Calculate voltage loss across wiring runs in electrical circuits. Critical for power delivery, lighting systems, motor circuits, and safety compliance with electrical codes.
Last Updated: 5/6/2026
Source voltage: 120V/240V (residential), 480V (industrial), 277V (commercial)
Full load current drawn by the device or branch circuit
Distance from source to load (calculator accounts for round-trip return path)
Smaller number = thicker wire = lower resistance
Voltage drop is the decrease in electrical potential (voltage) that occurs as electrical current flows through a conductor (wire) due to the conductor's resistance. This is a fundamental phenomenon governed by Ohm's Law (V = I × R) and is crucial in all electrical systems from household wiring to industrial power distribution. When current flows through a wire with finite resistance, some of the voltage is "lost" or dissipated. For example, a 120V circuit supplying a distant light fixture might deliver only 114V to the bulb if the wire run is long and the wire gauge too small. This 6V drop (5% loss) causes the bulb to be dimmer than designed, wastes energy as heat in the wire, and can shorten the lifespan of sensitive equipment. The problem worsens with longer distances and higher currents—a Tesla vehicle charger drawing 48A over 100 feet of inadequate wire could waste hundreds of watts and create a fire hazard. Engineers carefully manage voltage drop by selecting appropriate wire gauges, minimizing run distances, and in critical systems, overdriving voltage slightly at the source to compensate at the load.
The National Electrical Code (NEC) provides guidelines on voltage drop to support safe system design and reliable equipment operation. NEC Article 210 recommends branch circuits maintain no more than 3% voltage drop to the outlet, with combined voltage drop (feeder + branch circuit) not exceeding 5%. These guidelines exist because excessive drop causes motors to run hot (reducing efficiency and lifespan), lights to dim below design brightness, heating elements to produce less heat, and sensitive electronic loads (computers, programmable logic controllers) to malfunction or shut down. In practice, electricians and engineers use this calculator to evaluate whether their wire gauge and run length combinations align with NEC guidelines before installation. Industrial applications, telecommunications, and renewable energy systems (solar arrays, wind turbines) all rely on voltage drop analysis to ensure proper power delivery. Temperature effects also matter—wire resistance increases with temperature, so ambient conditions and current-induced heating further increase voltage drop, requiring de-rating factors and margin in design calculations.
Input the nominal voltage at the source of your circuit. Standard values in North America: 120V, 240V (residential); 208V, 277V, 480V (commercial/industrial). Use the voltage measured at the breaker or source, not the rated nameplate voltage, for accuracy.
Specify the maximum current that the circuit will draw under full-load conditions. For a device: check the nameplate (e.g., microwave 12A, electric heater 20A). For a branch circuit: use the circuit breaker rating or calculate from the load power (I = P / V). Always use full-load current for worst-case voltage drop analysis.
Measure the distance from the source (breaker, power supply) to the load. The calculator automatically accounts for the return path (hence doubling the length in resistance calculations). Always measure along the actual wire path (walls, conduit, etc.), not straight-line distance.
Choose the AWG (American Wire Gauge) you plan to use. Lower AWG = thicker wire = lower resistance = less voltage drop. Compare the calculated voltage drop percentage against NEC limits (3% for branch circuit; 5% total). If the drop exceeds limits, select a heavier gauge wire and recalculate.
Scenario: An electrician is installing a new outlet for a microwave oven 100 feet (round-trip distance) from the main breaker panel. The circuit will be protected by a 20A breaker (standard for kitchen circuits). Determine the voltage drop using #12 AWG wire and verify NEC compliance.
The calculator accounts for the round-trip circuit: current flows from the source through the hot conductor to the load, then returns through the neutral (or ground) conductor. Total wire resistance = (resistance per foot) × (one-way distance) × 2. You input one-way distance; the calculator automatically doubles it.
NEC Article 210 recommends: (1) Branch circuit voltage drop ≤ 3% to the outlet, and (2) Combined voltage drop (feeder + branch circuit) ≤ 5%. These limits ensure equipment operates within design specifications and safe temperatures. Exceeding limits causes overheating, dimming lights, and equipment malfunction.
Start with the current rating (matches circuit breaker size and tool/appliance nameplate). Enter your distance and voltage into this calculator. Try candidate gauges (thicker = lower gauge number) until voltage drop meets NEC limits. Account for temperature derating if running in hot conduit or outdoor heat.
Voltage drop is V = I × R. With fixed resistance, doubling the current doubles the voltage drop. High-current circuits (240V welder at 50A, large motor at 30+A) need thicker wire (lower AWG) to keep voltage drop reasonable. That's why electric vehicle chargers and main service feeds use thick cables.
Yes. Copper resistance increases ~0.39% per °C above 25°C. A wire in a hot attic or embedded in insulation will have higher resistance. The NEC derating table reduces allowable ampacity for high-temperature environments. For precision work, multiply calculated resistance by the derating factor.
Power loss = I² × R (watts). Energy lost as heat in the wire. In a 50-amp solar circuit 150 feet away with undersized wire, losses could exceed 1000W, wasting money and potentially overheating the wire insulation. Minimize power loss by using heavier gauge wires or shorter runs.
Aluminum has higher resistance (~1.68× copper) for the same diameter, so it requires a larger gauge. NEC allows aluminum if properly sized. This calculator uses copper values; for aluminum, move down 2 AWG sizes (e.g., use #10 aluminum where #12 copper would work). Aluminum is cheaper but more maintenance-prone (oxidation).
Motors draw more current and overheat; lights dim below brightness; heating elements produce less heat; power supplies and electronics may malfunction or shut down. Incandescent bulb brightness drops with voltage (halving the voltage roughly halves the light output). Always keep voltage drop within limits to ensure proper equipment operation and lifespan.
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