I just went through this on my current project. The electrical installation must comply with the National Electric Code (NEC).
For an 800A circuit, the NEC (Table 310.16) allows the use of two parallel 600 kcmil conductors, each is rated at 420A at 75°C. If you could use 90°C conductors you could downsize each to 500 kcmil, each rated at 430A. This would save in material expenses during installation (conductors and conduits).
The problem is that, per the NEC, conductors must be matched to the temperature rating of the terminal to which it is connected. i.e. 75°C conductors must be connected to 75°C terminals and 90°C conductors must be connected to 90°C terminals.
The reason that MCCB spec sheet doesn't mention terminal temperature is that when connecting to breakers rated above 100A the conductor must be rated for at least 75°C. [ NEC 110.14(C)(1)(b)]
(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75°C (167°F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors
So, if no temperature is listed it is 75°C.
From the article rootboy cited: "No distribution or utilization equipment is listed and identified for the use of 90 °C wire at its 90 °C ampacity." BTW, Jim Pauley, the engineer who wrote that article is now President of the NFPA, the organization which writes the National Electric Code. Manufacturers simply don't make 90°C equipment.
I also have to wonder what customer is planning on doing on the other end as you run into the same problem at both ends of course.
I would definitely read the article by Jim Pauley that rootboy posted as it does explain the situation very well.
Now, to file under the "customer is alway right," if they insist they want to do this, that article shows how you can use a set of terminals rated for 90°C, your customer connects here. Then you connect these terminals using 90°C conductors but sized as if they 75°C (2 x 600 kcmil from my above example) to the MCCB.
I would of course lobby heavily for your customer not to do this. Sure they save a few pennies up front, but for the lifetime of the equipment they will have to endure the costs of wasted electricity because of voltage drop. 500 kcmil wire has about 15% higher resistance than 600 kcmil.
Side note: I believe the Part Number you posted only includes the frame and no trip unit. i.e. it's not a complete part number for a breaker.
