I'm specifying cable for a VFD application and came across this table regarding NEC ampacities and corresponding cable size.
What I found interesting is that the ampacity increased with temperature for any environment (Free Air, Single Tray, Conduit). I would have thought that the current would decrease with increasing temperature.
In addition, does this mean at room temperature the current carrying capability would be lower than the ratings @75C and 90C ?
You're looking at it from the wrong direction.
The more current you put through a cable, the hotter it gets. Push too much current through, and the insulation starts to deteriorate. Wire insulation, such as PVC or XLPE (cross-linked polyethylene) will have a rated maximum temperature that it can withstand for many years before it starts to deteriorate. Go above that temperature, and the insulation gradually breaks down.
Looking at the top line of the table, if you have a 14AWG cable with insulation rated for a maximum of 75°C, then you can only put 13A through it without damaging the insulation. If you swap it for a cable rated for 90°C, then you can push the current up to 15A.
But be aware that many electrical accessories aren't rated for 90°C, so a very hot wire can also damage the things it's connected to.
Those aren't increases in capacity at higher ambient temperatures, the temperatures are the ratings for the cable.
So if the cable, terminations and equipment in your system are rated for 90 degrees, you use the higher ampacity value, if they're rated at 75 degrees, you use the lower one.
See this page.
That's not right. DC resistance of a conductor actually rises with temperature, which means at higher temperatures you will lose more power into heat in the cabling, which will heat it up even more.
The tables just mean that if wire temperature limit is 75 degrees, less current is allowed to pass through it, than in the case of allowing more current until wire heats up to 90 degrees.
Those numbers you're looking at are established in NEC 310.15(B)(16).
If you had looked at the original source material, and continued to read onward from there, you would have found NEC's treatment of high ambient temperature environments. Which would answer your question.
(They work as you'd expect).
The section you are reading assumes sensible ambient temperatures, and is talking about the thermal rise in wires from ampacity (resistance). It is limiting ampacity so that wires grouped in conduit and/or packed in wall insulation will not exceed their insulation rating, which varies.
