Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of power factor. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Power factor is 1.0 for a purely resistive load and approaches zero for a purely reactive one. Motors and transformers have low power factor, which is why they draw more current than their wattage suggests.
It matters for sizing because conductor and breaker ratings are based on current, not on watts. A 10 kW load at 0.8 power factor draws 25% more current than at unity.
Common misuse
Sizing a cable from the kW figure alone. At a low power factor the current — and therefore the cable — must be larger.