Phase rotation
The order in which the three lines of a supply reach their peaks. It decides which way a three-phase motor turns, and swapping any two lines reverses it. BS 7671 calls the check phase sequence, and it's part of initial verification on multi-phase circuits, usually done with a phase rotation tester.
Using Phase rotation in context
Reviewed
A distinction worth remembering
A calculation can go wrong even when the arithmetic is correct if line voltage is substituted for phase voltage. Draw the supply and load connection, label the quantities and state whether the load is balanced before choosing a formula.
Questions to take further
- How can I revise Phase rotation beyond the definition?
- Explain what Phase rotation describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- Professional Electrician: single-phase and three-phase power supplies (Enkin, 2024) — The basic difference between the two supplies, smoother power, higher capacity and balancing loads.
- Voltimum: basic three-phase supplies (Scolmore, 2025) — Where three-phase supplies come from: windings spaced 120° apart.
- legislation.gov.uk: Electricity Safety, Quality and Continuity Regulations 2002, regulation 27 — Declared low voltage supply of 230 V between phase and neutral in Great Britain, unless agreed otherwise.
- Professional Electrician: demand for three-phase supply in the UK (GivEnergy) — Most UK homes on a single-phase supply with one fuse, and why a battery maker says demand is growing.
- NIE Networks: connections FAQs — Northern Ireland: machines up to 17 kW on a single-phase supply, three-phase above that.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- Open University: relationships between electrical quantities — The relationship between voltage, current and resistance and the limits of an ohmic model.
Where Phase rotation fits in the learning path
The Inspection and Testing in Depth section covers these units:
- Reading Test Results
- The EICR from Arrival to Handover
- Testing Tricky Installations
- Coding Drills by Room
Curriculum mapping: C&G 2391-52 and 2365 Unit 304.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- Three-Phase PowerElectrical Science
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Electrical Science — quiz previewFree · Electrical Science
3. Return to the meaning
After reading or practising, explain the idea in your own words, then check what you remembered.
Recall a related term
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.
Compare related terms
Open a meaning to compare its role with Phase rotation, or follow its full entry.
AC — Alternating Current
Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.
Read the AC entryThree-phase supply
A supply with three line conductors whose voltages reach their peaks one after another. On a standard 400/230 V system there's 400 V between any two lines and 230 V from any line to neutral. It's used for bigger loads such as motors and large boards, and the lines are identified by brown, black and grey.
Read the Three-phase supply entryMotor starter
The unit that starts and stops a motor, usually a contactor with an overload relay. The relay looks for sustained excess current, such as from a jammed machine, and its setting comes from the maker's data. A fuse or circuit breaker upstream deals with fault current and has to ride through the start, so the two are chosen together.
Read the Motor starter entryA definition is a start. Using it in a real question is where it sticks.