How does distance affect wire gauge?

Longer wire runs create more voltage drop, so they generally require a larger conductor with a lower AWG number. This calculator uses the one-way distance and doubles it internally for the outgoing and returning conductors.

What voltage-drop percentage should I use?

Three percent is a common design target for a branch circuit, while five percent is commonly used for the combined feeder and branch circuit. Local codes, equipment requirements, and the authority having jurisdiction may require different limits.

Does this calculator replace an electrical code calculation?

No. It is a voltage-drop sizing estimate, not a complete conductor-selection calculation. Verify ampacity, insulation temperature, conduit fill, ambient temperature, termination ratings, overcurrent protection, and local code requirements before installation.

Wire Gauge Calculator by Distance

Wire Gauge by Distance: Why Run Length Matters

Choosing wire size is not just about matching the conductor to the circuit’s amperage. As the distance from the source increases, resistance increases and the load receives less voltage. Excessive voltage drop can reduce equipment performance, cause motors to overheat, dim lights, and create nuisance shutdowns.

This calculator estimates conductor size using the common voltage-drop relationship for a two-conductor circuit. It accounts for the full circuit length by multiplying the one-way distance by two, then calculates the conductor area required to keep voltage loss below your selected percentage. The result includes required circular mils and an approximate American Wire Gauge (AWG) value.

The default example uses a 120-volt, 15-amp copper circuit running 100 feet one way with a 3% voltage-drop limit. Remember that the approximate AWG result is not a substitute for selecting an actual standard conductor size. Choose the next larger conductor, which means the next lower AWG number, that satisfies the calculation. Confirm the final selection against NEC ampacity tables and local requirements. NEC Article 210.19(A) includes a 3% branch-circuit voltage-drop recommendation in its informational notes, but voltage drop is only one part of safe wire sizing.

How to Calculate Wire Gauge by Distance (step by step)

Step One: Find the Maximum Allowable Voltage Drop

Convert the selected voltage-drop percentage into volts by multiplying system voltage by the percentage and dividing by 100.

maximum voltage drop = system voltage × maximum drop percentage ÷ 100

120 V × 3% ÷ 100 = 3.6 V

Step Two: Convert One-Way Distance to Total Circuit Length

A typical two-conductor circuit carries current out to the load and back to the source, so multiply the one-way distance by two.

total circuit length = one-way distance × 2

100 ft × 2 = 200 ft

Step Three: Calculate the Required Conductor Area

Use the resistivity constant for the conductor material, the load current, the one-way distance, and the allowable voltage drop. The calculator applies the distance multiplier directly in this formula.

required circular mils = 2 × conductor constant × current × one-way distance ÷ maximum voltage drop

2 × 12 × 15 A × 100 ft ÷ 3.6 V = 10,000 circular mils

Step Four: Convert Circular Mils to Approximate AWG

Convert the required circular-mil area to an approximate AWG value. A lower AWG number represents a thicker conductor.

approximate AWG = 36 − (39 ÷ 2) × log10(required circular mils ÷ 25) ÷ log10(92)

36 − (39 ÷ 2) × log10(10,000 ÷ 25) ÷ log10(92) = 10.2 AWG

For this example, select the next larger standard conductor size, typically 10 AWG copper, then verify ampacity and all installation conditions before using it.