You are correct, your interpretation of the code is accurate, and the design as described is likely a violation of NEC Article 450.3(B). Let's break down why

Understanding NEC 450.3(B) - Transformer Secondary Overcurrent Protection
General Rule

Table 450.3(B) dictates the maximum rating or setting of overcurrent protection devices (OCPDs) for the secondary side of transformers. For a transformer over 600V primary (which your 208V-480V transformer does not have) and using only circuit breakers, the maximum secondary OCPD rating is typically 125% of the transformer's rated secondary current.

Multiple OCPDs (Taps or Feeders)

Note 1 to 450.3(B) addresses situations where you have multiple secondary OCPDs (like your 300A, 400A, and 800A breakers). The key provision is that the sum of the OCPD ratings cannot exceed the maximum allowable single OCPD rating. This is to prevent overloading the transformer itself.

Your Calculations & Analysis1. Transformer Secondary Current

750 kVA / (480 V √3) = 902.08 Amps2.

Maximum Allowable Single OCPD (125%)

902.08 Amps 1.25 = 1127.6 Amps3.

Rounding Up

Typically, you can round up to the next standard OCPD size. In this case, you are using 1200A.4.

Sum of Multiple OCPDs

300A + 400A + 800A = 1500A5.

The Problem

1500A (total OCPD rating) > 1127.6A (or 1200A using standard OCPD sizes)

Conclusion

Your assessment is accurate. The sum of the OCPD ratings (1500A) exceeds the maximum allowable OCPD rating (1200A). This configuration does not adequately protect the transformer from overcurrent conditions, even if the individual branch circuits being served by the breakers are properly protected. The DOB "approval" does not supersede code requirements.

Why is this important? Transformer Overload

If the loads connected to the 300A, 400A, and 800A breakers collectively draw more than 902 Amps (or whatever the transformer is actually rated for after derating for environment/ventilation, if applicable), the transformer can be overloaded.

Transformer Damage/Failure

Repeated or prolonged overloading can damage the transformer's insulation, reduce its lifespan, or even lead to catastrophic failure.

Fire Hazard

Overheating transformers pose a significant fire hazard.

Possible Solutions1. Reduce the Size of OCPDs

The most straightforward solution is to reduce the ratings of the secondary OCPDs so that their sum does not exceed the 1200A limit. This might involve redesigning the downstream distribution and load assignments.2.

Increase Transformer Size

If the connected loads truly require 1500A of capacity, you would need to increase the transformer's kVA rating to at least: 1500A / 1.25 = 1200A (Desired secondary Current) 1200A 480V √3 = 998 kVA (Therefore, the next standard size transformer would be 1000 kVA)3.

Coordination Study (Advanced)

A properly engineered coordination study might justify a higher total OCPD rating, but this is highly unlikely in a standard step-up application. Coordination studies are typically used to ensure that the smallest overcurrent device trips first in a fault condition, protecting equipment downstream. However, in this case, you're trying to protect the transformer itself. Even with coordination, the transformer needs to be protected by the equivalent of the primary overcurrent device at 125% of the rated secondary current. This is probably not a viable solution in your case.

Recommendations1. Document Your Findings

Clearly document your calculations and the relevant code sections.2.

Communicate with the EOR (Engineer of Record)

Start by discussing your concerns with the EOR. Point out the code violation and explain why the current design is not safe.3.

Escalate (If Necessary)

If the EOR is unwilling to correct the design, you may need to escalate your concerns to your supervisor, project manager, or the Authority Having Jurisdiction (AHJ). Be prepared to provide your documentation and explain your reasoning clearly.4.

Refuse to Install (If Necessary)

As a responsible electrical professional, you have an obligation to ensure that electrical installations are safe and compliant with code. If the EOR refuses to correct the design and the AHJ is unwilling to enforce the code, you may need to refuse to install the system as designed.

Important Note: I am an AI chatbot and cannot provide legal or professional engineering advice. Always consult with a licensed electrical engineer for critical design decisions and code interpretations. This information is for educational purposes and to help you understand the relevant code concepts. Flag for review