Voltage Drop

Overview

Voltage drop occurs as electrical current flows through conductor resistance, reducing the voltage available at the load. The National Electrical Code (NEC) recommends limiting voltage drop to 3% on branch circuits and 5% total (feeder plus branch). Accurate voltage-drop calculations help select the correct conductor size and ensure equipment operates within its rated voltage range.

Key Formulas

Ohm's Law approach:

$\Delta V = I \times R$

where resistance of a conductor is:

$R = \frac{\rho \times 2L}{A}$

The factor of 2 accounts for both the supply and return conductors in a single-phase circuit.

Circular-mils approach:

$\Delta V = \frac{K \cdot P \cdot L \cdot I}{A}$

This form uses specific resistivity K in Ω·circular-mils/ft and wire area A in circular mils, avoiding the need for separate resistivity conversion.

Simplified copper formula:

$\Delta V = f \cdot I \cdot L$

where f is a precomputed factor (volts per ampere per 1000 ft) from the conductor table below, I is load current in amperes, and L is one-way wire length in feet.

Variables

Specific Resistivity (K Values)

Calculator

Unit Converter

Source Examples

Example - Voltage Drop: for a single-phase copper circuit, voltage drop is calculated from conductor resistance, current, and one-way length with the return path included by the phase constant. Example - Specific resistivity and Voltage Drop: use the material K value in ohm-circular-mils per foot with conductor area in circular mils to avoid mixing SI and AWG units.

Copper Conductor Voltage Drop Factors

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Copper Conductor - Voltage Drop

Engineering Notes

• Temperature matters. Conductor resistance rises with temperature. Use the higher K value for circuits operating above 121 °F (49 °C).
• Parallel conductors. For parallel runs, divide the total current by the number of parallel paths and calculate each path separately.
• NEC recommendations. The NEC (NFPA 70) suggests a maximum 3% voltage drop on branch circuits and 5% total (feeder plus branch combined).
• Aluminum conductors. Aluminum has roughly 1.6× the resistivity of copper; the K values and factors above reflect this difference.
• Conduit and skin effect. At higher frequencies or in ferromagnetic conduit, effective resistance increases. The formulas here assume DC or 60 Hz in non-ferrous raceways.
• Simplified formula limits. The factor-based method is calibrated for copper at standard building-wire temperatures. For aluminum or elevated-temperature designs, use the circular-mils formula with the appropriate K value.

References

• Engineering ToolBox — Voltage Drop