You're on the right track, but let's clarify a few points about control transformer protection, especially according to the National Electrical Code (NEC) in the US, as that's the most common reference. Also, always consult your local codes and regulations, as they take precedence.

Understanding Overload Protection

Overload protection protects the transformer from sustained currents exceeding its rating. This is critical because prolonged overloading will cause the transformer to overheat and eventually fail. Short-circuit protection, on the other hand, protects the transformer from fault currents due to short circuits, which are significantly higher.

NEC and Control Transformer Protection

The NEC (specifically Article 450) outlines the requirements for transformer protection. Here's a simplified breakdown: Primary Protection

Your understanding of 300% is partially correct. NEC 450.3(B) (2023) allows for primary-only protection up to 300% of the transformer's rated primary current if the primary overcurrent device also protects the secondary. However, this ONLY applies under specific conditions where meeting the tighter requirements will cause nuisance tripping.

Secondary Protection

You're correct that secondary protection up to 250% is permitted under certain circumstances. Specifically, if the transformer primary is protected at 125% of its rated current or less, then secondary protection is not mandatory. But, if the primary protection is set higher than 125%, then secondary protection is typically required.

The General Rule

The general rule is that transformers should be protected against overload at no more than 125% of their rated primary current. This applies to both primary and secondary protection. The allowances of up to 300% and 250% are in place to avoid nuisance tripping under typical applications.

Calculating Transformer Currents

First, you need to calculate the rated primary current of your transformer: VA = Volts x Amps so Amps = VA / Volts Primary Current = 50 VA / 208 V = 0.24 Amps (approximately)Applying the Rules to Your Transformer1. Primary Protection at 125%

125% of 0.24 Amps = 0.3 Amps Ideally, you'd want a fuse or circuit breaker rated as close to 0.3 Amps as possible without nuisance tripping. This might mean using a 1/2 Amp fuse, or even a 1 Amp if nothing smaller is available, and if it proves reliable.2.

Primary Protection above 125% (Up to 300%)

300% of 0.24 Amps = 0.72 Amps. However, if you use an overcurrent device on the primary larger than 125% (say, a 1 Amp fuse), you MUST ensure that the secondary side is also protected.3.

Secondary Protection (Required if Primary is > 125%)

First, calculate the secondary voltage. You'll need the nameplate information on the secondary side (e.g., 120V, 24V, etc.). Let's assume it's a 120V secondary for this example. Secondary Current = 50 VA / 120 V = 0.42 Amps (approximately) 250% of 0.42 Amps = 1.05 Amps In this scenario, you would protect the secondary with an overcurrent device (fuse or circuit breaker) rated at or below 1.05 Amps. A standard 1 Amp fuse would likely be used.

So, What Protects from Overloading? The overcurrent protection devices (fuses or circuit breakers) on either the primary, the secondary, or both, protect the transformer from overloading. If the current draw exceeds the rating of these devices, they will trip (or blow), disconnecting power to the transformer.

Important Considerations



Inrush Current

Transformers have a high inrush current when first energized. This is a temporary surge of current that can be several times higher than the normal operating current. This is why the NEC allows for overcurrent protection exceeding 125% under specific circumstances, to avoid nuisance tripping from the inrush. Time-delay fuses or slow-blow circuit breakers are often used to handle inrush current.

Feeder Conductor Protection

Don't forget to ensure that the conductors feeding the transformer primary are also appropriately sized and protected, and are rated higher than the overcurrent device used for the transformer's primary.

Application

Consider the load connected to the secondary of the transformer. Is it a motor, a resistive load, or electronic equipment? The type of load affects the appropriate type and rating of the overcurrent protection devices.

Example Scenario and Recommendation

Let's say you want the simplest and most straightforward approach, adhering strictly to the 125% rule:1.

Primary

Use a fuse rated no higher than 0.3 Amps, as discussed. Given the difficulty in finding this rating, a 1/2 amp fuse might be used in practice.
2.

Secondary

If you're using a common 120V secondary, you would protect it with a fuse rated no higher than 125% of the 0.42A current, or 0.525A. Again, a 1/2 amp fuse might be the most practical solution if this transformer has a high inrush currentThis configuration (125% or less protection on the primary and secondary) will provide the best protection against both overloads and short circuits. If you select higher protection, you risk premature failure of the transformer due to overload.

Disclaimer: This information is for general guidance only and should not be considered a substitute for professional electrical engineering advice. Always consult with a qualified electrician or engineer to ensure that your electrical installations comply with all applicable codes and regulations.* Double check the VA rating, primary and secondary voltage of your exact model before selecting protection. Flag for review