Voltage Drop Calculator
Voltage drop is the loss of electrical pressure over distance. Too much drop causes dim lights, overheating motors, and tripped breakers. NEC recommends keeping voltage drop under 3% for branch circuits.
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How to Use This Calculator
- Select your source voltage.
- Enter the load current in amps.
- Enter the one-way wire distance in feet.
- Select the wire gauge.
- Select single or three phase.
How Voltage Drop Calculation Works
Electrical resistance in wire causes voltage to drop as current travels the length of the conductor. Longer runs, smaller wire, and higher current all increase voltage drop. NEC recommends a maximum of 3% drop for branch circuits and 5% total (feeder + branch). Copper wire resistance values are from NEC Table 9 at 75°C. For aluminum conductors, multiply resistance by approximately 1.6.
Formula
Single phase: V_drop = 2 × Length × Current × Resistance per 1000ft ÷ 1000. Three phase: V_drop = √3 × Length × Current × R/1000ft ÷ 1000. Percentage = V_drop ÷ Source Voltage × 100.