kW · kVA · AmpsConverter

kW (kilowatts) is real power — the actual energy consumed and converted to heat, light, or mechanical work. kVA (kilovolt-amperes) is apparent power — the total power drawn from the supply, including both useful (real) and reactive power. The ratio of kW to kVA is the power factor. A purely resistive load (heater, incandescent lamp) has a power factor of 1.0, so kW = kVA. An inductive load (motor, transformer) has a power factor below 1.0, so kVA > kW. Electricity meters measure kWh (real energy), but cables, transformers, and switchgear must be rated for kVA.

Power factor (pf) is a dimensionless number between 0 and 1 representing the ratio of real power (kW) to apparent power (kVA). A low power factor means higher current for the same useful power output — increasing cable losses, voltage drop, and the risk of overloading cables and switchgear. Industrial premises with lots of motors often pay a power factor penalty from their DNO if pf falls below 0.95. Power factor correction (adding capacitor banks) brings pf back toward 1.0, reducing kVA demand and saving money.

In a balanced three-phase system, the three line voltages are displaced by 120° from each other. The relationship between line-to-line voltage (400V in the UK) and line-to-neutral voltage (230V) is: VL = VN × √3. Similarly, the total three-phase apparent power is S = √3 × VL × IL, where VL is the line voltage and IL is the line current. The √3 factor accounts for the 120° phase displacement — you cannot simply add three single-phase powers because they do not peak simultaneously.

Use the motor's full load amps (FLA) from the nameplate or from this calculator, then apply a derating factor for grouping, ambient temperature, and installation method per BS 7671 Appendix 4. As a rule of thumb: choose a cable rated for at least 125% of FLA for continuous motor loads (to allow for starting current and thermal overload protection tolerances). For a 4kW motor at 400V 3-phase (pf 0.85): FLA ≈ 8.5A — a 2.5mm² cable (rated 21–26A depending on installation) is typically adequate after derating.

Full load amps is the rated current a motor draws at its nameplate power output. It is stamped on the motor nameplate and is the value used for cable sizing, overload protection setting, and contactors. The nameplate also shows the service factor (typically 1.0–1.15), which indicates how much the motor can be overloaded continuously. When sizing protective devices, always use the nameplate FLA, not a calculated value — manufacturing variations mean the nameplate figure can differ slightly from the formula.

Resistive loads (heating elements, incandescent bulbs): pf ≈ 1.0. LED drivers and switching power supplies: pf ≈ 0.9–0.95 (with PFC). Single-phase motors at full load: pf ≈ 0.75–0.85. Three-phase induction motors at full load: pf ≈ 0.85–0.92. Three-phase motors at part load: pf drops significantly — a motor running at 50% load may have pf ≈ 0.65. Welding sets and fluorescent lighting with magnetic ballasts: pf ≈ 0.5–0.7. Variable speed drives (VFDs) improve power factor because they control the motor's reactive power draw.

A typical motor nameplate shows: rated power (kW or HP), rated voltage(s) and connection (e.g. 400V Delta or 230/400V Star/Delta), rated frequency (50Hz in UK), rated current (FLA), speed (RPM), power factor (cos φ), efficiency class (IE2, IE3), service factor, frame size, insulation class (typically F or H), enclosure rating (IP class), and weight. The dual voltage rating (e.g. 230/400V) means the motor can be connected in delta for 230V or star for 400V. Always check which connection matches your supply before commissioning.