Ring Final CPC Calculator
About This Calculator
Purpose:
This tool helps electricians and testers quickly calculate and verify CPC (r2) values for ring final circuits, following the formulas and test requirements in IET Guidance Note 3 and the latest UK Wiring Regulations: BS 7671:2018+A3:2024 (18th Edition, Amendment 3).
Usage: Enter your measured r1 or rn, select cable sizes, and compare your actual CPC result against the calculated value and pass criteria. Designed for training, EICR, and on-site verification.
Usage: Enter your measured r1 or rn, select cable sizes, and compare your actual CPC result against the calculated value and pass criteria. Designed for training, EICR, and on-site verification.
This is a DEAD TEST.
You must always follow full safe isolation procedures before testing, including verifying the circuit is fully isolated at the consumer unit. Never attempt resistance measurements on a live circuit.
You must always follow full safe isolation procedures before testing, including verifying the circuit is fully isolated at the consumer unit. Never attempt resistance measurements on a live circuit.
- References: IET Wiring Matters (Mar 2020), Guidance Note 3 (18th Edition, Amendment 3), Table 2.
- Who should use it? Qualified electricians, apprentices, electrical inspectors, and educators.
- UK Standards: Suitable for 230 V installations under current regulations: BS 7671:2018+A3:2024 (18th Edition with Amendment 3, as of July 2024).
Disclaimer:
This calculator is for guidance only and must not replace professional judgement or statutory verification required by law. Always confirm results using approved test instruments. No liability is accepted for misuse, incorrect input, or errors in field conditions. For final confirmation, refer to the latest edition of BS 7671 and IET Guidance Notes.
This calculator is for guidance only and must not replace professional judgement or statutory verification required by law. Always confirm results using approved test instruments. No liability is accepted for misuse, incorrect input, or errors in field conditions. For final confirmation, refer to the latest edition of BS 7671 and IET Guidance Notes.
Ring Final CPC (r2) Calculator
Calculate CPC resistance on a ring final:
Enter measured r1 (phase) or rn (neutral) and select cable sizes.
Calculates r2 (CPC) per BS 7671 / GN3. Compare your actual r2 to calculated and see if it passes!
How it works:
r2 = r1 × (CPC size / phase size)
For 2.5/1.5 mm² T&E, use 0.6 × r1.
See IET Guidance Note 3, 18th Edition Table 2 (Page 63).
For 2.5/1.5 mm² T&E, use 0.6 × r1.
See IET Guidance Note 3, 18th Edition Table 2 (Page 63).