Voltage Drop Calculator

Voltage Drop Calculator

Enter your conductor, distance, voltage, and load to estimate voltage drop in volts and percent. This works well as a practical field and exam calculator for common runs.

System Type Single-Phase Three-Phase

Single-phase uses 2 × K × I × L ÷ CM. Three-phase uses 1.732 × K × I × L ÷ CM.

Conductor Material Copper Aluminum

System Voltage 120 V 208 V 240 V 277 V 480 V

Wire Size 14 AWG 12 AWG 10 AWG 8 AWG 6 AWG 4 AWG 3 AWG 2 AWG 1 AWG 1/0 AWG 2/0 AWG 3/0 AWG 4/0 AWG 250 kcmil 300 kcmil 350 kcmil 400 kcmil 500 kcmil

One-Way Distance (feet)

Load Current (amps)

Target Threshold 3% branch circuit target 5% feeder + branch target

Quick Note

This is a practical estimate for common copper and aluminum conductor runs. It is great for fast planning and exam prep.

Calculate Voltage Drop Reset

Voltage Drop

0.00 V

Percent Drop

0.00%

Estimated Load Voltage

0.00 V

Resistance Used

0.0000 Ω

This calculator uses standard circular mil areas and common K values for copper and aluminum. It is a practical estimating tool, not a substitute for full design review, local code requirements, temperature correction, conduit fill adjustments, or engineering sign-off.

If you have ever searched for a voltage drop calculator, you were probably in one of two moods. Either you were on a job trying to make sure a long run was not going to be a problem, or you were studying and hoping this topic would somehow go away.

Bad news, it does not go away. Good news, it is actually not that bad once you see what is going on.

Voltage drop is just the loss of voltage that happens as current moves through a conductor. The longer the run, the smaller the wire, and the higher the current, the more voltage you lose along the way.

Why electricians care about voltage drop

On paper, voltage drop sounds like a math problem. In real life, it shows up as equipment not running the way it should.

Motors can struggle. Lights can dim. Sensitive equipment can act weird. You might still have a legal installation in some situations, but the performance can still be lousy.

That is why electricians do not just care about whether a wire can survive the amperage. They also care about whether the load is actually getting the voltage it needs.

What a voltage drop calculator is really checking

A voltage drop calculator usually takes a few basic inputs. Voltage, current, conductor material, wire size, and length of run.

From there, it estimates how much voltage you lose across that conductor. Then you can look at the result as a raw voltage number or as a percentage.

That percentage is what most people pay attention to first. It gives you a quick reality check on whether the run looks reasonable.

The basic pattern behind voltage drop

You do not need to memorize some giant mystery formula to understand the idea. Just remember the pattern.

• Longer run = more voltage drop
• More current = more voltage drop
• Smaller wire = more voltage drop
• Copper usually performs better than aluminum at the same size

That is really the heart of it. If somebody doubles the length, the voltage drop gets worse. If somebody upsizes the conductor, the voltage drop gets better.

Why long runs get people in trouble

A lot of wire sizing questions feel easy until the run gets long. You can have a conductor that is technically okay for ampacity, but still not great once the distance gets big enough.

That is where people start bumping up the wire size. Not because the breaker changed, but because they want to control voltage drop.

This is one of those things that makes more sense in the field than in a classroom. If the equipment at the far end is starving for voltage, nobody cares that your original wire choice looked fine for ampacity alone.

The recommendation electricians usually hear

The common recommendation is to keep voltage drop to about 3 percent on a branch circuit and 5 percent total for feeder plus branch circuit combined.

That gets repeated a lot because it is a good practical target. It helps keep equipment running the way it should instead of barely hanging on.

So when you use a voltage drop calculator, that percentage is usually the first thing you compare against.

A simple example

Let’s say you have a 120-volt circuit feeding a load a long way from the panel. The farther that run goes, the more resistance the conductor adds, and the more voltage gets eaten up on the trip.

If the number comes back higher than you want, the usual fix is pretty simple. Upsize the conductor.

Bigger wire means less resistance, and less resistance means less voltage drop. That is why electricians will sometimes go larger than the minimum ampacity size on long runs.

Where people get mixed up on exams

Exams love voltage drop because it sounds scary enough to make people panic. Usually the trap is not the math itself. It is forgetting what the question is really asking.

Sometimes they want the voltage drop percentage. Sometimes they want the corrected conductor size. Sometimes they just want you to know that long runs may require upsizing.

If you stay calm and look at the length, load, and conductor size, the question usually gets a lot less dramatic.

The bar-stool version

Here is the easy way to think about it. Electricity hates long skinny trips.

The farther it has to go, and the smaller the wire you give it, the more voltage you lose along the way. So if the run is long and the load matters, check the drop and size the wire like you mean it.

That is really what a voltage drop calculator is helping you do. It is just giving you the fast answer instead of making you work it all out by hand.

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