UK Electrical Calculators Guide
Quick electrical calculations compliant with BS 7671
In one sentence: WireWrite ships six free UK electrical calculators — cable sizer, maximum demand, voltage drop, circuit check, earth fault loop impedance and solar string — all BS 7671 + IET On-Site Guide compliant and free to use without sign-up at wirewrite.co.uk/calculator.
Cable Sizer — BS 7671 Appendix 4
Pick your circuit type, enter length and load, and get the recommended cable size with a voltage drop check in a single pass. Table 4D5 thermoplastic singles + Table 4F3A SWA.
Maximum Demand — IET On-Site Guide Appendix A
Calculate total maximum demand for single-phase households using IET OSG Table A2. Pre-loaded with typical UK appliance loads and diversity factors, recommends a main switch rating.
Voltage Drop — BS 7671 Regulation 525
Shows pass/fail against BS 7671 Regulation 525 / Table 4Ab limits: 3% lighting, 5% other. Toggle between Public LV supply and Private LV supply to apply the correct ceiling.
Circuit Check
Verify an existing circuit is compliant. Enter cable size, breaker, length, load and Zs — get an instant pass/fail against each BS 7671 limit.
Earth Fault Loop Impedance — BS 7671 Tables 41.3 / 41.4
Check measured or calculated Zs against BS 7671 Tables 41.3 and 41.4 with the Cmin = 0.95 voltage factor. MCB Types B, C and D with prospective fault current.
Solar String Sizing — IET CoP for Grid-Connected Solar PV
Calculate min/max modules per string with temperature correction, MPPT voltage window, and the system voltage limits adopted by the IET Code of Practice for Grid-Connected Solar PV.
Ohm’s Law
Solve for voltage, current, resistance or power given any two values. Quick V = IR and P = VI cross-check from anywhere in the wizard.
Disclaimer: All calculations are guidance only. Always verify results against BS 7671 and the IET On-Site Guide and apply professional judgement before commissioning any installation or issuing an Electrical Installation Certificate.
Frequently asked
Are the WireWrite UK calculators free?
Yes. All six calculators (cable sizer, maximum demand, voltage drop, circuit check, earth fault loop impedance and solar string) are free to use with no sign-up. They are independent of the certificate platform and do not require a paid WireWrite subscription.
Which standard do the UK calculators use?
Cable sizing uses BS 7671 Appendix 4 (Table 4D5 for thermoplastic singles in conduit, Table 4F3A for SWA). Voltage drop applies BS 7671 Regulation 525 / Table 4Ab: 3% for lighting, 5% for other uses. Maximum demand follows the IET On-Site Guide Appendix A Table A2 single-phase household method. Solar string sizing follows the IET Code of Practice for Grid-Connected Solar PV with cold-Voc factors. Earth fault loop impedance is checked against BS 7671 Tables 41.3 and 41.4 with the Cmin = 0.95 voltage factor.
What does the Cable Sizer calculate?
The Cable Sizer recommends a cable size for a given circuit type, length and load. It checks both current-carrying capacity (BS 7671 Appendix 4) and voltage drop in a single pass and warns if a smaller cable would breach either limit. Typical results: 2.5mm² for 32A ring finals, 6mm² for showers up to 8.5 kW, 10mm² for an EV charger.
How does the Voltage Drop calculator handle Public vs Private LV supply?
BS 7671 Regulation 525 sets 3% for lighting and 5% for other uses on Public LV supplies (the DNO-fed household case). On Private LV (generator-fed or sub-board) the limits scale up. The calculator exposes both modes so you can run the check against the correct ceiling without re-deriving the percentages.
How does the Earth Fault Loop Impedance calculator work?
Enter the measured or calculated Zs for the circuit, the protective device type and rating, and the calculator returns a pass or fail against BS 7671 Tables 41.3 and 41.4. It applies the Cmin = 0.95 voltage factor so the limits match the corrected values most multimeters report, supports MCB Types B, C and D, and shows the resulting prospective fault current.
Can I use the calculator results to issue an EIC?
The calculators are guidance tools. The values they produce inform your design and verification but you remain responsible for the final compliance decision. Always cross-check critical results against BS 7671 and the IET On-Site Guide directly and apply professional judgement before commissioning an installation or issuing an Electrical Installation Certificate.
Do the calculators work on a phone in the field?
Yes. The calculators are responsive and work in any mobile browser. They do not require sign-in, so you can open wirewrite.co.uk/calculator on site without unlocking a paid account. The wider WireWrite app also installs as a Progressive Web App on Android and iOS for offline access to your saved drafts.
Use the WireWrite calculators in your full workflow
Issue BS 7671 EIC, Minor Works and EICR certificates with the same standards data behind every calculation. 30-day free trial, no card.