Cold cable, and correcting for ambient temperatureSame cable, 40 degrees. What changes?

What "cold cable" means

Every calculation so far has been for a cold cable. Imagine the cable still on the drum, or installed but not yet energised and commissioned.

That is the state the installation is in when you turn up to do the dead tests — wired, but never switched on. Once it is in service and carrying load, the cable warms up and the resistance rises.

Correcting for ambient temperature

Guidance Note 3 · Appendix B · Table B2 · p.175, "Ambient temperature multipliers (α) to Table B1". Table B1 is at 20 °C. If the ambient temperature on the test day is different, the calculation gains one more term:

The Table B2 multiplier corrects the 20 °C figure to the ambient temperature you are testing in. At 20 °C the multiplier is 1, so there is nothing to apply — you only reach for it above or below 20 °C.

mΩ/m for 4/1.5 = 16.71
Length            = 10 m
Table B2 at 40 °C = 1.08

R = (16.71 × 10 × 1.08) ÷ 1000
  = 180.5 ÷ 1000
  = 0.1805
  = 0.18 Ω   (against 0.17 Ω at 20 °C)

Resistance rises with temperature — but not in proportion, the way it does with length. Going from 20 °C to 40 °C adds about 8% (the 1.08); it does not double it. At 15 °C ambient the resistance would come out lower than the 20 °C figure.