In 5.1.2 of appendix 4 page 309 (BS 7671) there are details of the different formulas for calculating grouping factors, and equation 4 has a factor of 0.48 Ib squared (sorry can't seem to add a superscript 2), does anyone know what this 0.48 represents? I have quite a good library of text books and some of them refer to this formula but not one states where this factor is derived from. Neither can I find it on the web.

Cheers SL

I think it will be a correction factor in the equation, there are examples of its use in the IEE Electrical Installation Design Guide - calculations for electricians, p.49.

From a quick read, using the equation will allow grouped cables to be smaller, as it is very unlikely for them all to be subject to overload at the same time.
The example there shows Ib divided into the Cg.. factors, and giving a resulting It of 27.5A, the '0.48' equation gives a result of 24.7A for the same circuits, with the result being a cable capable of carrying 27.5A is needed, the larger result being the one required.

Comparing the results from Equation 2, and 3&4, Eq2 gives a current of 33.3A, Eq3&4 27.5A, so a smaller cable size is available if you do the longer calculations.

So, being as there is a lot of squaring and square root in the equation, a correction factor would be my best assumption.

The second part of the equation, looks like that for the calculation of an RMS value for heating effect. It could be a fudge factor from thermal modelling, as mathematicians are prone to do. But who would use the calculations in the practical sense, when tables are available?

Regards

Hi Gents

Thanks for your views on this and confirming basically what I thought anyway. My reason for inquiring was purely curiosity as I was a bit bored over the Xmas period. Sad I know!

Cheers and happy new year SL

Id say its a Nusselt Number, Look up Churchill, Prandt, Rayleigh.

If you think you were bored, you will be

Hello.

1.45 is the current causing effective operation of the protective device (see 433.1.1). In the derivation of the appendix 4 formula 1.45 ends up being squared and appears in the denominator, this is the same as multiplying by [1 divided by 1.45 squared] which equals 0.4756.... which rounds up to 0.48.