Calculate voltage drop in electrical circuits instantly with our free NEC-compliant calculator. Ensure your circuits meet the 3% branch and 5% feeder requirements for safe, efficient operation.
Voltage drop is the reduction in electrical potential (voltage) as current flows through the resistance of conductors in an electrical circuit. This phenomenon is critical in electrical design because excessive voltage drop can cause equipment malfunction, reduced efficiency, overheating, and premature failure of electrical devices. Understanding and calculating voltage drop ensures your electrical installations operate safely and efficiently while maintaining NEC compliance.
The National Electrical Code (NEC) establishes voltage drop recommendations to ensure proper equipment operation and energy efficiency. According to NEC Article 210.19(A), branch circuits should not exceed 3% voltage drop at the furthest outlet, while feeders should remain under 5% voltage drop. The combined voltage drop for both feeder and branch circuits should not exceed 5%. These requirements prevent insufficient voltage at equipment terminals, which can cause motors to overheat, lights to dim, and sensitive electronics to malfunction.
Voltage drop calculations are essential when sizing conductors for long circuit runs, high-current loads, or when circuit performance is critical. Factors affecting voltage drop include conductor material (copper vs. aluminum), wire gauge, circuit length, load current, and whether the circuit is single-phase or three-phase. Professional electricians use voltage drop calculators to determine appropriate wire sizes that balance code compliance, installation costs, and system performance requirements.
Enter circuit parameters to calculate voltage drop
NEC recommends maximum 3% voltage drop for branch circuits and 5% for feeder circuits. Combined feeder and branch circuit voltage drop should not exceed 5% total.
Voltage drop calculation follows a straightforward formula based on conductor resistance, circuit length, and load current. For single-phase circuits, the formula is: VD = 2 × K × I × L / CM, where VD is voltage drop, K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amperes, L is one-way length in feet, and CM is the circular mil area of the conductor.
The multiplier "2" accounts for current traveling through both the hot and neutral conductors in a complete circuit. For three-phase circuits, the formula adjusts to: VD = 1.732 × K × I × L / CM, where 1.732 (square root of 3) represents the three-phase power factor.
To determine voltage drop percentage, divide the voltage drop by the system voltage and multiply by 100. Compare this percentage against NEC recommendations: 3% maximum for branch circuits (from panel to outlet) and 5% maximum for feeder circuits (from service entrance to panel).
The National Electrical Code provides voltage drop recommendations in Article 210.19(A) for branch circuits and Article 215.2(A) for feeders. These are recommendations, not strict requirements, but following them ensures optimal equipment performance and energy efficiency.
Exceeding these recommendations can cause dimming lights, motor overheating, equipment malfunction, and reduced energy efficiency. Sensitive electronic equipment may require even stricter voltage regulation, sometimes limiting voltage drop to 2% or less for optimal performance.
VD = 2 × K × I × L / CM
VD = 1.732 × K × I × L / CM
The 1.732 factor (√3) accounts for the phase relationship in three-phase power systems. Three-phase circuits experience less voltage drop than single-phase circuits for the same load, making them more efficient for high-power applications.
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Professional electrical conduit fill calculations per NEC standards