Busbar trunking
A factory-made run of busbars in a casing, with power tapped off along its length. It suits a long workshop where machines get moved, because you can feed them from new tap-off points on the same run. BS 7671's Appendix 8 covers it, rather than the ordinary cable tables.
Using Busbar trunking 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 Busbar trunking beyond the definition?
- Explain what Busbar trunking 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 Busbar trunking fits in the learning path
The Commercial and Specialist Systems section covers these units:
- Three-Phase Distribution and Motors
- Fire Alarm and Emergency Lighting Circuits
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.
The system that carries or encloses wiring along its route, such as conduit, trunking, tray or basket. It helps support and protect cables and keeps runs organised for installation and maintenance. The material and fixing method must suit the location and the cables; containment alone does not prove a circuit safe.
Containment
Read the Containment entryCompare related terms
Open a meaning to compare its role with Busbar trunking, or follow its full entry.
Containment
The system that carries or encloses wiring along its route, such as conduit, trunking, tray or basket. It helps support and protect cables and keeps runs organised for installation and maintenance. The material and fixing method must suit the location and the cables; containment alone does not prove a circuit safe.
Read the Containment 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 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.