Power factor

On an AC supply, inductive loads — motors, transformers, fluorescent ballasts — draw current that lags the voltage. That lagging current still flows in the cables and switchgear, but part of it does no useful work; it only builds and collapses the magnetic field each cycle.

Power factor is the ratio of the useful (working) power to the total power the supply has to deliver. A figure of 1.0 means everything drawn does work; a poor figure like 0.7 means the supply is moving far more current than the actual load would suggest.

Poor power factor inflates the current in everything upstream — cables, contactors, the transformer, the supply. That eats switchboard capacity, adds heating and voltage drop, and on larger installations the supplier bills for it. A board that 'trips on load' but the equipment seems modest can be a power-factor problem hiding in plain sight.

Correction adds capacitors that draw a leading current to cancel the lagging current of the motors, so the supply only has to deliver close to the working power. On modern sites with lots of electronic loads, correction has to be chosen carefully — get it wrong and you can over-correct or create resonance.

Inductive loads pull the current to lag (the usual case). Capacitive loads — long cable runs lightly loaded, or too much correction — push it to lead. Either way away from 1.0 wastes capacity. The aim is to sit close to unity at the load's normal operating point.