Electrical Engineering Basics: Power Factor, Voltage Drop, and Wire Resistance
How power factor, voltage drop, and wire resistance together determine whether an electrical circuit is sized and configured correctly.
Three numbers determine whether an electrical circuit will perform as expected: how effectively power is actually being used, how much voltage is lost getting there, and how much resistance the wire itself contributes.
Power factor: real power versus apparent power
Power factor is the ratio of real power (the power that does useful work) to apparent power (voltage times current). A power factor below 1.0 means some of the apparent power isn't being converted to useful work — common with inductive loads like motors — and utilities sometimes penalize industrial customers for poor power factor since it strains the grid without proportional useful output.
Voltage drop: what's lost over distance
Every wire has resistance, and current flowing through that resistance causes a voltage drop proportional to the wire's length and gauge. Long circuit runs need thicker wire specifically to keep voltage drop within an acceptable range (commonly under 3-5%) at the load end.
Wire resistance: the root cause of voltage drop
Resistance equals resistivity times length divided by cross-sectional area (R = ρL/A). A 100-meter copper wire with 4 mm² cross-section has a resistance of about 0.42 ohms — this is exactly the number that, combined with current draw, produces the voltage drop over that run.
Sizing a circuit correctly
Power factor affects how efficiently a load uses supplied power; wire resistance and voltage drop determine whether that power actually arrives at full strength. Undersized wire on a long run is one of the most common and preventable electrical design mistakes. Try the power factor calculator, voltage drop calculator, and wire resistance calculator together when sizing a circuit.