Voltage Drop Calculator
Calculate the voltage drop and percentage drop across a copper wire run given its gauge, length and current.
Good
1.32% drop is within the commonly recommended 3% ceiling for branch circuits — the higher this reads, the more a thicker gauge wire pays for itself in delivered power.
- Percent Drop1.32%
- Voltage at Load118.41 V
Within the commonly recommended 3% voltage drop limit.
About the Voltage Drop Calculator
This calculator estimates how much voltage is lost along a copper wire run given its gauge, length and the current it's carrying — the check an electrician runs before wiring anything with a long cable path, like a detached garage, a well pump, or a workshop out past the house.
Wire isn't a perfect conductor — every foot of it has a small amount of resistance, and over a long enough run at high enough current, that resistance quietly eats into the voltage that actually reaches the load. This tool sizes that loss before the wire is bought and run, since undersizing a long circuit is expensive to fix after drywall is up.
Enter the wire gauge, the one-way length of the run, the expected current draw and the system voltage, and it returns both the voltage lost and the percentage drop — the number that actually determines whether a gauge is adequate.
How it’s calculated
Voltage drop is calculated from the wire's resistance per 1000 feet (a standard published figure for each AWG copper wire gauge), the one-way length of the run, and the current flowing through it. The length is doubled in the formula because current has to travel out to the load and back through the return conductor — a 50-foot run means 100 feet of total conductor.
The percentage drop is just the voltage lost divided by the system voltage. Electricians commonly target keeping voltage drop under about 3% for branch circuits, since drops beyond that can cause dim lighting, motors that run hot or underperform, and other symptoms of an undersized wire run.
Frequently asked questions
What voltage drop percentage is acceptable?
3% is a widely used guideline for branch circuits, and 5% is often cited as a combined limit for a feeder plus branch circuit together — beyond that, equipment can underperform and the National Electrical Code offers this as a recommended (not always mandatory) design target.
How do I fix excessive voltage drop?
The two main levers are using a thicker (lower AWG number) wire gauge, which has less resistance per foot, or shortening the run if that's practical. Reducing the current draw isn't usually an option since that's dictated by the load itself.
Why does wire length matter twice as much as I'd expect?
Because current has to complete a full circuit — it flows out through one conductor and back through another — so a 100-foot one-way run actually involves 200 feet of wire resistance total. This calculator's length input is one-way and doubles it internally to account for that.
Does thicker wire always solve voltage drop?
Generally yes — thicker wire has lower resistance per foot, which directly reduces voltage drop for the same length and current. The tradeoff is cost and physical size, so the goal is usually the thinnest gauge that still keeps drop within an acceptable range, not the thickest wire available.
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