Three-phase supply
A supply with three line conductors whose 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.
Using Three-phase supply 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 Three-phase supply beyond the definition?
- Explain what Three-phase supply 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 Three-phase supply fits in the learning path
The Electrical Science section covers these units:
- Circuit Fundamentals
Curriculum mapping: C&G 2365 Units 202/302.
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- Three-Phase PowerElectrical Science
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Recall a related term
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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 Three-phase supply, 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 entryLine voltage
The voltage between two line conductors on a three-phase supply: 400 V on a standard 400/230 V system. On that star-connected supply it's √3, about 1.73, times the phase voltage. People also use the phrase loosely for the 230 V on a single-phase line conductor, so check which one is meant.
Read the Line voltage entryPhase voltage
The voltage across one phase of a three-phase system. On a star-connected 400/230 V supply that's line to neutral, so 230 V. In a delta-connected load there's no star point, and each winding takes the full line voltage. Mixing the two up is an easy way to get a three-phase sum wrong.
Read the Phase voltage entryPhase 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.
Read the Phase rotation entrySubmain
A cable feeding one distribution board from another, such as a run from the main board to a board in an outbuilding. Submain is the everyday word: BS 7671's Part 2 calls it a distribution circuit. Because it carries a whole board's load, it's sized for that board's demand, and its protective device has to work with the devices below it.
Read the Submain entryA definition is a start. Using it in a real question is where it sticks.