How to use the voltage drop calculator
The formula
VD = 2 × K × I × L ÷ CM
K is the resistivity constant: 12.9 for copper, 21.2 for aluminium (ohm-circular mils per foot). I is the load current in amps, L is the one-way distance in feet, and CM is the wire's circular-mil area from the standard AWG table. Percentage drop is simply VD ÷ source volts × 100.
How to use it
Pick the conductor material and size, measure the one-way run length (not out-and-back — the formula already doubles it), enter the actual load current and the supply voltage, then press Calculate.
Why it matters
Every foot of wire has resistance, so some voltage is always lost as heat along the run. Long runs on small wire are the classic problem: the tool at the far end sees much less than the panel voltage.
Example
100 ft of 12 AWG copper carrying 10 A on a 120 V circuit: VD = 2 × 12.9 × 10 × 100 ÷ 6,530 = 3.95 V, which is 3.29% — just over the 3% branch-circuit guideline. Enter exactly those values above to see it.
Frequently asked questions
What formula does this calculator use?
The standard single-phase formula VD = 2 × K × I × L ÷ CM, where K is 12.9 for copper and 21.2 for aluminium, L is the one-way length in feet, and CM is the conductor's circular-mil area. The factor 2 accounts for the round trip out and back.
How much voltage drop is acceptable?
Common guidance is no more than 3% on a branch circuit and 5% total including the feeder. Equipment may still run above that, but motors run hotter and lights dim.
Does doubling the wire size halve the drop?
Not exactly — drop falls in proportion to circular-mil area. Going from 12 AWG (6,530 CM) to 10 AWG (10,380 CM) cuts the drop by about 37%.
What about three-phase circuits?
For balanced three-phase, replace the factor 2 with √3 (about 1.732): VD = 1.732 × K × I × L ÷ CM. This calculator covers single-phase runs.
More free calculators
Ohm's Law Calculator
Enter any two of volts, amps, ohms, or watts and get the other two — the most-used formula on the job.
Wire Size Calculator
Find the smallest AWG conductor that keeps voltage drop under 3% and safely carries your load current.
Watts / Amps / Volts Converter
Convert between watts, amps, and volts for DC and single-phase AC circuits, with power factor.
Electricity Cost Calculator
See what an appliance really costs to run per day, month, and year from its wattage and your kWh rate.
Conduit Fill Calculator
How many THHN wires fit in an EMT conduit at the standard 40% fill? Pick a gauge and trade size to find out.
Resistor Color Code Calculator
Read any 4-band resistor: choose the band colours and get the resistance value and tolerance.
Voltage Divider Calculator
Work out the output voltage of a two-resistor divider from the input voltage and resistor values.
Series & Parallel Resistance
Combine up to five resistors in series or parallel and get the total equivalent resistance instantly.
Battery Life Calculator
Estimate how long a battery will run a load, from capacity in mAh and the current the load draws.