Extension lead plug getting warm? advice please?

[EFLImpudence]

Two there is a correction factor of 0.917 when working out the volt drop for the final ring, so worked out at 16.5197 mV/A/m not 18 hence the slightly different figure,

If you are going to that degree of accuracy, it should be noted that the actual starting mV value is 17.568mV/A/m, not 18.

 

[JohnW2]

In a way suppose it is temperature, the it is all to do with cable not being loaded to the max, the calculation was in the appendix of the 17th Edition I remember it used sq root etc, so a bit complex.

I suspect that you are talking about Equation 6 in section 6.1 of Appendix 4. However, as 6.1 says, that equation is only applicable when ambient temperature is 30°C or higher, which is very rarely going to be the case in the UK.

As you say, one of the effects of that equation (when it is applicable) is (very reasonably) that the design value of the resistivity of the cable (i.e. mV/A/m) will reduce if the design current of the circuit is less than the tabulated current-carrying-capacity of the cable (as installed), since the conductor temp at design current will be then lower than the temp on which the tabulated mV/A/m figures are based. However, it's not clear as to what equation/calculation one is meant to use when ambient temp is less than 30°C. Indeed, even if (despite what is said about applicability) one tries to use the equation 'as is' when ambient temp is less than 30°C, values for Ca (ambient temp correction factor), which is one of the terms in the equation, are only given for temps down to 25C in Table 4B1.

I also find it difficult to know what figures for Ib and It (design current and tabulated cable CCC respectively) one is meant to use in that equation in the case of a ring final circuit. If you think about it a bit, I think you will find that it is far from obvious. If one to take the design current as 32a (or 26A, although I've never seen anyone but you {on the basis of some presentation} suggest that is acceptable) and looks up the CCC of 2.5mm² cable (potentially only 20A for a ring final compliant with regs), the design current could then be considerably in excess of the 'CCC'!

I therefore am not quite sure what one is meant to do.

Kind Regards, John

 

[ban-all-sheds]

it used sq root etc, so a bit complex.

Partly your imagination.