I wouldn't get involved in a technical decision with the client on this one. The 0.8 factor for the impedances in Tables 41.2 to 41.4 of BS 7671 is actually provided for you in the guidance in BS 7671 itself, Appendix 3 (page 410 in the Brown 2022 book).

I would answer the query by replying to that effect.

The 0.8 factor is to be used with respect to look impedances in Tables 41.2 to 41.4. If you use the 'maximum measured loop impedance' tables in OSG or GN3, the 0.8 factor is already taken into account for you, and you can just use the loop impedance value from those tables.

You could also answer the query regarding older circuits that used to be compliant, that, in earlier Editions of BS 7671 and the Wiring Regulations, higher impedances were permitted, but they have been adjusted in later 16th Edition (for change from 240 to 230 V as the nominal voltage), and in 17th Edition with the inclusion of C_min. This means that some circuits that used to be compliant are no longer considered to meet the latest safety recommendations.

Thank you, I have explained to them the added derating factor of Cmin which was not likely taken into account on the previous inspection, and that we have to ensure compliance to 0.8 of the maximum Zs. As MCCB's aren't covered in the wiring regulations due to the vast differences in tripping characteristics I just wanted to verify that any max Zs; even if given by the manufacturer, is taken at ambient temperature and not at full load would still be required to be taken at 0.8 as per appendix 3. They are being reasonable but are now worried about the cost of correcting the issues if what I'm saying is correct as some of the given values are extremely low. 0.03 at 100% for example a Zs they are unlikely to achieve at the end point especially when taken at 80%. 

Bees2699 said:

even if given by the manufacturer, is taken at ambient temperature and not at full load would still be required to be taken at 0.8 as per appendix 3.

Yes, although the conductor temperature (70 deg C vs ambient of 30 deg C) is what's important. The manufacturer's loop impedance values for 30 deg C align with the Tables I quoted in BS 7671.

On an EICR, if you don't have the original design data regarding circuit design, assumed fault currents and disconnection times, it's not possible to do a "re-design" on the hoof, so I guess the only option would be an "FI" (which definitely leads to "unsatisfactory") and doesn't help the client ? If you did have that data, you could take it into account.

Bees2699 said:

even if given by the manufacturer, is taken at ambient temperature and not at full load would still be required to be taken at 0.8 as per appendix 3.

Yes, although the conductor temperature (70 deg C vs ambient of 30 deg C) is what's important. The manufacturer's loop impedance values for 30 deg C align with the Tables I quoted in BS 7671.

On an EICR, if you don't have the original design data regarding circuit design, assumed fault currents and disconnection times, it's not possible to do a "re-design" on the hoof, so I guess the only option would be an "FI" (which definitely leads to "unsatisfactory") and doesn't help the client ? If you did have that data, you could take it into account.