Electrical calculator
Wire Size Calculator
Find the smallest listed copper gauge that meets the entered voltage drop target, then perform the separate ampacity and code checks.
Sage’s guided lesson
Let’s understand the result before using it
Estimate copper AWG wire size from current, one way distance, system voltage, and allowable voltage drop, with safety limitations. I’ll show you what the numbers mean, how to check the math yourself, and where real-world conditions can change the answer.
Start with the system
What this calculator is actually doing
The calculator tests common copper wire sizes using their approximate resistance. It chooses a gauge that keeps estimated round trip voltage drop at or below the target. A voltage drop result does not establish conductor ampacity.
Voltage drop is only one sizing condition. Ampacity, insulation temperature, terminals, bundling, environment, and code may require a larger conductor.
Now for the arithmetic
How to do the math yourself
Maximum allowed drop = source voltage × allowed percent ÷ 100. Maximum circuit resistance = allowed drop ÷ current. The calculator compares that resistance with each gauge over twice the one way distance.
Worked example
A 12 V circuit carries 10 A for 25 ft one way with a 3% target.Allowed drop = 12 × 0.03 = 0.36 VMaximum round trip resistance = 0.36 ÷ 10 = 0.036 ΩEach supported AWG is checked until its estimated resistance is low enough.Use the calculated circular mil requirement to choose the next larger standard conductor, never the next smaller one.
I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, Why did the belt get arrested? For holding up a pair of pants. Yeah, I don’t see the connection either, but I think it is cute!
Before you build, wire, mix, or run it
Tips, shortcuts, and mistakes to avoid
- Use the maximum continuous load and evaluate startup or surge current separately.
- Long low voltage runs often need much larger wire for voltage drop than ampacity alone would suggest.
- After selecting for drop, verify ampacity, insulation, temperature, bundling, terminals, conduit fill, overcurrent protection, and code requirements.
Connection resistance, conductor temperature, harmonics, startup current, and installation method change actual performance.
How we got here
A little history and background
American Wire Gauge originated in the nineteenth century from wire drawing practice. In AWG, a smaller gauge number means a larger conductor, which can feel backwards until you remember that repeated drawing steps made the wire thinner.
What the result does not tell you
The tool estimates copper conductor size for a simple two conductor run. It does not cover aluminum, AC impedance, three phase formulas, parallel conductors, code ampacity, motor circuits, fault current, or installation conditions.
Frequently asked questions
Why can the recommended wire be larger than an ampacity chart suggests?
Ampacity limits heating, while this calculator limits voltage drop. Long or low voltage runs may require a larger conductor.
Does a smaller AWG number mean smaller wire?
No. In the AWG system, smaller numbers mean larger conductors.
Can I use the result as a final wiring specification?
No. A qualified design must also satisfy all code, protection, temperature, installation, and equipment requirements.
That wraps up the lesson
Class is over. It is officially cheese fries time.
You now know more about how the Wire Size Calculator actually works, not just which buttons to press. That earns cheese fries, Sage’s favorite way to celebrate a finished lesson.
Use the result within its limits
Do not use this result alone to select building, vehicle, marine, battery, or machinery wiring. Verify all applicable safety and code requirements.