UK Why don't values in the OSG table I1 match those calculated?

[ConnorLondon]

Hi everyone. I am really puzzled by this, but I suppose there is an obvious answer / reason!

Anyway, Table I1 in the OSG says the mΩ/m value for copper with a CSA of 25mm2 is 0.727. However, when I calculate the R value using the formula:

I get 0.688... Does anyone know why the values aren't the same?

Thanks Connor

 

[Timbo]

What does The Book say?
That is what matters. BS7671.

 

[pc1966]

The conductivity value stated on Wikipedia, etc, will be for pure crystal copper, not the annealed stuff with traces of oxygen and other elements used for power cables. I'm not sure if that is enough to explain the difference, but in any case for design work you should use the figures from the cable manufacturing standards, which as @Timbo points out, as the figures that count in the regulations.