Voltage Drop Calculator
Estimate conductor resistance and voltage drop by cable size, material, and run length.
Estimate the voltage drop across a wire run based on its length, gauge (AWG), material, and the current it carries — a key check for electrical safety and code compliance.
How Voltage Drop Calculator Works
Every wire gauge and material has a known resistance per 1,000 feet (copper conducts better than aluminum, and thicker wires have lower resistance). The calculator looks up that resistance value, then multiplies it by your wire run length — doubled, because current has to travel out to the load and back through the circuit, so both directions of wire contribute resistance.
Voltage drop is then simply current multiplied by that total circuit resistance (Ohm's Law: V = IR). The result is shown as both a raw voltage figure and a percentage of your system voltage, since electrical codes typically express acceptable drop as a percentage rather than a fixed number of volts.
See It In Action
Who Uses Voltage Drop Calculator and Why
- Checking whether a long extension cord or sub-panel feeder run will deliver enough voltage to a workshop, shed, or barn before installing it.
- Verifying a proposed wire gauge for a new circuit meets National Electrical Code voltage drop recommendations before pulling wire.
- Comparing copper versus aluminum wire options for a long run to see how much the material choice affects voltage loss.
- Diagnosing why equipment at the end of a long circuit run seems underpowered or dim.
Mistakes to Avoid
- Entering the one-way distance to the load without realizing the calculator already doubles it internally to account for the return conductor — entering an already-doubled length overstates the drop.
- Mixing up copper and aluminum resistance values by leaving the material selector on the wrong option — aluminum has meaningfully higher resistance than copper at the same gauge, so this changes the result substantially.
- Ignoring the percentage figure and only looking at the raw voltage number — a 6V drop means very different things on a 12V system versus a 240V system, and code thresholds are expressed as percentages for exactly that reason.
Tips for Best Results
- If your calculated drop exceeds about 3%, try the next thicker wire gauge (a lower AWG number) rather than just accepting the result — it's usually the cheapest fix short of shortening the run.
- For long outdoor runs (well pumps, detached garages), check both the voltage drop and the wire's ampacity rating — a wire can pass one check and fail the other.
Fixing Common Problems
My percentage drop seems too high even for a short run. — Double-check you selected the correct AWG gauge and material — a mismatch here (especially picking a thinner gauge than you actually have) is the most common cause of an unexpectedly high result.
I'm not sure which system voltage to enter. — Use your actual supply voltage for that circuit — 120V for a standard US household branch circuit, 240V for large appliances, or the DC voltage of a battery system — since the percentage result is calculated relative to whatever you enter.
Terms Explained
AWG (American Wire Gauge): A standardized numbering system for wire thickness — lower numbers indicate thicker wire with lower resistance.
Voltage drop: The reduction in voltage that occurs as current travels through a conductor's resistance, leaving less voltage available at the load than at the source.