To determine whether a 100A or 125A breaker is required for the primary side of a transformer, we need to know the secondary voltage and current. We also need to understand the continuous load. Here's a breakdown of why a 125A breaker might be needed, and the factors involved

Why a 125A breaker might be required:

The question highlights the issue of "continuous load." Code (like the NEC) mandates that overcurrent protection (breakers and fuses) be sized to handle at least 125% of the continuous load. A continuous load is defined as a load that is expected to operate at its maximum current for 3 hours or more. 125% Rule

If the load is continuous, the breaker needs to be sized to at least 125% of the load current. This is to prevent nuisance tripping.

What Information is Needed to Determine the Correct Breaker Size?1. Transformer kVA Rating

We must know the kVA (kilovolt-amperes) rating of the transformer. This tells us the transformer's capacity.2.

Secondary Voltage and Current

If you know the secondary voltage and current, you can calculate the kVA.3.

Continuous Load

Is the load on the secondary side considered continuous? If so, the 125% rule applies.4.

Transformer Impedance (Z)

The transformer impedance is important for calculating fault current.

Let's illustrate with examples



Example 1: Calculating Primary Current from kVA Assume a 30 kVA transformer with a 277V primary. Primary Current (I_primary) = kVA 1000 / (Primary Voltage √3)Note: We use √3 when we have a three-phase system. We don't know at this point if it is three-phase or single-phase. I'll assume single-phase for the sake of the calculation.I_primary = 30 1000 / 277 = 108.3 A If the load is NOT continuous

A 100A breaker would be too small. You would need a breaker rated for at least 110A (to cover 108.3A). You would likely need a 125A breaker, because breakers are only available in standard sizes.

If the load IS continuous

The minimum breaker size becomes: 108.3 A 1.25 = 135.4 A In this case, you would need a 150A breaker (as 135A is not a standard breaker size).

Example 2: A Smaller Transformer Assume a 25 kVA transformer with a 277V primary. I_primary = 25 1000 / 277 = 90.3 A If the load is NOT continuous

A 100A breaker might be acceptable, but right on the margin.

If the load IS continuous

90.3A 1.25 = 112.9 A. A 125A breaker would be required.

Why the 280V Primary?

The slightly higher primary voltage of 280V (instead of 277V) will result in slightly lower primary current. However, this small difference usually isn't significant enough to change the required breaker size.

Important Considerations



NEC (National Electrical Code)

All electrical installations must comply with the NEC (or your local electrical code). The NEC has specific requirements for transformer protection, including sizing of overcurrent devices.

Transformer Nameplate

The transformer's nameplate provides critical information, including the kVA rating, primary and secondary voltages, and impedance. Always refer to the nameplate.

Inrush Current

Transformers have a high inrush current when initially energized. The breaker must be able to withstand this inrush without tripping. This is another reason why the 125% rule (or higher) is often applied. Some breakers are specifically designed to handle transformer inrush.*

Coordination

Overcurrent protection should be coordinated. This means that the breaker closest to a fault should trip first, minimizing the disruption to the rest of the system.

In summary:

Without knowing the transformer's kVA rating and whether the load is continuous, it's impossible to definitively say whether a 100A or 125A breaker is correct. The 125% rule for continuous loads is a critical factor. You need to perform the calculations based on the actual transformer parameters and the nature of the load. Consulting with a qualified electrician is highly recommended. They can perform the necessary calculations, consider all relevant code requirements, and ensure the installation is safe and compliant. Flag for review