AWG copper (6530 CM). VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V. At 120V = 6.6% drop (too high! Need larger wire). 10 AWG (10380 CM) = 5.0V drop = 4.2% (better but still high). 8 AWG = 3.1V = 2.6% (acceptable).
What wire gauge should I use to minimize voltage drop?
NEC recommendations (3% max for 120V = 3.6V drop): 15A circuit: 14 AWG up to 50ft, 12 AWG 50-100ft, 10 AWG 100-150ft. 20A circuit: 12 AWG up to 50ft, 10 AWG 50-100ft, 8 AWG 100-150ft. 30A circuit: 10 AWG up to 40ft, 8 AWG 40-80ft, 6 AWG 80-120ft. Factors: Current (higher = larger wire), distance (longer = larger wire), voltage (240V allows smaller wire than 120V for same power). Aluminum: Need 1 size larger than copper. Rule of thumb: When in doubt, go larger - cannot be too big (except cost), can be too small (safety hazard).
How does voltage drop affect different types of equipment?
Motors: Most sensitive. 5% drop reduces efficiency 10%, causes overheating, premature failure. Compressors, pumps critical. Lights: Incandescent dim noticeably (10% drop = 30% less light). LED less sensitive but still affected. Electronics: Power supplies compensate but reduced efficiency. Computers, TVs usually fine. Appliances: Heaters, stoves less affected (resistive loads). Refrigerators, AC affected (motors). EV chargers: Charge slower, potential errors. Acceptable levels: Motors 3% max, lights 5% acceptable, general circuits 3% NEC code. Measure: Use voltmeter at equipment, compare to source. If >3% drop, upgrade wire or reduce load.
What is the difference between voltage drop in AC and DC circuits?
DC circuits: Simple calculation, only resistance matters. Formula: VD = 2 × R × I × L / CM. Used in: Solar systems, batteries, automotive (12V/24V). AC circuits: Resistance AND reactance (inductance, capacitance). For most residential: Resistance dominates (power factor near 1), use same formula. For large motors: Account for power factor. Formula adds cosine factor. Three-phase: Different formula (√3 factor). Practical difference: AC usually similar to DC for voltage drop in residential. Industrial/commercial: Must account for power factor, use more complex calculations. When to use which: Residential 120/240V single-phase = DC formula works. Industrial/motors/3-phase = AC formula required.