Somewhere between 0.94 and 0.87 for air temperature other than 30.

Currently my Turkeys internal temperature is 58.3 degrees.

Basting starts at 68.

Merry Xmas.

Correction factor for your turkey is 0.61 at 58.3C. Some might say 0.5!

It is more complex than you might like to think - in a simple world, the rating would fall to zero at 70C, as no additional heating would be permissible as it would already be at max temp, and at the reference temperature the rating would be 100% of the book value. As heat  goes as I2 R one might expect a parabolic (x= y2 type) curve to link the two, or at least nearly, as a small correction resistance does change a bit with temp.

Actually if you plot it out, that is almost what the tables suggest - in fact it is a little way out - at 50 degrees the max heating to permitted to reach 70C is half that at 30C, so you may expect 70.7 % of the current rating, and when you are 3/4 f the way from refernce temp to maximum already - 60C, then the dissipation should be 1/4 so the current rating halved.
But this only works where the cooling is by conduction through contact with solids, and heat flow is in linear proportional to temp difference. If cooling is by convection however, then the air speed changes as things warm up, so cooling improves disproportionately well for increasing temperature difference. So use the tables for conduction cooling, but take great care with baskets and  so forth, where the aim is to get a decent temperature gradient and to drive  convection of the cooling gas.

Mike

So the 4B1 numbers should have some leeway as an installed cable can have many different types of heatsink along its route. Copper and insulating material quality would have an impact too. 

So the numbers quoted are simple finger in the air but work OK.

I take Mikes point that it is not as simple as Table 4B1 might suggest and that is why,perhaps, there is no note on how it should be used. So if the ambient temperature is 38C do you use the factor for 35C or for 40C? 
Setting aside Mikes comment and looking at the incremental steps in the table, I would suggest the selection should be for 35C.

Now this is related to a 2396 exam question that I will post in full later. 

I am posting this under threat of being dragged off to a gulag somewhere in the far-flung corners of Siberia!

A final circuit is to be installed to supply a 400V three-phase 28Kw motor for a conveyor system. The motor has a power factor of 0.85.

The circuit is 36m long and is to be installed using four-core armoured 90°C thermosetting insulated cable with copper conductors. One of the cores of the cable will be used as the cpc.

The cable is to be supported on a horizontal perforated cable tray in an ambient temperature of 38°C and touching one other similar circuit which has a conductor operating temperature of 70°C. The circuit is to be connected to a switchfuse at the installation origin. Circuit protection is to be provided by BS88-2gG fuses. Voltage drop for this circuit must not exceed 6v

The installation forms part of a 400/230v TPN TN-C-S system with an external earth fault loop impedance and measured and recorded as 0.05Ω.

Show all calculations.

Determine for the final circuit the

1. Design current (3 marks)

1. Rating of protective device (In) (1 mark)

1. Method of installation reference (2marks)

1. Rating factors that apply (4 marks)

1. Table number and column used to select current carrying capacity within BS 7671 (2 marks)

1. Minimum size of live conductors for current carrying capacity and voltage drop (5 marks)

1. Actual voltage drop (3 marks)

he hee 

well, this will fail the CandG but maybe an OK method in real life.

KVA = KW/PF = 28/.85 = 33kVA
each phase 1/3as 3 phases so  11KVA/phase
Each phase 11000/230= 47,7 A at nominal load
call it 50A per line
Motor Overloads set to 50, backed by fuses a lot higher, or it wont survive switch on. If adjacent circuit is already running at 70C then no heating of any kind at all from this one is permitted as it will overheat the existing cable so do not risk it,  cut the cable ties or undo the cleats and refit, with it moved it over a bit so not touching. Try and get at least one  cable dia of gap for thermal independence.

Are you able to terminate this cable in a way you can run it to 90C? if not then that is a red herring, use figures for 70C max

38C derater from graph 0,9 so that 50A becomes 55A - you need a cable that would be OK on a tray carrying 55A at 30C

Now look very hard at the holes in the tray - more than 30% holes, cols E or F. less than 30%  holes - and that is actually  most univolt/CEF/TLC type  stuff, then really it should be column C.

so depending on that  result 6mm or 10mm swa ;-)  I'd go for at least 10mm anyway, and get 1.6-1.9milliohms per core per metre, so at 36m and 50 A a bout 3,5 volts drop per core, and more like 5-6V off the 400V phase to phase voltage.

16mm would be a  choice if grouping cannot be mitigated.

Deliberately Contentiously, Mike.

Deliberately contentious. 

My workings slightly different but similar result.

40.42A per phase pf=0.85 (no mention of efficiency)

46.48A per phase In = 50A

Cg = 4C1-3 (0.88) Method 4A2-31 E or F

Ca = 4B1 (0.96)

It = 46.48 (Ib) x Cg x Ca = 54.91

5.1.2 For groups allow Ib instead of In, where simultaneous overload is unlikely. 

Vd for 10mm swa 4 x36x46.48/1000 = 6.69V

Vd for 16mm swa 2.5x36x46.48/1000 = 4.18V

So 16mm wins the Volt drop requirement.

I could of course be completely wrong!