You've correctly interpreted the intent of the NEC regarding transformer overcurrent protection (OCP). Let's break it down: 450.3 informational note on Secondary OCP

This note is clear: secondary OCP is specifically designed to protect the transformer itself from overload and fault conditions within the transformer. It's not inherently designed to protect the secondary conductors connected to the transformer.

Primary OCP Implication

While the informational note explicitly addresses secondary OCP, the overall context of 450.3 and the paragraph that follows implies the same principle applies to primary OCP. The code is focused on ensuring the transformer's internal safety.

Key Paragraph that Follows the Informational Note

This is the crucial part! The Code explicitly states the OCPD is not designed to necessarily protect the conductors, and therefore the designer must verify proper protection of the conductors as well.

Therefore, you are correct in your understanding



Primary OCP is primarily for transformer protection

The main function of primary OCP is to protect the transformer from overloads and faults originating within the transformer itself (or reflected from the secondary side).

Conductor Protection is a Separate Consideration

The NEC emphasizes that the primary and secondary OCP may not* adequately protect the primary and secondary conductors connected to the transformer. Therefore, the installer/designer must separately ensure that the conductors are properly protected by their own overcurrent devices (fuses or circuit breakers) that are sized according to the conductor ampacity and the applicable rules in Article 240.

In summary

Think of transformer OCP as "internal" protection for the transformer itself. Conductor protection is "external" protection for the wiring connected to the transformer, and must be verified independently. This means you need to:1.

Calculate Transformer OCP

Size the primary OCP according to 450.3(B) and the secondary OCP according to 450.3(C). These sections provide rules that relate the OCP rating to the transformer's kVA rating and voltage.
2.

Calculate Conductor Ampacity

Determine the required ampacity of both the primary and secondary conductors based on the transformer's kVA and voltage.3.

Choose Conductor Size

Select conductor sizes based on the calculated ampacity, considering derating factors (temperature, number of conductors in a raceway, etc.).4.

Verify Conductor OCP: Ensure that the overcurrent devices protecting the primary and secondary conductors are sized to protect the conductors, according to Article 240. Often, the transformer OCP sized for the transformer itself is larger than what's needed to protect the conductors, requiring additional OCP devices.By following this process, you ensure both the transformer and its connected conductors are adequately protected from overcurrent and fault conditions. Flag for review