LV System Designer
Build a low-voltage installation as a single-line diagram — supply, switchboard, distribution boards, sub-mains and final circuits — then calculate the whole system to BS 7671 in one pass. Maximum demand rolls up, fault levels and earth-loop Zs propagate down, cables auto-size, and discrimination and breaking capacity are checked between devices in series. A first-pass design aid — not a substitute for a full ProDesign model.
This is a desktop tool — build your network on the sheet, then read the results in the panel and schedules. Every value is indicative; verify against your own copy of BS 7671 and manufacturer data before issue.
Click an issue in the status bar to jump to it on the sheet. Drag the sheet to pan, scroll to zoom, Fit to frame everything. Add buttons wire the new element under your current selection; use Connect to wire two existing elements. Each board is a busbar — every circuit gets its own breaker and cable. A distribution board's final circuits are tucked away — click a board (look for the blue N ▸ badge) to open its own diagram.
Single-purpose calculatorsthe designer's engines, on their own
The LV System Designer is a first-pass design aid to BS 7671:2018+A2:2022. Its reference data (cable current ratings, mV/A/m, conductor resistances, correction factors, k-values and device data) is an indicative subset of BS 7671 Appendix 4 / Chapter 41–54 and manufacturer literature. Prospective fault current uses a simplified single-source impedance method; discrimination is a generic trip-band check, not a manufacturer-tested study. Every value marked VERIFY must be confirmed against your own copy of the standard and manufacturer data before a design is issued. The design remains the responsibility of the engineer.
One Model for the Whole Installation
Instead of checking circuits one at a time, the LV System Designer models the whole distribution network the way design software like Trimble ProDesign does — a single-line diagram of the supply, switchboards, distribution boards, sub-mains and final circuits. As you build, maximum demand with diversity rolls up from the loads to the source to size each sub-main, while prospective fault current and earth-fault loop impedance Zs propagate down from the origin through every cable and board. Each board is drawn as a busbar, and every outgoing circuit gets its own protective device and cable.
Every Cable Runs the Five Checks
Each connection — sub-main or final circuit — is sized and checked to BS 7671: the Ib ≤ In ≤ Iz coordination, current-carrying capacity with derating (Ca·Cg·Ci), cumulative voltage drop from the origin against the 3 % / 5 % limits, earth-fault loop Zs against the maximum for the device, and the adiabatic CPC check. On top of that the designer checks each device's breaking capacity against the fault at its terminals and gives an indicative discrimination verdict between devices in series.
Built From Our BS 7671 Toolset
The designer reuses the same engines as our single-purpose calculators — so you can still reach for the cable sizing calculator, fault level calculator, maximum demand calculator or selectivity checker when you only need one answer. Print / Save as PDF produces a whole-system report with a summary, per-board schedules and a VERIFY register. For a full, verified distribution design, a discrimination study or a stamped submission pack, see our ProDesign cable calculation service.