View the thread, titled "Zs and temperature correction" which is posted in Electrical Wiring, Theories and Regulations on Electricians Forums.

Hi, as an aspiring sparky I'm needing help with the temperature adjustment formula for Zs. I'm using GN3 (17th). I understand that the values given in Appendix A - Table A4 (for BS 60898) are assumed at a test ambient of 10C whereas the values in BS7671 are at the "normal operating temperature" of the conductor (70C) and adjusted by using a divisor of 1.24.

The example given under Table A6 in Appendix A is - 32A Type B 60898 at an ambient of 25C gives a Zs of 1.23Ω

I understand how they get this - BS7671 gives 32A Zs as 1.44 (at 70C), divided by 1.24 = 1.16Ω (at 10C), then using table A6 an adjustment factor of 1.06 accounts for the test temperature being 25C, gives 1.16 x 1.06 = 1.23 Ω.

The problem I have is that under Appendix A "A3 Methods of adjusting tabulated values of Zs" the example there is "twin and CPC 70C thermoplastic protected by 6A type B 60898 at 20C". They give the answer as 7.67 (the Zs from BS 7671) divided by 1.20, gives a Zs of 6.39Ω They get the 1.20 divisor from Table B3 in Appendix B which accounts for the conductor "incorporated in a cable or bunched".

Using the first formula the Zs comes out at 7.67 divided by 1.24 = 6.18Ω (at 10C), then using Table A6 to account for test temperature of 20C multiplying by 1.04 gives a Zs of 6.43Ω

I know that difference is very small between these two Zs values but I don't understand why they use two different calculations - what am I missing?
 
and on a 6A MCB, which is most likely to be lighting, when will the cable ever get even close to 70 deg.? we're in the real world. IET are all up their own arses, making new hoops for us to jump through, consolidating their gold plated pensions. screw 'em.
 

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Zs and temperature correction
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Electrical Wiring, Theories and Regulations
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KeenPensioner,
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