Let's break down your situation and address the requirements of 225.35.

Understanding the Requirements 225.35: Ungrounded Conductors Brought to a Building or Other Structure

This section of the NEC specifically deals with situations where you're extending power (in your case, 480V) from one building to another. It emphasizes the requirements for a disconnecting means (your main breaker) at the receiving building.

"Suitable for Service" Disconnecting Means

The key phrase is "suitable for service." This means the disconnecting means (the main breaker) must be rated and configured to safely disconnect all ungrounded conductors (the hots) supplying the building or structure. It implies more than just interrupting current; it means being robust enough to handle the voltage, available fault current, and environmental conditions at the service entrance.

Neutral is NOT Required for 480V, 3-Phase Loads

The important point here is that you're dealing with 480V equipment that does not require a neutral. This is typical for many industrial motors, heaters, and other loads that operate directly on the 480V 3-phase voltage.

Does the Main Breaker Need a Neutral Connection? No. Because your plumbing equipment does not need a neutral and the system is derived from a 480V wye configuration, the main breaker in your 480V distribution panel does not need a neutral connection. The requirement in 225.35 for a "suitable for service" disconnect is simply to have a disconnect that is rated and intended to be the main disconnect for the building or structure.

Important Considerations and Code Requirements1. Feeder

The feeder conductors to the 480V distribution panel must be sized appropriately for the load and comply with NEC Article 215. Consider voltage drop calculations.
2.

Overcurrent Protection

The overcurrent protection (OCPD) in the supplying panel (277/480V SES) must be sized to protect the feeder conductors to your 480V distribution panel. NEC Article 240 covers this.3.

Grounding Electrode System (GES)

You must establish a proper grounding electrode system (GES) at the new 480V panel's location. This is crucial for safety and fault current protection. Refer to NEC Article 250, particularly Parts III and IV. This involves bonding the equipment grounding conductors to a grounding electrode (e.g., a ground rod, building steel, or concrete-encased electrode).4.

Equipment Grounding Conductors (EGCs)

All metal enclosures, raceways, and non-current-carrying metal parts of the equipment connected to the 480V system must be properly grounded via an equipment grounding conductor (EGC). Size the EGCs according to NEC 250.122.
5.

Bonding

Bonding is essential. Bond the neutral conductor at the source (the 277/480V SES panel) to the grounding electrode system at that location. In your 480V distribution panel, do not* bond the neutral (since you don't have one). Instead, bond the equipment grounding bus to the enclosure.6.

Wiring Methods

Choose appropriate wiring methods (e.g., rigid metal conduit, EMT, cable) suitable for the environment and voltage. Comply with NEC Chapter 3.7.

Disconnecting Means (Individual Equipment)

Each piece of plumbing equipment must have its own disconnecting means (e.g., a local disconnect switch) within sight of the equipment.8.

Arc Flash Hazard

Perform an arc flash hazard analysis to determine the appropriate personal protective equipment (PPE) requirements for personnel working on or near the 480V equipment. Follow NFPA 70E guidelines.9.

Labeling

Properly label all panels, disconnects, and equipment with their voltage and applicable warnings.

In summary: The main breaker in your 480V panel only needs to switch the ungrounded conductors. Because you're dealing with a 480V, 3-phase system that doesn't need a neutral, the main breaker does not need a neutral connection. However, you must ensure proper grounding and bonding of all equipment and enclosures, establish a grounding electrode system, and comply with all other relevant NEC requirements. Finally, ensure you get the design reviewed and signed off by a qualified electrical engineer. Flag for review