The cable sizing and circuit design app for electricians who want BS 7671:2018+A2 compliant results in minutes — not hours of manual table lookups.

Every electrician knows the drill. You need to verify a cable size for a certificate, so you pull out your copy of BS 7671, turn to Appendix 4, cross-reference current-carrying capacity tables, apply derating factors, check voltage drop, calculate Zs, and verify disconnection times. One circuit can eat up twenty minutes of careful arithmetic — and a single slip means a non-compliant design.

iCertifi’s Circuit Designer does all of that in under two minutes. It is a fully integrated BS 7671:2018+A2 cable sizing calculator built directly into the iCertifi iOS app, designed specifically for UK electricians, electrical contractors, and domestic installers who need reliable, regulation-compliant circuit design on site or in the office.

Circuit Designer uses a guided 5-step wizard interface that walks you through every design parameter. No guesswork, no missed variables, no hunting for the right table. Just answer each step and the app handles the maths.

The earthing system selection is critical because it determines how automatic disconnection of supply (ADS) is verified. TT systems, for example, rely on RCD-based protection rather than Zs values alone — and Circuit Designer handles this distinction automatically.

The app automatically calculates design current (Ib) from your power input. Select from pre-configured templates for common circuits — ring finals, radial sockets, lighting, showers, cookers, and EV chargers — so standard installations take seconds to set up.

Ring final circuits are handled intelligently: the app automatically doubles the current-carrying capacity and applies the /4 voltage drop calculation method required by the regulations.

Circuit Designer references the correct BS 7671 cable data table for your selected cable type. Whether you are using Table 4D5 for flat Twin & Earth, Table 4D4A for SWA PVC, Table 4E4A for SWA XLPE, Table 4D1A for singles in conduit, or Tables 4J1/4J3 for MICC cables — the right current-carrying capacities and resistance values are applied automatically.

This is where most manual calculation errors occur. Getting derating factors wrong — or forgetting to apply one entirely — leads to undersized cables and failed inspections. Circuit Designer calculates and applies every factor for you:

• Ca — ambient temperature correction factor
• Cg — grouping correction factor
• Ci — thermal insulation correction factor
• Cc — BS 3036 semi-enclosed fuse factor (where applicable)

The final step brings everything together. Circuit Designer verifies that your chosen protective device will disconnect within the required time by calculating Zs (Ze + R1+R2) and checking it against the maximum Zs values from BS 7671. You get a clear pass or fail for ADS compliance — no ambiguity.

Once you complete the five steps, Circuit Designer produces a full set of BS 7671:2018+A2 compliant results. Here is what it calculates for every circuit:

Circuit Designer includes current-carrying capacity and resistance data from the BS 7671 cable tables that electricians use daily:

You no longer need to flip between Appendix 4 tables to find the right current rating or mV/A/m value. Circuit Designer selects the correct table, finds the right column for your reference method, and returns the result.

This is where Circuit Designer goes beyond a standalone calculator. Every circuit you design can be saved directly to a distribution board schedule in iCertifi. The app automatically populates the fields that matter for certification:

• Cable size and CPC size
• Protective device type and rating
• Maximum Zs value
• Reference method

Those completed schedules then flow directly onto your EIC, EICR, and Minor Works certificates. No retyping. No transcription errors. Design the circuit, verify it complies, and the certificate data is already filled in.

Circuit Designer does not just give you a cable size and leave you guessing. Every calculation produces detailed output that shows the full methodology:

• Full calculation breakdown — see exactly how every result was derived
• Design notes — the app explains the methodology and any assumptions made
• Pass/fail indicators — clear visual confirmation for each compliance check
• Warnings and recommendations — flagged issues with guidance on how to resolve them
• PDF export — export calculation sheets as professional PDFs for your records or to submit alongside certificates
• Full audit trail — every design parameter is recorded for future reference

If a Qualifying Supervisor or building control officer asks how you arrived at a cable size, you have the documentation to back it up — produced instantly, on site, from the same app you used to create the certificate.

Circuit Designer was built from conversations with working electricians who told us what they needed: something fast, accurate, and directly tied to their certification workflow. It is not a generic engineering calculator adapted for UK regs — it was designed from day one around BS 7671:2018+A2.

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