Table 4B1 gives rating factors for temperatures other than 30C. The lowest temperature is 25C. If the ambient is say 15C then can the table be extrapolated?

I would have thought so, as the temperature coefficient is constant.

""The correction factor is given by {1 + 0.004(ambient temp - 20 °C)} where 0.004 is the simplified resistance coefficient per °C at 20 °C given by BS EN 60228 for copper and aluminium conductors"
(IET 183)
IET. ON-SITE GUIDE BS 7671:2008 (2011).

So one assumes the current carrying capacity would relate directly to this. This was shown on a video somewhere when a 1mm cable cooled sufficiently (i.e. very very cold) can carry 1000 amps.
Hope this helps

First decide if the cable is protected from overload and short circuit or just short circuit.

For low ambient temps where overload protection is required you need a different factor from that where short circuit protection only is needed

Eg:

At 15C, you would need a factor of 1.09 if overload protection is offered and 1.17 if short circuit protection only (for cables constrained to 70C

If overload is a consideration and excluding BS 3036 fuses, use:

Ca = 1/1.45 sqr root( (to - ta)/(tp-30))

Where

to = max conductor temp under overload
ta = ambient temp
tp = max permitted operating temp


the max conductor temp under overload is found from:

to = tr +1.45^2(tp - tr)

where tr is the reference temp - usually 30C

If it's for short circuit only the factor is as above but omit the 1/1.45 and change to for tp

regards

OMS

-------------------------
Failure is always an option

OMS, thanks for the calculation. Where is the formula from?

I've found a table in my Schneider Electrical Installation Guide that comes up with the exact same figure.

Where is it from ? - it's just a few sums to manipuate data published with specific reference to find an answer to your question

You'll recognise the number "1.45" from 433.1.1 (iii) - the rest is just deriving a temperature from knowledge of other temps, ie Tp and Tr that the data is published with

regards

OMS

-------------------------
Failure is always an option