Power factor is a multiplier in voltage drop calculations that accounts for how efficiently an AC load uses current. When power factor is low, the current drawn from the supply increases even though the actual work being done stays the same. That higher current flowing through the resistance and reactance of wires produces a larger voltage drop between the source and the load.
Understanding this relationship matters if you’re sizing conductors, troubleshooting dim lights at the end of a long run, or trying to figure out why a motor isn’t getting the voltage it needs.
What Power Factor Actually Represents
In a DC circuit, voltage and current rise and fall together. In an AC circuit, they don’t always. Certain loads, especially motors and transformers, cause the current wave to lag behind the voltage wave. Power factor is a number between 0 and 1 that describes how far apart those two waves are. A power factor of 1.0 means voltage and current are perfectly in sync. A power factor of 0.5 means they’re significantly out of step, and roughly half the current flowing through the wire isn’t doing useful work at all.
That out-of-sync current still flows through the conductor. It still generates heat. It still creates voltage drop. But it doesn’t power anything at the load end. This is why power factor matters so much in voltage drop calculations: a low power factor means more total current for the same amount of real power delivered.
How Power Factor Changes Voltage Drop
The standard voltage drop formula for a single-phase AC circuit looks like this: voltage drop equals current multiplied by a combination of the wire’s resistance and reactance, weighted by the power factor. Specifically, the formula accounts for two components of the drop. One is proportional to the wire’s resistance times the cosine of the phase angle (the power factor). The other is proportional to the wire’s reactance times the sine of the phase angle.
In practical terms, this means the voltage drop has both a resistive piece and a reactive piece, and power factor determines how much each one contributes. At a high power factor (close to 1.0), the resistive component dominates because almost all the current is “in phase” with the voltage. At a low power factor, the reactive component grows larger because more of the current is out of phase, interacting with the wire’s inductance rather than its resistance.
For most building wiring in smaller sizes, resistance is much larger than reactance, so the resistive component of voltage drop dominates regardless. But in longer runs or larger conduit with bigger conductors, the reactance of the wire becomes more significant, and a poor power factor can substantially increase the total drop.
Why Low Power Factor Makes Things Worse
Here’s the core problem. If a motor needs 10 kilowatts of real power and operates at a power factor of 0.85, it draws a certain amount of current. If that same motor’s power factor drops to 0.60, it now draws roughly 42% more current to deliver the same 10 kilowatts. Since voltage drop is directly proportional to current, the drop through the supply wires increases by that same percentage.
This is not a theoretical concern. Electric motors running at less than full load have notoriously poor power factors. A small motor (under 5 horsepower) at full load typically has a power factor around 0.84. At one-quarter load, that same motor drops to a power factor of 0.50 to 0.60. Completely unloaded, it falls to 0.15 to 0.20. The motor is barely doing any work, but it’s still pulling reactive current through the wires and creating voltage drop.
Industries with large numbers of motors tend to have low facility-wide power factors. Textile plants commonly run at 0.35 to 0.60. Oil pumping operations sit around 0.40 to 0.60. Even hospitals typically operate at 0.75 to 0.80. All of that reactive current flowing through distribution wiring creates voltage drop that wouldn’t exist if the power factor were closer to 1.0.
The Two Parts of the Drop
If you look at a phasor diagram (a vector drawing of voltage and current), you can see why power factor splits the voltage drop into two pieces. When current flows through a wire, it creates a voltage drop across the wire’s resistance that lines up with the current direction, and a separate drop across the wire’s reactance that sits at 90 degrees to the current. The total voltage drop is the combination of these two vectors.
When the load has a lagging power factor (the typical case with motors and transformers), the current vector rotates away from the voltage vector. This rotation changes how the resistive and reactive drops add together. At unity power factor, they combine in the most straightforward way. As the power factor drops, the reactive component contributes more to the total, and the overall drop grows.
This is why voltage drop tables in the National Electrical Code and wire manufacturer catalogs often list values at specific power factors, commonly 0.85 or 1.0. If your actual load has a different power factor, you need to adjust. Using a table based on unity power factor for a facility running at 0.65 will underestimate the real voltage drop.
Leading Power Factor: A Special Case
Most loads cause a lagging power factor, where current lags behind voltage. But capacitive loads, including power factor correction capacitors and some solar inverter configurations, cause a leading power factor where current leads voltage. This flips the reactive component of voltage drop in the opposite direction.
In some cases, a leading power factor can actually produce a voltage rise instead of a voltage drop. This is a real concern with solar panel installations on long distribution feeders. The inverters can push the voltage at the end of the line higher than at the source. Utilities sometimes use deliberate leading power factor operation of solar inverters (setting reactive power output to about negative 20% of real power output) as a low-cost way to manage voltage on distribution networks without installing dedicated voltage regulation equipment.
Correcting Power Factor to Reduce Voltage Drop
If voltage drop is a problem and low power factor is contributing to it, improving the power factor at the load is often cheaper than upsizing the wire. Adding capacitors near a motor or at a facility’s main panel supplies the reactive current locally instead of pulling it through the entire length of the supply wiring. The current in the wires drops, and so does the voltage drop.
For example, improving a facility’s power factor from 0.65 to 0.90 reduces the supply current by about 28%. That translates directly to 28% less voltage drop in the feeders. The wires, transformers, and switchgear all run cooler, and the voltage at the load stays closer to nominal. This is why utilities charge commercial customers a penalty for poor power factor: it wastes capacity in the entire distribution system.
When you’re calculating voltage drop for a new installation, always use the actual expected power factor of the load, not 1.0. For motor circuits, use the power factor at the expected operating load, not the nameplate full-load value, since most motors spend much of their life running at partial load where the power factor is worse. For mixed commercial loads like offices, 0.80 to 0.90 is a reasonable starting assumption. For heavy industrial motor loads, 0.65 to 0.80 is more realistic unless power factor correction is already in place.