Okay, let's break down the OCPD (Overcurrent Protection Device) sizing for your transformer. You are on the right track! Here's the breakdown, incorporating NEC guidelines (if applicable to your location) and best practices

1. Primary Side OCPD (120/208V):
Transformer Primary Current

You've correctly calculated this: 30,000 VA / (208V 1.732) = 83.3 Amps

NEC (National Electrical Code) Rules (Assuming US based) This is critical, as local codes may differ. NEC 450.3(B) typically dictates overcurrent protection for transformers. It has two main options, depending on how your setup is designed and what the authority having jurisdiction (AHJ) will allow: Option 1: Primary Only Protection

You can protect the transformer with only a primary side OCPD. In this case, the maximum OCPD size is 125% of the primary current rating for a transformer with a primary current greater than 9 amps. 83.3 Amps 1.25 = 104.13 Amps.

Therefore, the next standard size up, and not exceeding 125%, is a 100 amp breaker. Option 2: Primary and Secondary Protection

You can also have a primary OCPD that is greater than 125% provided you have secondary protection. This option is used when the inrush current of the transformer trips a lower rated breaker. In this case the maximum primary OCPD size is 250% of the primary current rating. 83.3 amps 2.50 = 208.25

Therefore, the next standard size down, and not exceeding 250%, is a 200 amp breaker.2. Secondary Side OCPD (230/400V)



Transformer Secondary Current

You have the required table saw load, but we need to verify this current against the transformers rated output. 30,000 VA / (400V 1.732) = 43.3 Amps

Load Considerations

The table saw has a load of 25 amps, as you stated. However, the OCPD needs to protect the transformer and the conductors.

NEC Rules (again, assuming US)

The OCPD must be rated such that is does not exceed 125% of the transformers rated secondary current, and sized to carry the intended load. 43.3 Amps 1.25 = 54.13 Amps

Therefore, you could use a 50 or 55 amp breaker as it exceeds the 25 amps of the table saw, and does not exceed 125% of the transformers rated output.3. Important Considerations and Summary



Consult Local Codes



This is absolutely essential! Your local electrical codes and AHJ may have specific requirements that supersede or modify the NEC. Always check with your local electrical inspector. Conductor Ampacity

Ensure that the conductors on both the primary and secondary sides are properly sized for the calculated currents and are rated for the OCPD protecting them. The conductor's ampacity (current-carrying capacity) must be equal to or greater than the OCPD rating.

Transformer Nameplate

Always refer to the transformer's nameplate for specific voltage, current, and impedance information. This is crucial for accurate calculations.

Breaker Type

Consider using inverse time circuit breakers. These are designed to handle short-duration inrush currents (like a transformer energizing) without nuisance tripping.

In summary, with Option 1, based on the information provided



Primary OCPD

100 Amp Breaker

Secondary OCPD

50 or 55 Amp Breaker

With Option 2, based on the information provided



Primary OCPD

200 Amp Breaker

Secondary OCPD

50 or 55 Amp BreakerREMEMBER: This is an interpretation based on the information provided and general electrical principles. It is NOT a substitute for professional electrical engineering advice or compliance with local codes.

You must consult with a qualified electrician in your area to ensure that your installation is safe and code-compliant. Flag for review