Regulation 314: Division of Installation Into Circuits

Section 314 of BS 7671:2018+A4:2026 is printed under the heading Division of installation. It sits in Chapter 31, Part 3 (Assessment of General Characteristics), and runs to four regulations. Each one does a different job:

To prevent indirect energising of a circuit, what must be done?

Regulation 314.1(f) sets the objective — prevent the indirect energising of a circuit intended to be isolated — and Regulation 314.4 says what must actually be done about it. Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board, and the wiring of each final circuit shall be electrically separate from that of every other final circuit. On site that means no borrowed neutrals, no two circuits sharing a cable, and no arrangement in which isolating one way at the board leaves conductors in that circuit live from another way.

Circuit division is not just about calculating cable sizes and protective device ratings. It is about designing an installation that limits the consequences of a fault, allows safe maintenance, provides operational flexibility, and minimises nuisance tripping. A well-designed circuit arrangement means a fault on one circuit does not plunge the house into darkness, does not disable the fire alarm, and does not defrost the freezer.

Regulation 314.1 is clear: every installation shall be divided into circuits, as necessary, to achieve six objectives. The regulation lists them as items (a) to (f), and a compliant design must satisfy each one that is relevant to the installation:

The remaining regulations in Section 314 turn the broad objectives of 314.1 into concrete design rules. It is worth getting the numbering right, because each one means something different — here they are in full:

Which loads get their own circuit, and on whose authority

Section 314 is a set of objectives, not a shopping list of circuits, so it is worth being precise about where each everyday separation rule actually comes from:

Section 314 does not prescribe exactly how to divide circuits — it sets the objectives and leaves the specific design to the competent electrician. That is a feature, not a gap: it is also why an inspector can record a division of circuits as unsatisfactory without pointing at a numeric limit.

Each circuit must be designed to carry the maximum demand of the connected load. The protective device rating and cable size are selected from the design current (Ib) of the circuit, and Regulation 433.1.1 sets the coordination that follows: Ib ≤ In ≤ Iz.

Diversity applies to circuits too, not just to the main supply

A common misreading is that diversity belongs only at the origin. Regulation 311.1 says otherwise: in determining the maximum demand of an installation or part thereof, diversity may be taken into account — and Part 2 defines diversity in the same terms, as a means of determining maximum demand for an installation or part thereof, taking account of usage patterns. The cooker row above is exactly that. The On-Site Guide method takes the first 10 A of the appliance's rated current in full, adds 30% of the remainder, and adds a further 5 A if a socket-outlet is incorporated in the control unit. A 12 kW cooker draws roughly 52 A connected, which becomes about 28 A of design current — which is why a 32 A device is normal on a 12 kW cooker and a 52 A one is not.

Diversity is a design allowance, not a get-out. Once assessed, the circuit must still satisfy Ib ≤ In ≤ Iz, the voltage drop limit and the Zs limit for its disconnection time. For the dedicated high-demand circuits, see the sizing guides for the cooker circuit, the electric shower and the EV charger.

The choice between ring and radial circuits is one of the most common design decisions for UK electricians. Both are equally compliant with BS 7671 — the choice depends on the application, cable routing, and floor area.

Spurs: what Appendix 15 actually says

Spurs are the part of Appendix 15 that gets misquoted most. An unfused spur run in 2.5 mm² cable should feed one single or one twin socket-outlet only, and may be connected at the origin of the circuit in the distribution board as well as out on the ring. Where the connection is made in a junction box, the box should be to BS EN 60670-22 and — if it has screw terminals — it must remain accessible for inspection, testing and maintenance under Regulation 526.3, or use maintenance-free terminals instead.

If several extra socket outlets are needed, either extend the ring itself or fit a fused connection unit to BS 1363-4 with a maximum 13 A fuse. The number of socket outlets a fused spur can supply is not fixed by a rule: Appendix 15 makes it dependent on the load characteristics, having taken diversity into account.

Beyond ring vs radial, the design has to decide which loads go on which circuits. These are the conventions that discharge the 314.1 objectives in a normal dwelling.

Circuit division and RCD selection are the same decision

Regulation 531.3.2 requires RCDs to be selected and erected so as to limit the risk of unwanted tripping, and lists what must be considered: subdivision of circuits with individual associated RCDs, the use of RCBOs for individual final circuits in residential premises, and a hard ceiling — the accumulation of protective conductor currents and earth leakage currents downstream of the RCD shall be not more than 30% of the rated residual operating current. On a 30 mA device that is 9 mA across everything downstream. Both of the first two indents cross-refer back to Section 314, which is why the circuit schedule and the board layout have to be designed together.

Type matters as much as count. Regulation 531.3.3 restricts RCD Type AC to fixed equipment where it is known that the load current contains no DC components — the examples given are electric heating appliances and simple filament lighting with no electronic components. A modern dwelling has almost nothing that qualifies, so Type A is the working minimum, with Type F where a frequency-inverter appliance sits on a line-to-neutral circuit and Type B where smooth DC residual current is possible.

Here is a practical circuit design for a typical 3-bedroom semi-detached house. This is a starting point — adjust based on the specific property, customer requirements, and installed equipment.

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Commercial installations follow the same principles as domestic but with additional considerations for three-phase supplies, larger load diversity, and more complex circuit arrangements.

The design process for a commercial installation typically starts with a load schedule (every item of equipment and its power demand), followed by a diversity assessment, then circuit allocation (deciding which loads go on which circuits), and finally cable sizing and protective device selection for each circuit. BS 7671 permits the diversity step at Regulation 311.1 and defines the term in Part 2, but the worked allowance tables are in the IET On-Site Guide (Appendix A, Table A2) — which itself cautions that the values are guidance only, are not recently updated, and that appropriate allowances call for special knowledge and experience. Current-carrying capacity and voltage drop figures for the final step come from BS 7671 Appendix 4. The cable sizing calculator handles the cable sizing and protective device verification for each individual circuit.