CableSpec is a cloud-based cable sizing application for UK electrical engineers. Every circuit is checked to BS 7671 Appendix 4: current-carrying capacity, overload protection, voltage drop, earth fault loop impedance and protective conductor size, with the prospective fault current shown — plus professional PDF certificates and full project management.
Cable sizing to BS7671 Appendix 4 is a multi-step process with interdependent checks, a stack of correction factors and a growing library of cable types. Most engineers are still doing it on spreadsheets - or worse, from memory.
Enter the load, device and run - CableSpec selects the smallest cable size that passes every check.
Compare cable types, installation methods and protective devices side by side on any circuit.
Define cable routes to calculate BS7671 grouping factors (Cg) automatically for every circuit on the route.
Per-circuit calculation certificates and full project schedules with your company branding.
Share read-only project summaries with clients and reviewers. No login required.
CableSpec is structured around real-world design workflows. Define the supply parameters once at project level, then work through each circuit with the calculation engine handling all the BS7671 arithmetic.
Enter the project’s supply parameters - voltage, earthing system (TN-S, TN-C-S or TT), measured Ze and source. These apply automatically to every circuit in the project for Zs calculations.
Define each circuit’s design current, cable type, installation method, run length and protective device. Click Calculate - CableSpec runs every BS7671 check and selects the minimum compliant size.
Review the project schedule showing every circuit’s status. Download a per-circuit PDF calculation certificate, a full project PDF report, or an A4 landscape circuit schedule. Share the project with clients via a secure link.
Enter Ib, device and run length and CableSpec works up through the available cable sizes to find the smallest that passes every check. If no size passes, it tells you which check is failing, the smallest size that meets capacity and voltage drop, and what would fix it — an RCD, a lower device rating or a bigger CPC.
Start a circuit from a preset — ring final, lighting, EV charging, solar PV, motor, fire alarm, emergency lighting, heat pump or battery storage — with sensible defaults and the relevant regulation notes. Duplicate any circuit to start the next one.
Run alternative calculations on the same circuit with different cable types, installation methods or protective devices. Compare results side by side to find the most efficient or practical option.
Every calculation is saved as a numbered, timestamped version with its results and the engineer who ran it. Reopen any earlier version to see how the design evolved — useful for CDM, design reviews and revisits.
Define cable routes with one of 11 arrangements — bunched, enclosed, clipped, trays, ladder, buried direct or in ducts. CableSpec looks up the BS7671 grouping factor (Cg) for the number of circuits on the route, and flags every circuit on it for recalculation when the route changes.
Organise work by project, each with its own supply parameters. Bulk recalculate all circuits in a project after changing Ze or earthing type. Project dashboard shows instant pass/fail/pending status at a glance.
Professional A4 PDF for each circuit: all inputs, every correction factor, each check result with its regulation reference, step-by-step VD and Zs working, prospective fault current, and an engineer declaration with name, date and your saved signature.
Print-ready A4 landscape circuit schedule showing all key results across the project. Full project PDF with a compliance summary, project details, engineer declaration and the circuit schedule. CSV export for external processing.
Share a read-only view of any project with clients, principal designers or reviewers. Links last 30 days, need no login on the receiving end, and can be revoked at any time from your project view.
Upload your company or organisation logo. It appears automatically on circuit certificates, project reports and printed schedules, replacing the CableSpec wordmark with your own identity.
Every circuit is stamped with the BS7671 amendment in force when it was last calculated. Circuits calculated under a previous amendment are flagged on the project view, certificate and share page.
Changed Ze or earthing system after preliminary design? One click re-runs the engine across all circuits in a project and shows a pass/fail summary on completion.
CableSpec holds current-carrying capacity, mV/A/m and conductor resistance data for over 30 UK cable types, from BS7671 Appendix 4 tables — with manufacturer data for fire-resistant and mineral insulated cables, where BS7671 has no table. Single-phase and three-phase ratings are held separately.
PVC and LSF flat twin & earth and 3-core & earth, 1–16mm². Methods A, B, C and 100–103 (cables in insulated walls, floors and ceilings). Twin & earth is single-phase only.
70°C 6491X and 6181Y, 90°C 6181XY and LSF 6491B singles, 1–300mm². Methods A, B and C in conduit or trunking, and F in free air from 25mm².
70°C PVC and LSF and 90°C XLPE circular multicore, 1–300mm². Methods A, B, C and E.
70°C PVC and LSF and 90°C XLPE SWA, 2-core and 3/4-core, 1.5–300mm². Methods C, E, D1 and D3 (buried direct and in ducts). The armour is checked as the CPC.
90°C XLPE SWA with aluminium conductors, 2-core and 3/4-core, 16–300mm². Methods C, E and D1. Common for DNO feeders and large sub-mains.
PVC-sheathed light duty (1.5–4mm²) and heavy duty (1.5–25mm²) mineral insulated cable, methods C and E. Inherently fire resistant.
FP100, FP200, FP200 Gold, Fireforce PH30 and FP400 armoured, rated from manufacturer data. Used for emergency lighting, fire alarm and life-safety circuits.
60°C PVC flexible cords 3182Y–3185Y (0.5–4mm²) and H07RN-F rubber flexible (1–50mm²), methods C and E. Cords without a CPC are blocked where fault protection is needed.
CY (tinned copper braid screened), SY (steel wire braid) and YY (unscreened) flexible cables, 0.5–6mm², rated conservatively as 60°C cords. Used for instrumentation and machine wiring.
CableSpec is designed for electrical engineers and contractors who produce compliant cable sizing calculations for real projects - not as a back-of-envelope check, but as a documented, auditable design record.
CableSpec gives your cable sizing calculations the rigour they deserve - every Appendix 4 check, full correction factor working, and a professional PDF certificate per circuit.
Produce the cable schedules, project reports and client documents that tenders, IET Wiring Regulations Part 6 and CDM documentation require - without spreadsheet errors or missing checks.
CableSpec is part of the Construct Software product suite - specialist applications for the UK electrical contracting and building services sector. Single sign-on across all products.
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One licence gives you unlimited projects, unlimited circuits and all platform features. Contact us for current pricing and multi-user organisation licences.
Annual subscriptions. All prices exclusive of VAT. Full pricing details →
Everything you need to know before getting started with CableSpec.
CableSpec brings BS7671 Appendix 4 rigour to every cable sizing decision - with automatic correction factors, reactive VD, the right disconnection times and a professional PDF record of every calculation.