You're asking a good question about transformer protection, and it's important to get this right for safety and code compliance. Here's a breakdown of how to approach this, along with the relevant considerations

Key Considerations:
Transformer Protection

The primary goal of primary overcurrent protection on a transformer is to protect the transformer itself from overloads and faults on the secondary side, and from internal transformer faults. It is NOT primarily sized to protect the load on the secondary side.

Load Protection

The secondary overcurrent protection is sized to protect the load equipment.

NEC (National Electrical Code)

The NEC is the standard electrical code in the US, and it has specific requirements for transformer protection. Other countries have similar codes that must be followed. My advice below is generally based on the NEC, but you must consult your local electrical code for compliance.

Coordination

The primary and secondary protection devices should be coordinated to ensure that a fault on the secondary side trips the secondary device first, and only the primary device if the secondary device fails to clear the fault. This minimizes unnecessary outages to other loads served by the transformer.

Step-by-Step Approach


Transformer Primary Overcurrent Protection



Calculate Transformer Primary Current

KVA = (Volts x Amps x √3) / 1000 Amps = (KVA x 1000) / (Volts x √3) Primary Amps = (45 KVA x 1000) / (480 V x √3) = 54.1 Amps (approx)

NEC Guidelines (General - consult local code)

NEC Article 450 covers transformers. Typically, the NEC allows you to protect the primary side of a transformer at:

Up to 125% of the transformer's rated primary current if using a circuit breaker. Up to 250% of the transformer's rated primary current if using a fuse (under specific conditions). Applying the Guidelines



Breaker

54.1 Amps x 1.25 = 67.625 Amps. You would typically select the next higher standard breaker size, which would likely be a

70 Amp breaker. Fuse

54.1 Amps x 2.50 = 135.25 Amps. This is significantly higher, but permissible under specific conditions outlined in the NEC. It's less common, and requires careful coordination.

Important

This is a maximum allowed value. You can use a lower value if desired, and this is often a good idea to provide better protection. The goal is to choose a value that allows the transformer to handle its rated load without nuisance tripping, while still protecting against overloads and faults.2.

Transformer Secondary Overcurrent Protection (for the Load)



Calculate Load Current

KW = (Volts x Amps x √3) / 1000 Amps = (KW x 1000) / (Volts x √3) Load Amps = (33 KW x 1000) / (400 V x √3) = 47.6 Amps (approx)

Size for 125% of Load

47.6 Amps x 1.25 = 59.5 Amps. You'd likely use a

60 Amp breaker or fuse.3. Coordination



Secondary Device MUST Trip First

The 60A secondary breaker/fuse should trip before the 70A (or higher if using a fuse) primary breaker/fuse. This is essential for isolating faults to the specific load and minimizing downtime.

Time-Current Curves

To properly coordinate, you ideally need to consult the time-current curves (TCCs) for the specific breakers/fuses you are using. These curves show how long it takes for a device to trip at various levels of overcurrent. The secondary device's TCC should lie below the primary device's TCC.

Why NOT Size Primary Based on Load Directly

You can't directly size the primary overcurrent protection based on the 33kW load alone. Here's why:

Transformer Capacity

The transformer has a 45 KVA capacity. It could theoretically supply more than 33 kW. If you sized the primary protection only for the 33kW load, you wouldn't be protecting the transformer if you were to connect a bigger load to the secondary in the future.

Inrush Current

Transformers experience a very high inrush current when initially energized (typically 8-12 times the rated current for a very short duration). Sizing the primary protection too tightly based on the load would likely cause nuisance tripping due to inrush.

Example Summary



Primary Overcurrent Protection



70 Amp circuit breaker (based on 125% of transformer primary current). A fuse could be used at a higher rating, but coordination becomes more critical. Secondary Overcurrent Protection



60 Amp circuit breaker (based on 125% of load current).

Important Disclaimers



Consult a Qualified Electrician

This information is for general guidance only. You must consult with a licensed and qualified electrician for the actual installation and sizing of overcurrent protection. They will be familiar with local codes and can ensure a safe and compliant installation.

NEC (or Local Code) is Paramount

The NEC (or your local electrical code) is the final authority. Refer to it for the definitive requirements.

Transformer Manufacturer's Recommendations

The transformer manufacturer may have specific recommendations for overcurrent protection that should be followed.

Coordination Study: For larger systems or critical applications, a formal coordination study may be necessary to ensure proper selective coordination of overcurrent devices.In summary, protect the transformer based on its rated KVA on the primary, and the load based on its requirements on the secondary, ensuring proper coordination between the two. Always prioritize safety and compliance with local electrical codes. Flag for review