CableSpec Pro implements the BS7671:2018+A4:2026 Appendix 4 cable sizing methodology. Every circuit is evaluated against five pass/fail requirements — not just current-carrying capacity — with the prospective fault current calculated alongside. Here is exactly what the engine calculates and how.
The circuit’s design current must not exceed the protective device rating or the adjusted capacity of the cable.
The device rating must be co-ordinated with the cable. For BS 3036 fuses and buried cables, BS7671 requires a reduced rating; ring finals have their own rule.
Reactive voltage drop applied wherever BS7671 provides reactive data. Phase-aware: three-phase circuits use BS7671’s line-to-line values directly. Per-circuit limit of 3% lighting / 5% other, or custom.
Zs is calculated from Ze plus the temperature-corrected (R1+R2) and compared with the maximum for the required disconnection time. Missing device data is a failure, never an automatic pass.
The CPC must withstand the fault energy. CableSpec uses the device’s I²t let-through where it is held, otherwise selection to Table 54.7.
The prospective fault current at the load end is calculated from Zs and shown on the certificate. A warning is raised where it exceeds the selected device’s rated breaking capacity.
Ca, Cg, Cs and Cd (and the harmonic factor, where it applies) are applied to the tabulated rating to give Iz. Cf and Cc apply to the overload check only. Every factor is shown on the calculation certificate, and any of them can be overridden on a circuit.
Corrects for the actual ambient temperature, using the next tabulated temperature at or above it (conservative). For buried cables, the ground-temperature formula is used instead.
Applied when cables share a route with others. CableSpec looks up Cg from the route manager for the route’s arrangement and number of circuits, or accepts a manual override per circuit.
Cs = 1.0 (not touching), 0.75 (one side) or 0.50 (fully enclosed in insulation). Applied when a cable runs through or is surrounded by thermal insulation material.
Applied to buried cables laid deeper than the 0.7m reference depth: stepped values from 0.93 at 0.8m to 0.79 beyond 1.5m. No uprating is taken for shallower depths.
Applied automatically when a BS 3036 semi-enclosed fuse is selected: Cf = 0.725, so the device rating must not exceed 0.725 Iz. Buried cables likewise get Cc = 0.9 (Reg 433.1.203).
For underground installation methods (D1, D2, D3), CableSpec uses the analytical ground-temperature correction formula rather than a table lookup: Ca = √((Tmax − Tground) / (Tmax − Tref)) where Tmax is the cable insulation maximum temperature, Tref = 20°C (the reference ground temperature for the buried rating tables) and Tground is the ground temperature entered for the circuit (20°C by default). This follows the basis of BS7671 Appendix 4 Table 4B2.
Ring finals are sized to Reg 433.1.204 (Iz ≥ 20A for a 30/32A device). Zs uses the worst-case midpoint impedance Ze + (R1+R2)×L/4 from Appendix 15, and an optional spur adds its full (R1+R2). Voltage drop is taken with the load at the midpoint (L/4). The R1+Rn value is calculated and shown for reference, and the 63A socket-outlet rule sets the disconnection time.
For parallel cable installations, the design current is split equally per cable (Ib/n), the combined capacity is n×Iz, the loop impedance is divided by n, and the adiabatic fault energy splits as I²t/n² per CPC. A grouping advisory is generated reminding the engineer that Cg counts circuits not individual runs. Up to 6 parallel cables per phase are supported.
Single-phase and three-phase circuits are both supported. BS7671’s three-phase ratings and line-to-line mV/A/m are held separately for every cable type that has them and used directly — no conversion factor. Cable types without a three-phase rating are blocked for three-phase circuits. Zs and PFC always use Uo = 230V.
When third-harmonic content is entered for a three-phase circuit, the neutral current is calculated as 3 × h × Ib. From 15 to 33% a 0.86 rating factor is applied and the cable is sized on line current; from 33 to 45% it is sized on the neutral current with the 0.86 factor; above 45% it is sized on the neutral current. The minimum neutral size is reported (Reg 523.6.3).
An estimated operating temperature is calculated for each conductor size: θ = θambient + (Ib/Iz)² × (θmax − θambient). This uses the per-cable current (Ib/n for parallel installations), accounts for the actual ambient, and is shown on the calculation certificate as a useful indicator of thermal margin.
Any circuit can be sized with an alternative cable type, installation method or protective device without modifying the base circuit. Results are saved as named scenarios and displayed in a side-by-side comparison table showing the size, Iz, voltage drop, Zs, each check outcome and the overall result, with the best option highlighted. Previous scenarios are retained until explicitly deleted.
CableSpec Pro tracks voltage drop from the supply origin through every sub-main to each final circuit. The ΣVD is the sum of VD% across all upstream sub-mains plus the circuit's own VD. This is the value compared against the 3% (lighting) or 5% (other) limit from the origin - not just the circuit's own VD. Shown per-circuit in the board schedule's ΣVD column and in the accumulated VD card on the circuit result view.
For each circuit with a defined upstream sub-main, CableSpec Pro performs two selectivity checks: (1) energy selectivity (Reg 536.4) — the downstream device’s let-through I²t must be below the energy the upstream device needs to operate; (2) current selectivity — the fault current at the downstream device must stay below the upstream device’s instantaneous trip threshold. MCB curves B/C/D, BS 3871 types, MCCBs and fuses are covered where manufacturer data is held; otherwise the result is ‘unknown’. The fault level at the upstream board sets the context. Result: discriminates / partial / fails / unknown. Advisory only — it does not change pass/fail status.
CableSpec Pro’s device library covers the common UK protective device types. Maximum Zs values follow BS7671 Table 41.3 (Uo × Cmin / Ia). Manufacturer MCBs also provide I²t let-through data for the protective conductor and selectivity checks; a device must be selected for every circuit.
Hager, Schneider, ABB, Eaton and Legrand (B, C, D); Wylex and Crabtree (B, C). I²t data held. Generic curves too.
General-purpose gG fuses, 6–200A. Also BS 1361 cartridge fuses.
Semi-enclosed fuse. In ≤ 0.725 Iz applied automatically (Reg 433.1.202).
125–800A, with instantaneous trip settings. Common for sub-mains. Legacy BS 3871 MCBs also included.
Request a demo and we'll walk you through a complete calculation from project setup to PDF certificate - including the distribution network hierarchy and accumulated VD.