You've identified a common point of confusion in applying the NEC. Here's a breakdown and clarification

Understanding the Interaction of 370.80 and 110.14(C)
370.80 (Cablebus Ampacity)

This section specifically allows you to determine the ampacity of the conductors within the cablebus assembly using Table 310.17 (free air). This is because cablebus is designed to provide excellent ventilation and heat dissipation.

110.14(C) (Termination Temperature Limitations)

This is the key section causing the confusion. It's a general rule that applies to terminations for all equipment, unless the equipment is specifically listed and marked otherwise.

The Critical "Unless" Clause in 110.14(C)

The language "Unless the equipment is listed and marked otherwise" is what allows you to use the 90°C ampacity if:1. The Equipment is Listed

This is a critical starting point. The equipment (termination point - think panel, breaker, switchgear connection points) must be listed by a recognized testing laboratory (UL, ETL, etc.). This listing confirms the equipment's ability to safely handle the higher temperatures associated with 90°C conductors.2.

The Equipment is Marked

The equipment must be specifically marked as suitable for use with 90°C conductors at the termination point. This marking is the manufacturer's certification that the equipment (specifically the termination) can withstand the higher temperatures.3.

Manufacturer's documentation Documentation must support that the cablebus terminations (lugs) are rated and approved for 90C, otherwise the cable ampacity is limited by 110.14(C) to the 75C ampacity.

Why the Limitation to 75°C is Generally Required

The general limitation to 75°C is a safety precaution. Older or less expensive equipment terminations may not be designed to handle the higher operating temperatures of 90°C conductors. Using 90°C ampacities with terminations not designed for it can lead to overheating, insulation degradation, and potentially a fire hazard.

Your Example with 4000A Aluminum Cablebus: Analysis

Your example is valid if the following conditions are strictly met: The Aluminum Lugs are Listed and Marked for 90°C at the termination point

This is the most critical point. The lugs themselves must have the 90°C rating clearly marked. You need to verify this with the manufacturer's documentation and markings on the lugs themselves. A general "90°C insulation rating" of the cable is not sufficient.

Cable Ampacity

If the above conditions are met, then you can size the conductors in the cablebus using Table 310.17 (free air, 90°C column). As you said, 8 sets of 750 kcmil aluminum conductors provide 8 700A = 5600A.

Lug Ampacity

The lugs would need to be sized for the full 4000A load. If the lugs have a wire range of 750 kcmil to 1000 kcmil, you would use 1000 kcmil in your ampacity calculation for the lugs (even though the cable is smaller). If you used 8 sets of conductors in the cablebus, then each lug needs to handle 500A (8 sets of conductors with 500A per conductor set = 4000A). If the termination point is rated for 90C, then 1000kcmil provides 590A. If the equipment termination point is not rated for 90C, then 1000kcmil provides 545A.

Why the Seeming Inconsistency?

The code prioritizes safety. While you might have components rated for 90°C, the weakest link (often the termination) dictates the overall system temperature rating.

In summary: Cablebus conductors can be sized per 310.17 (free air). 110.14(C) is a general rule. The "unless" clause in 110.14(C) allows you to use 90°C ampacities ONLY IF the equipment terminations are listed and marked for 90°C. Always consult the manufacturer's documentation for all components to verify their temperature ratings.* Err on the side of caution. If there's any doubt, use the 75°C ampacity. Flag for review