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Wire Gauge Calculator

The minimum wire gauge for an electrical circuit is determined by the voltage drop formula VD = (2 x K x I x L) / CM, solved in reverse to find the smallest wire (highest gauge number) that keeps voltage drop within an acceptable percentage. K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way wire length in feet, and CM is the circular mil area of each AWG size. The NEC recommends keeping voltage drop at or below 3% for branch circuits. Enter your circuit details below to find the recommended wire gauge.

Quick Answer

A 120-volt, 20-amp copper circuit with a 50-foot one-way run needs approximately 10 AWG wire to stay within the 3% voltage drop recommendation.

Common Examples

Input Result
120V, 20A, 50 ft, 3%, copper 10 AWG (approximately 2.07% drop)
120V, 15A, 75 ft, 3%, copper 10 AWG (approximately 2.33% drop)
240V, 40A, 100 ft, 3%, copper 8 AWG (approximately 2.60% drop)
120V, 20A, 50 ft, 3%, aluminum 8 AWG (approximately 2.14% drop)
120V, 20A, 100 ft, 3%, copper 8 AWG (approximately 2.60% drop)

How It Works

This calculator uses the standard voltage drop formula for single-phase circuits, solved in reverse:

VD = (2 x K x I x L) / CM

Where:

  • VD = voltage drop in volts
  • K = resistivity constant of the conductor material in ohm-cmil/ft. For copper, K = 12.9. For aluminum, K = 21.2.
  • I = load current in amperes
  • L = one-way length of the wire run in feet (the formula multiplies by 2 to account for the full circuit, out and back)
  • CM = circular mil area of the wire, determined by the AWG gauge

The calculator tests each AWG gauge from 14 AWG (smallest) through 4/0 AWG (largest) and finds the smallest wire where the percent voltage drop stays at or below your specified maximum. This gives you the most cost-effective wire size that meets the voltage drop requirement.

Why wire sizing matters

Undersized wire causes excessive voltage drop, which leads to dimming lights, underperforming motors, and wasted energy as heat in the wire. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the farthest outlet. Choosing the correct wire gauge prevents these problems while avoiding the unnecessary cost of oversized conductors.

Copper vs. aluminum

Copper has a lower resistivity (12.9 ohm-cmil/ft) compared to aluminum (21.2 ohm-cmil/ft). For the same wire gauge and run length, aluminum produces approximately 64% more voltage drop than copper. When using aluminum conductors, you typically need to go up one or two wire sizes to match the performance of copper.

Worked example

For a 120V circuit with a 20A load and 50 feet of copper wire, with a 3% maximum voltage drop: The maximum allowable voltage drop is 120 x 0.03 = 3.6V. Starting with 14 AWG (4,110 CM): VD = (2 x 12.9 x 20 x 50) / 4,110 = 25,800 / 4,110 = 6.28V (5.23%). This exceeds 3%. Trying 12 AWG (6,530 CM): VD = 25,800 / 6,530 = 3.95V (3.29%). Still above 3%. Trying 10 AWG (10,380 CM): VD = 25,800 / 10,380 = 2.49V (2.07%). This is within the 3% limit. The recommended gauge is 10 AWG, with an estimated voltage drop of 2.49V and approximately 117.51V at the load.

Related Calculators

Voltage Drop Calculator

Calculate voltage drop, voltage at load, and percent drop for copper or aluminum wire runs.

Amps Calculator

Convert between amps, watts, and volts using the electrical power formula I = P / V.

Frequently Asked Questions

How is this different from a voltage drop calculator?
A voltage drop calculator tells you the voltage drop for a specific wire gauge you have already chosen. This wire gauge calculator works in the other direction. You specify the maximum acceptable voltage drop percentage, and the calculator recommends the minimum wire gauge needed to meet that requirement. It tests all standard AWG sizes and picks the smallest (most economical) wire that stays within your limit.
What voltage drop percentage should I use?
The NEC recommends 3% maximum voltage drop for branch circuits (from the panel to the outlet). For the total circuit from the service entrance to the farthest outlet, the recommendation is 5%. Most electricians use 3% as the standard target. For sensitive electronic equipment or long runs to outbuildings, some prefer a stricter 2% limit.
Why does aluminum wire need a larger gauge than copper?
Aluminum has higher electrical resistivity than copper (21.2 vs. 12.9 ohm-cmil/ft). This means aluminum wire has more resistance per foot at the same gauge, producing a higher voltage drop. To compensate, you need a larger aluminum wire (lower gauge number) to achieve the same voltage drop as a smaller copper wire. Aluminum is lighter and less expensive per foot, which can offset the cost of the larger size.
What if no standard gauge is sufficient for my circuit?
If even 4/0 AWG wire exceeds your voltage drop limit, you have several options: reduce the wire run length by relocating the panel or subpanel closer to the load, increase the source voltage (for example, running 240V instead of 120V halves the current for the same wattage), or run parallel conductors to effectively double the circular mil area. Very long runs to detached buildings often require a subpanel to keep wire sizes practical.
Does this calculator account for ampacity ratings?
This calculator sizes wire based on voltage drop only. Wire also has an ampacity rating, which is the maximum current it can safely carry without overheating. In many cases, especially for longer runs, the voltage drop requirement results in a larger wire than the ampacity requirement alone. For short runs, ampacity may be the limiting factor. Always verify that the recommended gauge also meets the NEC ampacity tables for your installation conditions.