Electrical calculator
Voltage Drop Calculator
Estimate how much voltage is lost in a copper supply and return path before the load receives power.
Sage’s guided lesson
Let’s understand the result before using it
Estimate two conductor copper wire voltage drop, load voltage, percentage loss, and wire heating from current and distance. 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
Every wire has resistance. Current flowing through the round trip conductor creates a voltage drop and dissipates heat. Low voltage systems are especially sensitive because a small absolute drop can be a large percentage of the supply.
Use one way distance in the input only when the formula accounts for the return conductor, as this calculator does.
Now for the arithmetic
How to do the math yourself
Round trip resistance = ohms per 1,000 ft × temperature factor × (2 × one way length ÷ 1,000). Voltage drop = current × resistance. Percent drop = drop ÷ source voltage × 100. Wire loss = current × drop.
Worked example
A 12 V load draws 10 A through 25 ft one way of 14 AWG copper.Round trip length = 50 ftResistance ≈ 2.525 × 50 ÷ 1,000 = 0.12625 Ω at the reference conditionDrop ≈ 10 × 0.12625 = 1.2625 V, or about 10.5%Keep conductor resistance and length precise, then judge the final percentage against the equipment requirement and applicable code.
I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, What do you call a pencil that's the boss? The ruler. 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 one way distance in the form; the calculator doubles it for the outgoing and return conductors.
- Design from the actual continuous and startup current, not only an average current.
- Conductor ampacity, insulation temperature, bundling, terminals, ambient temperature, and code rules must be checked separately.
Temperature, connections, material, power factor, startup current, and shared conductors can increase real voltage drop.
How we got here
A little history and background
Voltage drop became a practical design concern as electric lighting and power distribution expanded in the nineteenth century. Larger conductors reduced both lost voltage and wasted heat, a tradeoff that remains central to wiring design.
What the result does not tell you
The calculation uses typical copper DC resistance and a selected temperature factor. It does not establish ampacity, code compliance, AC reactance, connection resistance, aluminum conductor behavior, fault current, or protective device sizing.
Frequently asked questions
Why is the distance doubled?
Current travels to the load and back, so a two conductor circuit has twice the one way conductor length.
Is a low voltage circuit automatically safe?
No. Low voltage systems can deliver very high current, overheat conductors, start fires, or create arc hazards.
What percentage drop is acceptable?
That depends on the equipment, circuit, and applicable rules. Use the load manufacturer and electrical code guidance for the installation.
That wraps up the lesson
Class is over. It is officially cheese fries time.
You now know more about how the Voltage Drop 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
This is an estimate, not a wiring approval. Verify ampacity, insulation, overcurrent protection, grounding, environment, terminations, and applicable electrical rules.