Okay, let's break down your situation and the feasibility of your proposed solutions. You're essentially trying to optimize your cablebus feeder design to minimize conductor size while maintaining compliance with the NEC.

Understanding the Key NEC Sections 110.14(C): Termination Temperature Limitations

This is your primary hurdle. It states that the temperature rating associated with equipment terminals must be used when selecting conductor ampacity. Typically, equipment is marked for 75°C or 60°C, although higher temperature ratings are permitted for some equipment as specified by their marking.

310.15(B)(16) (formerly 310.16): Ampacities of Insulated Conductors Rated 0-2000 Volts

This is the standard ampacity table in the NEC. It's based on conductors installed in raceway or cable.

310.15(B)(17) (formerly 310.17): Ampacities of Single-Conductor Cables in Free Air

This table provides ampacities for single conductors run in free air, which allows for higher ampacities than conductors installed in conduit due to better heat dissipation.

370.80: Ampacity of Cablebus

This section covers the requirements for cablebus, including the ampacity of the conductors it uses. It references the other sections (like 310.15) for determining ampacity.

Analyzing Your Proposed Solution

You're proposing to use a busway-to-cable tap box to avoid the 75°C termination temperature limitations at the switchgear and equipment termination box. By connecting the busway flange directly to the switchgear bus and using a busway cable tap box, you're hoping to effectively extend the busbar and potentially use higher temperature ratings for the cablebus conductors.

Answer to Question 1: Can I size my cablebus with 90 deg C column now instead of 75 deg C? Potentially, yes, BUT with very specific conditions. Your logic of using the 90°C column is partially valid, but it hinges on the entire system being rated and designed for 90°C operation, and documentation to support it. Here’s a breakdown: The Weak Link

The weakest link in your system will dictate the temperature rating. If your equipment termination box is not specifically listed for 90°C terminations, then you're still limited to the temperature rating of the termination box.

Manufacturer's Listing and Instructions

You must have documentation from both the cablebus manufacturer and the equipment manufacturer stating that their products are rated for 90°C operation under the specific installation conditions. The burden of proof is on you to demonstrate that the entire assembly (cablebus, terminations, etc.) meets the 90°C requirements. This includes the insulation temperature rating of the conductors used.

Busway Tap Box

The busway tap box needs to be clearly rated and listed for 90°C operation for the continuous current rating you're planning.

Practical Consideration

Even if technically permissible, using 90°C conductors may not be economical. The increased cost of high-temperature rated conductors might outweigh the savings from reducing the conductor size.

Answer to Question 2: Can I use the 90 deg C column from the free air ampacity table 310.15(B)(17) or still have to use the 310.15(B)(16)? Am I still under temperature limitation per 110.14(C)? Again, Potentially Yes, IF and ONLY IF all conditions of 310.15(B)(17) are met. You are still under the temperature limitations of 110.14(C). Here's why: Free Air Requirements

310.15(B)(17) is specifically for single conductors installed in free air. This means there must be adequate spacing between the conductors to allow for proper heat dissipation. The cablebus system design must demonstrate compliance with the free air requirements, including proper conductor spacing.

Termination Temperature Limitations Still Apply

Even if you can use 310.15(B)(17) for the cablebus conductors themselves, you are still bound by 110.14(C) at the termination points (both at the tap box and at the equipment termination box) unless you can absolutely prove that those connections are rated for 90°C under the conditions of use.

Calculations

You need to perform calculations to ensure that the conductor temperature rise under maximum load will not exceed the conductor's insulation temperature rating and that the terminations are within their rated temperature limits. These calculations must be defensible and based on accepted engineering principles.

In Summary

Your plan is theoretically possible, but it's a high-risk approach due to the stringent requirements and the need for thorough documentation. You will need to show to the inspector that the entire assembly is listed and rated for 90 deg C continuously including the conductors, tap box, busway flange and equipment termination box.

Recommendations1. Verify Equipment Termination Temperature Rating

The absolute first step is to confirm the temperature rating of the terminals in your equipment termination box. This will likely be the limiting factor.2.

Consult with Manufacturers

Contact the cablebus and busway flange/tap box manufacturers to confirm their products are rated for 90°C operation under your specific installation conditions and load requirements. Get written documentation. Also, discuss 4000A+ cablebus solution with multiple manufacturers.3.

Conservative Design

Consider using a larger conductor size based on the 75°C ampacity tables for a simpler, more reliable, and easier-to-inspect installation. The cost savings of reducing conductor size may be negligible compared to the potential headaches of proving compliance with the 90°C requirements. This is especially true if you're not absolutely confident that you can maintain the "free air" conditions for the entire cablebus run.4.

Engineering Analysis

Engage a qualified electrical engineer to perform a detailed analysis of the cablebus system, including ampacity calculations, voltage drop calculations, and a thermal analysis to verify that all components will operate within their rated temperature limits.5.

Local AHJ Consultation

Discuss your proposed design with the local Authority Having Jurisdiction (AHJ) before* starting any installation work. They may have specific interpretations of the NEC that could impact your design.6.

Bus Duct

Consider traditional bus duct as an alternative. While potentially more expensive upfront, it can simplify the design, reduce the number of connections, and provide a more robust and reliable system.

Important Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. This information is for general guidance only. Always consult with a qualified electrical engineer and comply with all applicable codes and regulations. The NEC is subject to interpretation, and the final decision on compliance rests with the AHJ. Flag for review