AI Circuit DesignerBS 7671 Compliant Design

The AI Circuit Designer follows the full adiabatic cable sizing method from BS 7671 Appendix 4. It starts with the design current (Ib) for the circuit, selects a protective device rating (In) that is greater than or equal to Ib, then determines the required current-carrying capacity (It) by applying all relevant correction factors. These include the ambient temperature factor (Ca) from Table 4B1, the grouping factor (Cg) from Tables 4C1 to 4C6, the thermal insulation factor (Ci) from Table 52.2, and where applicable the semi-enclosed fuse factor (Cc) of 0.725. The designer then selects the smallest cable size from the appropriate table in Appendix 4 (for example, Table 4D5 for thermoplastic flat cable) whose tabulated current-carrying capacity (Iz) exceeds the required value of It. It then verifies the cable meets the voltage drop limits, earth fault loop impedance requirements, and adiabatic equation for fault protection. If any check fails, the designer automatically upsizes the cable and recalculates.

Yes. The AI Circuit Designer handles single-phase and three-phase circuit design. For three-phase installations, it calculates line currents, selects appropriate three-pole and four-pole protective devices (MCCBs, TP MCBs, TP+N RCBOs), sizes three-phase cables using the correct current-carrying capacity tables, and verifies voltage drop across all three phases. It understands the differences between balanced and unbalanced three-phase loads, calculates neutral current in unbalanced systems, and selects neutral conductor sizes accordingly. For commercial and industrial installations, it produces distribution board schedules showing phase allocation to achieve balanced loading across the three phases, and calculates diversity factors appropriate to the type of installation.

Yes. For every circuit, the designer verifies that the earth fault loop impedance (Zs) at the furthest point of the circuit does not exceed the maximum value permitted for the selected protective device. It uses the formula Zs = Ze + (R1+R2), where Ze is the external earth fault loop impedance of the supply and (R1+R2) is the resistance of the line conductor plus the circuit protective conductor, corrected for conductor operating temperature. The designer references the maximum Zs values from BS 7671 Tables 41.2 to 41.6 for different protective device types and ratings. If the calculated Zs exceeds the maximum, the designer either increases the cable size to reduce (R1+R2), selects a larger protective device rating where the design current permits, or recommends the use of an RCBO for additional fault protection.

EV charger circuits have specific requirements under BS 7671 Section 722 that the Circuit Designer addresses automatically. For a typical 7.4 kW single-phase charger drawing 32A, the designer applies a continuous load factor (the charger operates at rated current for extended periods, so the cable must be rated for sustained loading). It specifies Type A RCD protection as a minimum, or Type B where required by the charger manufacturer for DC fault current protection. The cable sizing accounts for the full route from the consumer unit to the charging point, including any derating for thermal insulation where the cable passes through insulated walls or loft spaces. For installations requiring a dedicated supply (such as a 22 kW three-phase charger), the designer includes the supply assessment and any upgrades needed to the main incoming supply. BS 7671 Section 722 also includes specific requirements for TN-C-S (PME) earthing systems: outdoor EV charging equipment on a PME supply requires particular consideration of the earthing arrangement and protective measures, as the PME earth may not be suitable without additional precautions. The designer flags this when the earthing system is identified as TN-C-S and the charger is located outdoors.

The Circuit Designer supports all earthing systems defined in BS 7671: TN-S (separate neutral and earth, typically older PME supplies with a lead-sheathed cable earth), TN-C-S (combined neutral and earth at the supply transformer, separated at the intake position — the most common arrangement for modern UK domestic supplies), and TT (no earth provided by the distributor, requiring a local earth electrode). For each earthing system, the designer applies the correct maximum disconnection times from BS 7671 Table 41.1 (0.4 seconds for final circuits not exceeding 32A, 5 seconds for distribution circuits), selects appropriate protective devices, and verifies earth fault loop impedance values. For TT systems, it accounts for the typically higher Ze values and recommends RCD protection where the earth fault loop impedance would otherwise be too high for overcurrent devices alone to provide disconnection within the required time.

Yes. When you describe an installation, the Circuit Designer generates a complete consumer unit schedule that maps directly onto the schedule of circuits section of the Electrical Installation Certificate (EIC). For each circuit, the schedule includes: circuit number, circuit description, protective device type and rating (MCB Type B/C/D or RCBO), cable type and reference (for example, 6242Y for twin and earth flat cable), cable size, design current (Ib), protective device rating (In), maximum permitted Zs, voltage drop in volts and as a percentage, and whether the circuit requires RCD protection. The schedule also specifies the recommended consumer unit make and model, the main switch rating, and SPD (Surge Protection Device) requirements under BS 7671 Section 443. The output is formatted so you can transfer the values directly onto the EIC without additional calculation.

The Circuit Designer automatically verifies voltage drop for every circuit against the limits set by BS 7671. The statutory requirement is in Regulation 525, and the numeric limits — 3% for lighting circuits and 5% for all other circuits — are stated in Appendix 4, Section 6.4 of BS 7671:2018+A4:2026. The designer calculates voltage drop using the millivolt-per-ampere-per-metre (mV/A/m) values from the cable data tables in Appendix 4, multiplied by the design current and the cable route length. For longer cable runs — such as an outbuilding supply or a submain to a separate distribution board — the designer accounts for the cumulative voltage drop across all sections of the circuit. If the calculated voltage drop exceeds the permitted limit, the designer automatically upsizes the cable to bring the voltage drop within limits and recalculates all other parameters to confirm compliance.

Yes. Regulation 411.3.4 of BS 7671:2018+A4:2026 introduces a mandatory requirement that, within domestic (household) premises, additional protection by an RCD with a rated residual operating current not exceeding 30 mA shall be provided for AC final circuits supplying luminaires. This is a new A4:2026 obligation — not present in earlier editions — and the Circuit Designer applies it automatically when designing a domestic installation. When producing the consumer unit schedule, the designer specifies RCBO protection (or RCD coverage from a split-load board) for every lighting circuit in a domestic property, not only for socket-outlet circuits. The requirement uses "shall" and is not subject to a risk-assessment exception; it applies to all domestic lighting circuits regardless of installation method.

Regulation 421.1.7 of BS 7671:2018+A4:2026 recommends the installation of arc fault detection devices (AFDDs) in AC final circuits of a fixed installation to mitigate the risk of fire due to arc fault currents. The regulation uses recommendatory rather than mandatory language, but many network operators and insurers treat it as a practical requirement for high-risk premises (houses in multiple occupation, care homes, and similar). Where the Circuit Designer recommends or specifies AFDDs for socket-outlet final circuits, this must be recorded on the Electrical Installation Certificate: Regulation 133.1.3 of BS 7671:2018+A4:2026 requires that certain equipment usages — including AFDD installation — shall be recorded on the appropriate Part 6 electrical certification. The consumer unit schedule output from the Circuit Designer flags AFDD-protected circuits and reminds you to declare this on the EIC.