The ratio of real power to apparent power in an AC circuit.
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.
Sizing a cable from the kW figure alone. At a low power factor the current — and therefore the cable — must be larger.
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