You're asking a very important and nuanced question about SCCR calculations under UL 508A. Here's a breakdown of the principles and how they apply to your situation

General Principles of SCCR Calculation in UL 508A
Overall Panel SCCR

UL 508A aims to ensure that the entire control panel can withstand a specific short-circuit current without creating a hazard. This is often accomplished by choosing components with adequate SCCR ratings and using current limiting protection.

Highest Value Wins

The overall SCCR of the control panel is usually dictated by the lowest SCCR of any component in the main power circuit, unless those components are effectively protected by a current-limiting protective device.

Component SCCR Responsibility

Each component manufacturer is responsible for determining and marking the SCCR of their device. This is often determined through testing per UL 508A.

Your Specific Scenario Breakdown


Primary Side of the Control Transformer (480V)

The 10A Class CC fuse protects the transformer and potentially downstream components. The SCCR of this primary circuit definitely factors into the overall panel SCCR calculation. The SCCR of the transformer and the downstream components from the transformer must be greater than the let through current of the 10A Class CC fuse.2.

Secondary Side of the Control Transformer (120V)

This is where it gets more complex. Here are the key considerations:

Transformer SCCR Contribution

The transformer itself must have an SCCR rating. This rating should be obtained from the transformer manufacturer.

Secondary Protection and Limitation

The 25A Class CC fuse on the secondary side provides a level of conditional SCCR protection. If the 25A fuse is truly current-limiting (and this must be verified based on the specific fuse's let-through current characteristics - consult the fuse manufacturer's data), it can potentially raise the effective SCCR of components downstream of it as long as: The downstream components are appropriately rated for the let through current of the 25A fuse. This let-through current is usually much lower than the interrupting rating of the fuse. The 25A fuse is suitable to protect these lower rated components.

Components with LOW SCCR (Convenience Outlet)

The convenience outlet with the 3A breaker having a 0.2kA SCCR is the critical point. Since it is in the circuit, the whole circuit SCCR is also limited to that amount unless you can apply a higher SCCR based on current limitation.

PSU

The 15A Class CC fuse is the same story as the 25A fuse. If it's truly current limiting, you might be able to utilize its limiting characteristics to increase the effective SCCR of components after the fuse.

Does UL Take Secondary Side SCCR into Account?Yes, absolutely. While the primary side protection is critical, the SCCR of components on the secondary side of the control transformer definitely factors into the overall panel SCCR calculation.

How to Calculate the Overall SCCR


Document Everything

Gather the SCCR ratings for all components in the power circuit, including: Main disconnect Incoming terminals/conductors Control Transformer (both primary and secondary ratings) Fuses (Type, Rating, and current-limiting curves/data) Branch circuit breakers PSU Convenience outlet Any other component carrying power.2.

Start from the Source

Begin with the available fault current at the panel's incoming terminals.3.

Evaluate Current Limiting

For each circuit protected by a fuse, determine the peak let-through current (Ipk) of the fuse at the available fault current. This information is critical and is found in the fuse manufacturer's documentation (time-current curves and let-through charts). If the Ipk of the fuse is lower than the SCCR of a downstream component, the fuse can potentially increase the effective SCCR of that component to a value no higher than the fuse's Ipk.4.

Consider Cascading/Series Ratings (With Caution)

Don't rely on cascading without thorough verification. UL 508A does not generally allow for series ratings or cascading of breakers or fuses in the same branch to increase SCCR. You need specific testing and listings to apply a series rating.5.

Identify the Weakest Link

The overall panel SCCR will be the lowest SCCR value of any component not adequately protected by a current-limiting device.

Specific Recommendations for Your Case



Convenience Outlet

The 0.2kA SCCR of the convenience outlet is almost certainly your limiting factor. You must address this. Here are possible solutions:

Replace the Outlet

Find an outlet that meets the SCCR you need.

Isolation Transformer

Add a small, separate isolation transformer to supply the convenience outlet. Size the transformer and OCPD to meet the load of the outlet.

Remove Outlet

If you do not need the convenience outlet then remove it.

Fuse Let-Through

Get the let-through current (Ipk) curves for your 10A, 25A, and 15A Class CC fuses. This is essential for determining if they provide any SCCR enhancement to downstream components.

Professional Consultation

Given the complexity, it's highly recommended to consult with a qualified electrical engineer experienced in UL 508A and SCCR calculations. They can provide a definitive SCCR calculation and ensure your panel meets all requirements.

Important Cautions



Assumptions are Dangerous

Don't assume a fuse is current-limiting. Verify with the manufacturer's data.

Component Changes

Any change in components after an SCCR evaluation necessitates a re-evaluation of the overall panel SCCR.

Field Modifications: Any field modifications that affect the power circuit require a re-evaluation of the SCCR.In summary, SCCR is a critical safety consideration, and you must account for the SCCR of all components in the power circuit, including those on the secondary side of the control transformer. The convenience outlet with the low SCCR is likely your limiting factor, and you need to address it. Using current-limiting fuses can potentially help, but you must* verify their let-through characteristics. Consulting with an expert is highly advisable. Flag for review