You're on the right track, but let's refine the approach and consider a few factors

Overall Assessment:

Your calculations and initial choices are generally sound, particularly regarding the primary side. However, the secondary side might benefit from a different type of overcurrent protection (OCP) device.

Let's break it down, focusing on the specific application (control transformer)

1.

Primary Side (480V)



Your Calculation

As you pointed out, 1000VA / 480V = 2.08A (nominal primary current).

UL508A Table 35.2

You correctly identified the multiplier of 2.5 for a control transformer primary. 2.08A 2.5 = 5.2A

Rounding Up

Rounding up to 6A is a reasonable and safe practice.

Class D MCB

A Class D MCB is a suitable choice for the primary. Class D breakers are designed to handle the inrush current associated with transformers. They trip faster than Type K but slower than Type B and C, so it is a good middle of the road breaker. If nuisance tripping becomes a problem, you could try a Class K breaker.

Conclusion

A 6A Class D MCB for the primary is likely a good selection.2.

Secondary Side (120V)



Your Calculation

1000VA / 120V = 8.33A (nominal secondary current).

UL508A Table 35.2

Again, correctly using the table. 8.33A 1.67 = 13.91A

Rounding Up

You rounded to 15A.

Class D MCB or Fuses?

This is where you should carefully consider the load connected to the 120V secondary.

Fuses (Recommended)

For the secondary side of a control transformer, fuses are often the preferred method of protection. This is because fuses provide better current limiting and can clear faults faster than MCBs, which can be critical for protecting sensitive control circuits and preventing damage to connected devices. Specifically, fast-acting fuses are often recommended.

MCB (if used)

If you're set on using an MCB, a Class D may work, but keep in mind the potential for nuisance tripping, especially if you have loads with high inrush currents on the secondary. Furthermore, the current-limiting characteristics of an MCB are generally not as good as a fuse.

Consider the Load



Type of Loads

What are you powering with the 120V? Relays? Solenoids? Control circuits? Each has different characteristics.

Inrush Current

What is the inrush current of those loads? Solenoids and some motors have significant inrush.

Coordination

How well does the secondary OCP coordinate with the branch circuit protection (if any) for the devices connected to the 120V?

Conclusion

A 15A fuse (fast-acting) is likely a better choice for the secondary. If using an MCB, a Class D might be acceptable, but you'll need to monitor for nuisance tripping.

Key Considerations and Recommendations



Fuses vs. MCBs for Secondary

Seriously consider using fuses, especially fast-acting fuses, on the secondary side of the control transformer. The improved current limiting and faster clearing times can provide better protection for the connected control circuits and devices.

Load Analysis

Carefully analyze the load connected to the 120V secondary. This will influence your final OCP choice. Understand inrush currents and potential fault conditions.

Coordination

Ensure that the primary and secondary OCP are coordinated. You want the secondary OCP to trip first in the event of a secondary-side fault.

UL 508A Requirements

Double-check all relevant UL 508A requirements for control transformers and overcurrent protection. Consult the latest edition.

Manufacturer's Data

Refer to the transformer manufacturer's documentation for specific recommendations on overcurrent protection.

Example: Fuse Selection

If you choose fuses for the secondary, consider using 15A, fast-acting fuses. Look for fuses specifically designed for control transformer protection. Manufacturers like Bussmann, Littelfuse, and Mersen offer these. The fuse's interrupting rating must be suitable for the available fault current.

Important Safety Note: Working with electrical systems can be dangerous. Always follow proper safety procedures and consult with a qualified electrician if you are not experienced. De-energize equipment before working on it, and use appropriate personal protective equipment (PPE).In summary, while your calculations are generally correct, the key is to prioritize safety and optimal protection of the control circuits and devices connected to the secondary. Fuses, particularly fast-acting fuses, are often a better choice than MCBs for the secondary side of a control transformer. Good luck with your project! Flag for review