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Wire Voltage Drop Calculator

Calculate voltage drop in a wire circuit.

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Inputs

V
A
ft

Your values stay on this device.

Results

Voltage Drop
Percentage Drop2.65%
Voltage at Load116.82

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Assumptions

  • The formula assumes a two-wire single-phase or DC circuit with a one-way entered length.
  • Resistance values are approximate and based on standard per-1000-foot tables for the listed gauges.
  • Voltage must be positive, and current and wire length cannot be negative.

Sensitivity preview

Voltage -10%: Voltage Drop 3.18 V

Voltage Current: Voltage Drop 3.18 V

Voltage +10%: Voltage Drop 3.18 V

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About this Voltage Drop

Voltage drop occurs when the internal resistance of the wire causes the voltage at the load to be lower than the voltage at the source. Excessive voltage drop can cause lights to flicker, motors to burn out, and equipment to malfunction. Use this calculator to ensure your wire gauge is sufficient for the distance and load.

Who this calculator is for

  • Electricians, installers, and hobbyists sizing wire runs for DC or single-phase circuits.
  • Anyone checking whether a long cable run will cause too much voltage loss at the load.
  • Builders comparing wire gauge and material choices before installation.

When to use it

  • Use it to estimate voltage loss over a long run to a motor, light, inverter, or accessory circuit.
  • Use it when deciding whether a thicker wire gauge is needed to stay under a target percentage drop.
  • Use it before choosing copper versus aluminum for the same load and distance.

How to use this calculator

  1. Enter the Voltage of the system (e.g., 120V, 12V).
  2. Enter the load Current (Amps).
  3. Enter the one-way Wire Length.
  4. Select the Wire Size (AWG) and material (Copper/Aluminum).

Key assumptions

  • The formula assumes a two-wire single-phase or DC circuit with a one-way entered length.
  • Resistance values are approximate and based on standard per-1000-foot tables for the listed gauges.
  • Voltage must be positive, and current and wire length cannot be negative.

Accuracy and testing

Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.

View accuracy page

How this calculator works

The formula used is VD = 2 × L × R × I / 1000 for single-phase DC or AC (assuming power factor ~1). * L: Length (one way) in feet. * R: Resistance per 1,000 ft (from NEC Chapter 9, Table 8). * I: Current in Amperes.

Inputs explained

Wire Size (AWG)

American Wire Gauge. Smaller numbers indicate thicker wires.

Material

Copper is standard. Aluminum is lighter and cheaper but has higher resistance (approx 1.64x copper).

Understanding your results

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for feeders/combined systems to ensure efficiency.

Example scenarios

Long Extension Cord

120V, 15A Load, 100ft 14 AWG cord. Drop = 7.58V (6.3%). This is too high; lights will dim.

Low Voltage Landscape

12V, 5A Load, 50ft 12 AWG wire. Drop = 0.79V (6.6%). Lights may be noticeably dimmer at the end.

Key Features

  • NEC Resistance Values
  • Copper vs Aluminum
  • Percentage Drop Alert

Tips & Best Practices

  • For long runs, always size up (use a lower AWG number) to reduce resistance.
  • Voltage drop implies energy loss as heat. In extreme cases, this can melt insulation.

Glossary of Terms

NEC
National Electrical Code. The standard for safe electrical installation in the US.
AWG
American Wire Gauge. A logarithmic stepped wire gauge system.

Frequently Asked Questions

What is an acceptable voltage drop?
The NEC recommends no more than 3% for branch circuits (the final run to the outlet/device) and 5% total voltage drop including the feeder.
Why does wire length matter?
Wire has internal resistance. The longer the wire, the more resistance the electricity faces, resulting in greater voltage loss over distance.
Is Aluminum wire worse than Copper?
Aluminum has about 60% of the conductivity of copper, so you typically need a larger gauge aluminum wire to carry the same current as a smaller copper wire.