You're on the right track. Let's break down the code considerations and rationale

Your Understanding and Concerns
Correct Primary Current Calculation

A 75 kVA, 480V, 3-phase transformer has a full-load primary current of approximately 90.2 Amps (75,000 VA / (480V √3)).

Oversized Disconnect

You're right to question the necessity of a 400A disconnect. It appears significantly oversized for the transformer's primary current.

Tap Rule Concerns

Tapping the 500 kcmil conductor down to a smaller 3 AWG raises concerns about meeting NEC tap rules.

NEC Considerations and Analysis1. Transformer Protection (Article 450) Primary Overcurrent Protection (450.3(B)): The primary side of the transformer needs overcurrent protection to protect the transformer itself. For transformers with a primary current of 9 amps or more, the overcurrent protection device must be rated at no more than 125% of the transformer's primary full-load current. In your case: 1.25 90.2A = 112.75A. So a 110A or 100A breaker would likely be the choice. Secondary Overcurrent Protection (450.3(B)): This is required when the primary overcurrent protection doesn't meet the requirements. Your secondary is protected by a 200A disconnect, which is fine. Transformer Size and Protection: Your setup is a common configuration, and Article 450 provides rules for proper protection.2. Feeder Tap Rules (240.21) General Requirements

The feeder tap rules permit you to tap conductors from a larger feeder to supply a smaller load, under specific conditions. These are designed to protect the smaller conductors from overcurrent.

Specific Tap Rule to Consider (240.21(B)(2) - 10-Foot Tap Rule This is potentially applicable to your scenario if the total length of the tap conductors does not exceed 10 ft and it is enclosed in a raceway. The ampacity of the tap conductors (3 AWG) must be: Not less than the calculated load of the transformer primary (90.2 A). 3 AWG copper is typically rated for 100A. Not less than the rating of the overcurrent device (the 100A breaker you proposed). More Complex Tap Rules

Longer tap lengths (e.g., 25-foot tap rule) exist but have more stringent requirements. It is unlikely that the 25ft tap would apply to this temporary situation.3.

Disconnecting Means (Article 430, Article 440, Article 450) Purpose

Disconnects provide a means to safely de-energize equipment for maintenance or in emergency situations.

Sizing

The disconnect needs to have an ampacity not less than the load it serves.

Your Situation

While a 400A disconnect would "work" (in that it wouldn't trip), it's clearly overkill and adds unnecessary cost. A disconnect rated for 100A would meet the requirements here, provided the transformer primary is protected by a breaker of that size or smaller.

Your Proposed Solution (Tapping and 100A Protection)



Feasibility

Yes, this can be done, but you must strictly adhere to the tap rules in 240.21. If you can meet the conditions of the 10-foot tap rule, this is a viable solution.

Important Considerations



Tap Length

Measure the actual tap length carefully. It's easy to underestimate.

Enclosure

The tap conductors MUST be installed in a raceway

Connections

Use approved connectors for tapping the 500 kcmil conductor. Properly torque all connections.

Overcurrent Protection

Ensure the 100A breaker is the correct type for the application (e.g., inverse time circuit breaker) and has an adequate interrupting rating.

Recommendations1. Simplified Approach (and possibly the best for a temporary setup)

Install a 100A circuit breaker in the existing 350A MCCB cubicle. This provides the code-required primary protection for the transformer. You may need a bucket replacement to accommodate the lower amperage breaker. This depends on the panel design. Run conductors (sized appropriately for 100A – 3 AWG would work) from the new 100A breaker to a 100A disconnect switch. The disconnect needs to be readily accessible.2.

If tapping is absolutely necessary

Strictly follow the 10-foot tap rule (240.21(B)(1)). Install a 100A breaker at the load end of the tap. Use proper tapping connectors for the 500kcmil. Document and label everything clearly.

Why the 400A Disconnect is Unnecessary (and Potentially Problematic) Cost

It's significantly more expensive.

Misleading

It could give a false impression of the actual load size. Someone could assume the circuit can handle much more current than it actually can, leading to overloads.

Code Issues

While not strictly a violation, it creates a system that doesn't clearly reflect the design. The NEC generally prefers systems where components are sized appropriately for the loads they serve.

Communication with the Project Manager

Explain to the PM: The Code Requirements

Show them the relevant sections of the NEC regarding transformer protection and tap rules.

The Cost Savings

Explain the significant cost difference between a 400A disconnect and a smaller, code-compliant solution.*

Safety and Clarity

Emphasize that properly sized components are safer and more clearly reflect the actual electrical load.

In summary, your intuition is correct. A 400A disconnect is almost certainly unnecessary. You can achieve a safe and code-compliant installation using a smaller disconnect and overcurrent protection sized appropriately for the transformer's primary current. Double-check all calculations, ensure you strictly adhere to the NEC, and document your work. Flag for review