CCC adjustments for short distances in long runs

The heat does indeed soak along the cable for a bit, so that the hot part benefits from the cooling further along, but it is only a matter of maybe a few tens of cable diameters, so probably only  a few inches rather than several feet.  A quick dive to come up through 100mm of loft insulation is probably OK,  running any distance in it, unwise, and if you really cannot re-route or improve the cooling then a step up in cable size for the tight section may be in order.


A related thing, allows you  to estimate  the conductor temperature for a less than fully loaded circuit, realise the temperature rise goes as the power, so square of current.


td = ta + ((Ib./Ita)^2, x (tp - tr))


Where


td = temperature of cable at actual design current

ta = ambient temp

Ib = design current (average)

Ita = tabulated current carrying capacity at ambient tr & max cable temp tp

tp = permitted temperature

tr = reference temperature.


E.g.  for a 27 amp cable (say clipped direct T/E 2.5mm) carrying only 13A in an ambient of 25C

(permitted temp is 70C and Reference temp is 30C.)


td = 25 + (13/27)^2,x(70-30) = 35C.

There's table 52.2 in BS 7671 - at least for cables up to 10mm² and insulation up to 0.5m (beyond that, it's 0.5x the clipped direct rating). So for 150mm the correction factor would be somewhere between 0.78 and 0.63.


You can always calculate section-by-section if you wish - rather than applying all the worst case factors simultaneously - so perhaps some saving if say thermal insulatation doesn't co-inside with large grouping or high ambient temperature.

   - Andy.

Length in insulation (mm)

De rating factor

50

0.88

100

0.78

200

0.63

400

0.51

>500

0.50

I agree make one hole per cable in the insulation if you can, one lot of factors is enough.

Table From
Online at DIYwikki

regards Mike

Thanks for the responses.  The specific of runs through short distances in insulation is well covered in these - thanks.


Is there anything in terms of more general guidance on treating relatively short distances of alternative installation methods on a cable run.  For longer sections the idea of calculating section-by-section is a good one as you can ascertain which has the 'worst case' set of conditions - but how do you choose which one to record on the certificate as the forms only generally support listing one - do you pick the one that is the majority of the length or the one with the worst case - or something else?