Table 4D1A

Table 4D1A has cables that when reference method F is used starting at 25mm^2, I cannot find a suitable explanation as to why 25mm^2 single insulated cables are okay to use at this CSA and not any less.

If I used 25mm^2 cable from this table the insulation is still the same as a smaller cable from the same table.

is there any justification that when the cable gets to 25mm^2 it somehow becomes acceptable to use?

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  • In effect you are using the change in resistance of the cable over temperature, and I'd caution that it is indeed easy if you do not mind being 10- 20% out with the deduced temperature rise;-)

    First you assume uniform current distribution and heating, which will be fine in small diameter cables, but  you also assume the temperature coefficient of resistance is linear with absolute temperature and classic phonon dominated Debye model, i.e there are no lattice dislocations from impurities in the copper or worse changes of  state, which is indeed true for pure soft copper, but not for all cable types. Try it with steel or brass conductors and be amazed....  Less seriously very modest amounts of impurity in copper have quite a serious effect on the residual resistivity ratio (RRR). (more info here )

    These sort of errors you can eliminate by measuring the resistance with a low current and putting the cable in an well stirred oven or oil bath, when the cable sample is at say 80C, then that is the resistance of that cable at 80C but it is no longer an easy experiment.

    And again with larger cables and layouts where flow and return AC currents are non-adjacent, you need to be careful of the inductive element to the voltage drop as that is not contributing to heating.

    I'd be happy with it as a quick and dirty estimator method, I'd be most disappointed if any of the figures in the official tables were derived that way without cross checking to proper temperature probe measurements.