Voltage Drop Calculator

Calculate the electrical voltage drop and percentage loss across wiring runs based on wire size and distance.

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Select wire material and gauge, enter the circuit voltage, current load, and run distance below to estimate electrical drop.

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A Guide to Voltage Drop and Wire Resistance

In electrical engineering and home wiring, voltage drop is the decrease in electrical potential along the path of a current flowing in an electrical circuit. Because all physical wires have some internal electrical resistance, some energy is lost as heat as current travels through them. If the wire is too thin or the run is too long, the voltage drop will exceed safe limits, causing connected devices to run inefficiently, overheat, or fail to start.

How Voltage Drop is Calculated

To calculate voltage drop in a single-phase circuit, the formula is: Voltage Drop = (2 × L × R × I) / 1000. In this equation, L is the one-way length of the wire run in feet, R is the electrical resistance of the wire material per 1,000 feet, and I is the current load in amperes (amps).

The factor of 2 accounts for the return path of the current (hot and neutral wires). Wire size (measured in AWG or mm²) and material (copper vs aluminum) directly establish the resistance value R. To convert run lengths between feet and meters, check out our translating measuring units tool.

To find percentage losses, use our percentage rates converter. For basic math operations, use our standard daily math helper.

Everyday Wiring Scenarios

Sizing Extension Cords: Using thin extension cords for heavy machinery (like table saws) causes significant drop, harming motors.
Solar Installations: Wiring solar panels to battery chargers across long distances requires thick cables to prevent power loss. You can check straight paths using our coordinate distance mapping tool.
Landscape Lighting: Low-voltage 12V garden lighting systems drop voltage rapidly over long runs, causing distant bulbs to appear dim.
Rounding Wire Gauges: Selecting standard commercial wire sizes requires rounding up, which you can verify alongside our rounding decimals and digits tool.

National Electrical Code Limits

The National Electrical Code (NEC) recommends that the maximum voltage drop for branch circuits should not exceed 3%, and the total drop for both the feeder and branch circuits combined should not exceed 5% at maximum load.

Adhering to these guidelines ensures equipment efficiency, prevents insulation breakdown, and protects properties from electrical fire risks. Our online calculator computes both absolute drop voltage and percentage loss values to verify code compliance.

Example of a Long Wire Run

Suppose you want to run a 120-volt circuit over a 100-foot copper wire (using 14 AWG wire) to power a 15-amp space heater.

The resistance of 14 AWG copper wire is approximately 3.07 ohms per 1,000 feet. Setting up the single-phase formula: Drop = (2 × 100 feet × 3.07 ohms × 15 amps) / 1000 = (9210) / 1000 = 9.21 volts. Subtracting this from 120V leaves 110.79V at the heater. The percentage drop is 9.21 / 120 = 7.68%. Because 7.68% exceeds the recommended 3% limit, you should increase the wire size to 10 AWG to reduce the drop to a safe 3.02% (3.63V). This example shows how voltage calculations protect electrical circuits.