Textbook lesson 23

Real, Reactive and Apparent Power

kW, kVAr, kVA and why power factor matters.

Training safety note: This is study material, not permission to perform electrical work. Practical activities must be done on approved training equipment or under licensed supervision with the current rules and workplace procedures.

In this lesson

Learning outcomes
Core theory
Trade application
Worked example
Workshop task
Fault-finding notes
Revision questions and answers

Learning outcomes

Explain the purpose of this topic in everyday electrical work.
Identify the circuit conditions that must be checked before relying on a reading.
Apply the relevant calculation, test or drawing interpretation in a supervised training scenario.
Recognise common apprentice mistakes and unsafe assumptions.
Record evidence in a form that another tradesperson can understand.

Core theory

Alternating current is not simply DC that changes direction. The voltage and current vary sinusoidally, and the timing relationship between them matters. Resistive loads draw current in phase with voltage. Inductive loads such as motors and transformers cause current to lag. Capacitive loads cause current to lead. This phase relationship affects power factor, voltage drop and heating.

RMS values let electricians compare AC to an equivalent DC heating effect. A 230 V RMS supply has a higher peak value, but the RMS value is what is used for most power calculations. Understanding this prevents confusion when apprentices see different values on oscilloscopes, labels and calculations.

Three-phase systems are used because they deliver smoother power and efficient motor operation. The relationship between line and phase values depends on star or delta connection. A neutral conductor carries the imbalance in a star-connected system, so harmonics, load balance and borrowed neutrals must be understood before assumptions are made.

Core law

P = V × I | E = P × t

Power tells you the rate of energy conversion. Energy tells you how much is used over time. Heating is proportional to current squared through resistance, so small increases in current or connection resistance can create serious heat.

Worked example

A 230 V heater draws 8.7 A. Power is about 2.0 kW. If it runs for 3 hours it uses about 6 kWh. If a loose connection has only 0.5 Ω resistance at 8.7 A, heat at that point is I²R = 37.8 W, concentrated in a tiny area.

Textbook depth: kW, kVA and kVAr

Real power, measured in watts or kilowatts, performs useful work and produces heat, light or motion. Reactive power, measured in var or kVAr, moves energy in and out of magnetic or electric fields. Apparent power, measured in VA or kVA, is the product of RMS voltage and RMS current before power factor is applied.

Power factor is the ratio of real power to apparent power. A poor power factor means more current is required for the same real power. More current means more voltage drop and more heating in conductors and equipment. Electricians need this concept for motors, large sites, generator sizing, UPS sizing and switchboard loading.

Example: A single-phase load takes 10 A at 230 V. Apparent power is 2.3 kVA. If power factor is 0.75, real power is 1.725 kW. The conductors still carry 10 A, not the lower current implied by kW alone.

Trade application

On site, this topic is rarely isolated. It connects to safety, drawings, protection, cable selection, terminations, testing and documentation. A good apprentice does not ask only “does it work?” They ask whether it is correctly supplied, correctly protected, correctly controlled, mechanically sound, suitable for the environment, and verifiable by inspection and test.

When using this material, build a notebook of standard methods. For each topic, write the normal value, the likely fault value, the test points, the instrument setting, and the action to take if the result is abnormal. This becomes a practical diagnostic map rather than a collection of memorised definitions.

Workshop practical

Use a training transformer or simulator to compare resistive, inductive and capacitive AC loads. Record supply voltage, load current, apparent power and real power where instruments allow. Draw the phase relationship as a simple phasor diagram.

Evidence to collect: labelled sketch, predicted readings, actual readings, explanation of differences, supervisor feedback and one improvement to your method.

Fault-finding notes

Confirm the complaint or task requirement in plain language.
Compare the installation against the drawing, label or expected circuit arrangement.
Prove whether supply is present at the correct point and under the correct condition.
Divide the circuit into smaller sections instead of testing random points.
After repair, test the protective measure, not just the load operation.

Common apprentice mistakes

Mistake	Why it matters	Better habit
Measuring voltage without a reference plan	The reading may be real, induced, back-fed or meaningless without a return path.	State the exact two points being measured and the expected value first.
Assuming a device is faulty because it is not operating	The fault may be supply, control, protection, return path, settings or mechanical load.	Prove each section of the circuit in sequence.
Recording only pass/fail	Future workers cannot see whether results were strong, marginal or abnormal.	Record actual values, conditions and instrument details.

Assessment standard

The assessor is looking for correct RMS language, recognition of phase angle, correct use of apparent/real/reactive power terms, and safe interpretation of single-phase and three-phase diagrams.

Revision questions

What should be proven before this task is attempted on real equipment?
Which measurement would best confirm the main idea of this lesson?
What reading or symptom would make you stop and ask for supervision?
How could a poor termination change the behaviour of this circuit?
What information should be recorded for handover or assessment evidence?