Calculate voltage drop across any wire run instantly. Enter your circuit details below to see voltage drop, percentage, and whether your run meets NEC guidelines.
What Is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as current flows through a wire. The longer the wire run and the higher the current, the more voltage is lost before it reaches the load. Excessive voltage drop causes lights to dim, motors to overheat, and equipment to underperform or fail prematurely.
NEC Voltage Drop Guidelines
The National Electrical Code recommends keeping voltage drop at or below 3% on individual branch circuits, and no more than 5% combined across the feeder and branch circuit. These are recommendations, not hard code requirements, but they represent accepted industry practice for reliable performance.
The Voltage Drop Formula
For single-phase circuits: VD = (2 × R × I × L) ÷ 1000
For three-phase circuits: VD = (1.732 × R × I × L) ÷ 1000
Where R is the resistance of the wire in ohms per 1,000 feet (from NEC Table 9), I is the current in amps, and L is the one-way wire length in feet.
Wire Gauge and Resistance Reference
Thicker wire (lower AWG number) has less resistance and produces less voltage drop. For long runs or high-current circuits, upsizing the wire gauge is the most effective way to keep voltage drop within acceptable limits.
Tips for Reducing Voltage Drop
If your calculation shows excessive voltage drop, consider upsizing your wire gauge one or two steps, reducing the length of the run where possible, splitting the load across multiple circuits, or increasing the supply voltage where the application permits.
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