Calculate the voltage drop across a wire run by entering your source voltage, load current (amps), one-way wire length, wire gauge (AWG), conductor material, and circuit phase. You get back the voltage drop in volts, percentage voltage drop, and voltage at the load end — so you can confirm your wiring meets the NEC's recommended 3–5% limit.
Voltage Drop
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Voltage Drop (%)
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Voltage at Load End
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Total Wire Resistance
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NEC Recommendation
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Voltage drop is the reduction in electrical potential (voltage) that occurs as current flows through a wire due to the wire's resistance (or impedance in AC circuits). The longer and thinner the wire, the higher the resistance, and the more voltage is lost before reaching the load.
For DC and single-phase AC circuits, the formula is: Voltage Drop = 2 × Length × Resistance per unit length × Current. The factor of 2 accounts for the full round-trip path (hot and neutral conductors). For three-phase AC circuits, the factor changes to √3 (≈1.732). Resistance values depend on the wire gauge (AWG) and conductor material (copper vs. aluminum).
The National Electrical Code (NEC) recommends limiting voltage drop to no more than 3% for branch circuits and 5% for the combined feeder and branch circuit. Exceeding these levels can cause poor equipment performance, overheating, and reduced efficiency.
Yes, significantly. Copper has lower resistivity than aluminum, meaning a copper wire of the same gauge will produce less voltage drop than an aluminum wire of the same gauge. Aluminum wiring typically requires a larger gauge to achieve the same performance as copper.
Wire gauge (AWG) determines the cross-sectional area of the conductor. Lower AWG numbers mean thicker wire with less resistance per foot, resulting in lower voltage drop. Choosing the correct gauge for your circuit length and current is essential to stay within NEC recommendations.
Power factor is the ratio of real power to apparent power in an AC circuit, ranging from 0 to 1. It only applies to AC circuits. A power factor less than 1 (common with motors and inductive loads) means more current is drawn for the same real power, increasing voltage drop. For purely resistive loads like heaters and incandescent lights, a power factor of 1.0 is appropriate.
Enter the one-way length of the cable run. The calculator automatically doubles this value to account for the full round-trip current path (both the supply and return conductors) when calculating voltage drop. Using the total round-trip distance as input would double-count the length.
Yes. Excessive voltage drop can cause motors to overheat and fail, lights to dim, electronics to malfunction, and circuit breakers to nuisance-trip. Consistently low voltage at the load end stresses equipment, shortens its lifespan, and reduces energy efficiency.