To tell Pirform that a circuit is a ring final circuit, just enter the word ring into the Circuit Description field.
Pirform looks at 5 separate aspects to determine whether the over current protective device you have selected is appropriate to the cable size in use. These are:
- Wiring type
- Reference method of installation
- Cable size
- Protective device rating
- Whether the circuit is a ring final circuit (Pirform assumes all circuits are radials until you tell it otherwise).
If your wiring type is A (PVC/PVC cables), your reference method is C (Clipped Direct), the cable size is 2.5mm and the protective device is 32A, Pirform will generate some intelligence informing you that the combination goes beyond the recommendations of BS 7671.
Entering the word ‘Ring’ into the circuit description will now Revert that intelligence.
Under Regulation 433.1.5, in Lehman’s terms, if the cable can carry 20A in its installed conditions, then a 30A or 32A protective device, combined with 2.5mm (or 1.5mm for MICC) is acceptable if the circuit is wired as a ring.
If your Reference Method is either 101 or 103 (Table 4D5), then the de-rating factor of the insulation brings the current carrying capacity down to 17A and 13.5A respectively.
Therefore a rethink is required here, and don’t be fooled by thinking upgrading to 4.0mm will do for Reference Method 103, as this de-rates the cable to 17.5A.