Voltage Drop Calculator
Over a long run, with the round trip counted.
Work out Voltage Drop. Over a long run, with the round trip counted. Says what it does not answer, as well as what it does.
One way — the formula counts the return conductor.
3% on a branch circuit and 5% overall are the usual recommendations.
Voltage drop
6.58%
7.90 V lost · 112.1 V at the load
The run is counted twice — current goes out on one conductor and returns on the other — so a 100 foot run is 200 feet of conductor. Leaving out that factor of two is the commonest error in this calculation, and it halves the answer.
How the Voltage Drop Calculator works
Every conductor has resistance, so voltage arriving at a load is lower than voltage leaving the panel. Over a short run it is negligible; over a long one it is why the lights dim and the motor runs hot. The formula's easiest mistake is the factor of two on single phase — the current goes out on one conductor and back on the other, so the run counts twice.
Also known as: wire voltage drop calculator · cable voltage drop calculator · 3 percent voltage drop · long run voltage drop
The calculation itself
Every conductor has resistance, so some voltage is lost getting to the load. The standard formula uses circular mils:
Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = √3 × K × I × L ÷ CM. K is 12.9 for copper and 21.2 for aluminium.
The 2 is the round trip. Current leaves on one conductor and returns on the other, so a 100-foot run is 200 feet of conductor. Omitting it halves the answer, and it is the commonest error in the calculation.
In practice
20 A over 100 feet of 12 AWG copper (6,530 circular mils) on a 120 V circuit:
VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 51,600 ÷ 6,530 = 7.90 V. That is 6.6% — well outside the usual 3% guidance, and the load sees 112 V rather than 120.
Going up to 8 AWG (16,510 circular mils) brings it to 3.13 V, or 2.6%, which clears the limit.
Three phase over the same run drops 6.84 V rather than 7.90, because √3 is smaller than 2 — one of several reasons long runs are often three phase.
Where the voltage goes
It is dissipated as heat in the conductor. So voltage drop is simultaneously a performance problem and an efficiency loss: the load underperforms and you pay for energy that warmed the cable.
Motors suffer most. Reduced voltage means reduced torque, which means the motor draws more current to do the same work, which increases the drop further. That feedback is why undersized motor runs fail rather than merely underperform.
Resistive loads simply do less — a heater at 6% low voltage delivers about 12% less heat, since power goes with the square of voltage.
Where the figure deceives
The 3% and 5% figures are recommendations in most codes rather than hard requirements, so a run can be technically compliant and still cause trouble.
The formula assumes DC-like behaviour and ignores reactance, which is fine for typical branch circuits and increasingly wrong for very large conductors and long AC runs.
It also assumes a constant load. Motor starting current is several times running current, so the drop at start is several times what this shows — which is why lights dim when the compressor kicks in.
Acting on it
Measure the actual cable route, not the straight-line distance. Runs follow walls and joists and are routinely half again as long as they look.
Check the drop at starting current for anything with a motor, not just at running current.
On a long run, upsizing at installation is cheap relative to pulling the cable again later.
Frequently asked questions
How do I calculate voltage drop?
For single phase: 2 × K × amps × length ÷ circular mils, with K being 12.9 for copper and 21.2 for aluminium. Three phase uses the square root of three in place of the two.
Why is the run counted twice?
Because the circuit is a loop. Current travels the length of the run to the load and the same length back, so the conductor length in the formula is twice the one-way distance.
What is an acceptable voltage drop?
Common practice is 3% on a branch circuit and 5% overall including the feeder. Those are recommendations in most codes rather than hard requirements, but equipment misbehaves beyond them.
Does voltage drop waste energy?
Yes — the dropped voltage is dissipated as heat in the conductor. It is both an efficiency loss and the reason an undersized run gets warm.
Does aluminium drop more than copper?
Yes, by roughly 64% for the same cross-section, which is why aluminium runs are sized larger. The constants in the formula are exactly that ratio: 21.2 against 12.9.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/voltage-drop-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Voltage Drop Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
Related calculators
Wire Size Calculator
The smallest gauge that holds the drop — not an ampacity answer.
OpenOhm's Law Calculator
Any two of volts, amps, ohms and watts give the other two.
OpenWatts to Amps Calculator
Both directions, with the power factor that trips breakers.
Open