BS EN 60228 gives a simplified resistance coefficient per ºC for copper and aluminium conductors at 20 ºC of {1 + 0.004 (ambient temperature - 20 ºC) }.*
BS 7671 takes this to mean that at typical cable operating temperatures, the approximate resistance-temperature coefficient of 0.004 per ºC **
This means that for every ºC difference in a value of a resistance or impedance at a reference temperature, then the corrected value will be:
{ reference resistance / [1 + 0.004 (reference resistance temperature - corrected resistance temperature ) ] } ***
BS 7671 gives reference values for maximum permitted Zs (including factors such as Cmin) at a reference temperature of 70ºC ****
The maximum Zs values quoted in Appendix B of the On-Site Guide and Appendix A of GN3 assume an ambient temperature of 10ºC *****
As the reference temperature is 70ºC and the ambient temperature is assumed to be 10ºC, the equation above states:
Corrected value of resistance = { reference resistance / [ 1 + 0.004 ( 70 - 10 ) ] }
= { reference resistance / [ 1 + ( 0.004 * 60 ) ] }
= { reference resistance / [ 1 + 0.24 ] }
= { reference resistance / 1.24 } ******
So for example, a 32A BS EN 60898 type B circuit breaker has a maximum Zs of 1.37Ω *******
So the corrected value is 1.37 / 1.24 = 1.1048Ω
Of note: 1 / 1.24 = 0.80645. So rather than divide by 1.24, you could instead multiply by 0.80645 ********
Appendix 14 (informative) of BS 7671 advises that the requirements of Regulation 411.4.5 or 411.5.4 are considered to be met when Zs (measured) = 0.8 x [ ( U0 * Cmin ) / Ia ] *********
So if you want to work out to the 4th significant figure what your maximum acceptable measured Zs value is, then by all means use 0.80645 as your multiplier. Of course, your calibrated thermometer (to measure the ambient temperature) must have an equivalent degree of accuracy, as must your calibrated test instrument. Good luck with that.
And whilst we're on the subject of accuracy... does it strike anyone as odd that the maximum permitted disconnection times given in regulations 411.3.2.2 - 411.3.2.4 are only to 1 significant figure? And that they are "round numbers" (particularly 411.3.2.3 and 411.3.2.4 ("5s" and "1s")? These - and the nominal voltage of 230, and Cmin of 0.95 (presumably because 0.94 looked too ugly) - are where all the other values come from.
I'm sure there a good technical reasons for choosing these seemingly random maximum disconnection times. I'll just look it up in my copy of "Commentary on the IET Wiring Regulations." It must be here somewhere.... I ordered it in February, after all, and the estimated arrival date is October 2015. I'm sure it will be here any day now....
* ref table B2 of GN3
** ref note in section 6.1 of Appendix 4 of BS 7671 (p 337)
*** because maths
**** ref NOTE 3 of 411.4.6, 411.4.7, 411.4.8 and 411.8.3
***** ref p 119 of both publications - huh, freaky!
****** because maths
******* ref Table 41.3 of BS 7671
******** because maths
********* ref Appendix 14 of BS 7671 (p 452)
