Submain
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.
Using Submain in context
Reviewed
A distinction worth remembering
A consumer unit is where the supply splits into circuits, with protection for each. The main switch turns all the circuits off, but the meter tails stay live. A circuit breaker protects one circuit's cable from too much current. An RCD trips when current leaks away, and an RCBO does both jobs for one circuit. A smart new enclosure doesn't prove those devices were well chosen, or that the circuits behind them have been verified.
Questions to take further
- How can I revise Submain beyond the definition?
- Explain what Submain 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
- Electrical Safety First: Best Practice Guide 1, replacing a consumer unit — Issue 5.2, September 2026. When a unit needn't be replaced, what's recommended, checks before the work, pre-work surveys, defects, minimum tests, certificates and notification.
- Professional Electrician: replacing a consumer unit in a dwelling (NICEIC) — 19 February 2021. Testing before work, telling the client early, shared neutrals, and low insulation resistance tripping RCDs once circuits are reconnected.
- legislation.gov.uk: Building Regulations 2010, regulation 12 — In England, the electrical work in a home that has to be notified to building control (a new circuit, a replacement consumer unit, or work on existing circuits in a special location), and what counts as a special location.
- IET: consumer-unit enclosures — Non-combustible enclosures or cabinets, and why older units need assessment rather than automatic replacement.
- IET: inspection and testing FAQs — Inspection, test interpretation and protecting equipment during testing.
- IET: RCD selection, types and testing — Residual-current waveforms, device selection and the distinction from overcurrent protection.
Where Submain 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
- Consumer UnitsInstallation
- Cable SizingInstallation
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Cable Selection — quiz previewPlus · Installation
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.
A distribution assembly for a domestic installation, bringing together switching and circuit protective devices. Circuits may have separate RCBOs or share residual-current protection. BS 7671 requires a non-combustible enclosure or suitable enclosing cabinet arrangement for consumer units within domestic premises. An older plastic enclosure is not, by itself, proof that the installation is unsafe.
Consumer unit
Read the Consumer unit entryCompare related terms
Open a meaning to compare its role with Submain, or follow its full entry.
Consumer unit
A distribution assembly for a domestic installation, bringing together switching and circuit protective devices. Circuits may have separate RCBOs or share residual-current protection. BS 7671 requires a non-combustible enclosure or suitable enclosing cabinet arrangement for consumer units within domestic premises. An older plastic enclosure is not, by itself, proof that the installation is unsafe.
Read the Consumer unit entryMaximum demand
The assessed greatest simultaneous demand of an installation or part of it. Connected loads, their operating patterns and justified diversity allowances inform the assessment. It helps determine supply and distribution capacity; it is not a guarantee that actual demand can never be higher, and a change of use may require reassessment.
Read the Maximum demand entrySelectivity — Discrimination
Arranging protective devices so only the one nearest a fault operates and everything else stays on. Older books and plenty of people on site call it discrimination. It can be total, or partial up to a stated current, so check the makers' curves or selectivity tables rather than just comparing ratings.
Read the Selectivity 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 entryA definition is a start. Using it in a real question is where it sticks.