UK electrical design tool

BS 7671 cable size calculator for quick UK circuit design checks

Build an initial cable sizing assessment for single-phase and three-phase circuits with current, voltage drop, and adiabatic calculations in one clean workflow.

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Designed for practical pre-design reviews

  • Supports 1φ and 3φ supply assumptions.
  • Includes earthing, installation method, grouping and insulation adjustments.
  • Offers four load input modes including an appliance basket.
  • Highlights default voltage drop targets: 3% lighting and 5% for other circuits.

BS 7671 cable sizing calculator

Set the installation assumptions below to estimate a candidate cable size and review key design checks.

Design results

Ib — Design current24.00 A

Ib entered directly: 24.00 A

Selected cable4 mm² Cu

First size where tabulated current ÷ correction factor ≥ In (32 A)

Corrected Iz36.00 A

Iz = It × Ca × Cg × Ci × Cm = 36 × 1 × 1 × 1 × 1 = 36.00 A
It = tabulated current (PVC) for 4 mm²; Ca = ambient temp factor at 30°C; Cg = grouping factor for 1 circuit(s); Ci = thermal insulation factor; Cm = installation method C factor

Voltage drop6.60 V / 11.50 V limit

Vd = (mV/A/m × Ib × L) ÷ 1000 = (11 × 24.00 × 25) ÷ 1000 = 6.60 V
Limit = 5% × 230 V = 11.50 V
mV/A/m from BS 7671 Appendix 4 for 4 mm² Cu (single-phase)

Ib ≤ In ≤ IzPass

24.00 ≤ 32 ≤ 36.00 — BS 7671 Regulation 433.1

Voltage dropPass

6.60 V vs 11.50 V limit — BS 7671 Appendix 12

Adiabatic (CPC size)Needs ≥ 16.50 mm²

S = (I² × t)^0.5 ÷ k = (3000² × 0.4)^0.5 ÷ 115 = 16.50 mm²
k = 115 for Cu/PVC conductor (BS 7671 Appendix 3, Table 54.2–54.4). Selected cable: 4 mm² — must be ≥ 16.50 mm²

Zs / disconnectionNot yet implemented

Automated Zs verification requires tabulated maximum Zs values for MCB devices on TN-C-S systems (BS 7671 Tables 41.2–41.4). Verify manually against the appropriate table for your protective device.

Assumptions and limitations

What this tool calculates

  • Design current (Ib) from your chosen load input method
  • Corrected current-carrying capacity (Iz) using BS 7671 Appendix 4 correction factors
  • Voltage drop against default BS 7671 Appendix 12 limits (3% lighting, 5% other)
  • Minimum CPC size using the adiabatic equation (BS 7671 Regulation 543.1.3)

Known limitations

  • Zs / disconnection time check not implemented. You must verify maximum earth fault loop impedance manually against BS 7671 Tables 41.2–41.4 for your protective device type and rating.
  • Cable data covers 1.5–95 mm² only. Sizes above 95 mm² are not included in the current table.
  • Single correction factor per cable run. The tool applies one set of conditions throughout — it does not model cables that pass through different environments (e.g. partly clipped, partly in insulation).
  • No diversity or harmonics. Motor starting currents, harmonic loading and demand diversity are not modelled.
  • Voltage drop uses resistive mV/A/m values only. Reactive component is not included — results may be optimistic for longer runs or larger cables.

Data sources

  • Current ratings: BS 7671:2018+A2:2022 Appendix 4, Tables 4D1A–4E4A (simplified)
  • Correction factors: BS 7671 Tables 4B1, 4B2, 4C1
  • Voltage drop (mV/A/m): BS 7671 Appendix 4 (resistive component)
  • Adiabatic k values: BS 7671 Tables 54.2–54.4

⚠️ Important notice

This tool is a preliminary design aid only. Results must be verified by a qualified electrician against the current edition of BS 7671, manufacturer data, and the specific conditions of your installation before any work is carried out or certified. Do not use this tool as the sole basis for any electrical design or certification.

How the calculator works

The app calculates design current (Ib) from your chosen load input method, applies simplified correction factors for ambient temperature, grouping, insulation and installation reference method, then identifies the first cable size whose corrected current-carrying capacity (Iz) can satisfy the selected protective device rating (In).

It then compares voltage drop against default limits and calculates the minimum adiabatic protective conductor size using bundled k-values. The outputs are intentionally explicit so engineers, estimators and contractors can see why a draft selection passes or fails.

Ideal use cases

Use the tool for early-stage sizing of lighting circuits, socket radials, ring finals, motors and submains. It is useful as a lead-generation landing page or customer-facing design helper for UK electrical contractors and consultants.

Always confirm final cable selection against full BS 7671 tables, product manufacturer data, thermal insulation details, fault protection requirements and any diversity or harmonics assumptions applicable to the real project.

Frequently asked questions

What does this BS 7671 cable size calculator check?

It estimates design current, proposes a cable size from bundled reference data, and shows current-carrying capacity, voltage drop and adiabatic pass/fail cards.

Can I use this tool for formal certification?

No. It is a fast preliminary design aid and should be verified against the current edition of BS 7671, manufacturer data and your project-specific assumptions.

Why is the Zs check marked as a placeholder?

This version reserves the workflow and UI for Zs and disconnection verification, but does not yet ship the full tabulated protective device data set needed for automated confirmation.