Power factor explained

The three powers

Alternating current loads that store energy in a magnetic or electric field — motors, transformers, ballasts, some switch-mode supplies — draw current that is out of step with the voltage. That out-of-step current still flows in the cable, still heats it, and still counts towards the protective device rating, but it does no useful work.

• • True power (kW) — the part that does work: heat, light, torque. What the energy meter bills.
• • Reactive power (kVAr) — the part that shuttles back and forth charging and discharging fields. Does no work, but occupies capacity.
• • Apparent power (kVA) — the vector sum of the two. This is what the cable, the MCB and the supply actually see.

Why it changes the design

Take a 3 kW load on a 230 V single-phase supply:

pf 1.00 (resistive heater)

13.0 A

pf 0.95 (mixed / good LED)

13.7 A

pf 0.85 (heat pump)

15.3 A

pf 0.70 (uncorrected fluorescent, small motor)

18.6 A

pf 0.50 (cheap LED driver, welding set)

26.1 A

Same kilowatts, double the current. At pf 0.5 that 3 kW load has moved from a comfortable 16 A circuit in 1.5 mm² to needing 32 A protection and 4 mm² — and volt drop, which is proportional to current, has doubled with it. This is the single most common way a paper design under-sizes a real installation.

Typical values by load type

Use the equipment data plate or manufacturer's data wherever it exists. The figures below are design guidance only, for use when nothing better is available.

Leading, lagging and distortion

• • Lagging — inductive loads (motors, transformers, ballasts). Current lags voltage. By far the most common in UK installations.
• • Leading — capacitive loads, or over-correction. Current leads voltage. Over-corrected banks on lightly loaded sites can cause voltage rise and upset generator or UPS control.
• • Distortion power factor — with switch-mode supplies, LED drivers, VSDs and IT loads the current is not a clean sine wave at all. The true (total) power factor is displacement × distortion, so a load that looks like cos φ 0.98 can have a total pf nearer 0.6. Third-harmonic currents from these loads add in the neutral of a three-phase circuit, which is why the neutral is sized as a current-carrying conductor and Table 4Ab derating applies.

Correction — when it's worth it

Domestic installations are not billed for reactive power, so correction is almost never justified in a house. On commercial and industrial supplies the DNO or supplier may levy an availability or reactive charge, and poor power factor eats transformer and cable capacity you have already paid for.

• • Fixed capacitors at the load — simple, cheap, best for a single large motor running continuously. Never fit across a motor fed by a VSD.
• • Automatic capacitor banks — switched steps at the main panel, controlled by a relay measuring site pf. Standard for varying commercial loads.
• • Detuned / harmonic-filtered banks — required where there is significant harmonic content, otherwise the capacitors and supply impedance can resonate and destroy the bank.
• • Active harmonic filters — electronic correction of both displacement and distortion. Expensive, but the only real answer on heavily non-linear sites.

BS 7671:2018+A4:2026 Chapter 81 and Appendix 17 treat power factor correction as part of the energy efficiency assessment of an installation, alongside load profiling, transformer sizing and conductor selection.