Okay, let's break this down step by step.1. Buck-Boost Transformer Necessity Yes, a buck-boost transformer is needed. If your appliance requires 230V, and you only have a 120/208V source available, you must use a transformer to increase the voltage. A buck-boost transformer is the typical (and often cost-effective) solution for this type of voltage transformation.2. Transformer Sizing Apparent Power (kVA) is Key

You've correctly identified that you need to size the transformer based on the apparent power (Volt-Amps or VA) of the load, not just the wattage.

2300 Watt Load

Assuming a power factor close to 1, 2300 Watts is approximately 2300 VA. It's highly recommended to confirm the power factor of the appliance. If the power factor is, for example, 0.8, the VA would be 2300 / 0.8 = 2875 VA.

Transformer kVA

A 3 kVA transformer should be sufficient if the 2300W load is close to unity power factor. However, due to safety margins and potential future load increases, it's often good practice to round up to the next standard size (if available and economical) For this example, let's stick with 3kVA.

Consider starting current inrush as well, large motor loads may require 500%-600% of the amperage draw on startup.3. Primary Overcurrent Protection (OCPD) - Correct Calculation and Considerations

Your primary current calculation is mostly correct, but let's refine it: Transformer Primary Current Calculation

Transformer kVA = 3 kVA = 3000 VA Primary Voltage = 208V Primary Current (without safety factors) = 3000 VA / 208V = 14.42 Amps

NEC 450.3(B) for Transformers

The National Electrical Code (NEC) provides specific rules for transformer overcurrent protection. You usually can't just use a 1.25 factor.

Primary Only Protection

This is generally the simplest approach. You protect the primary only, and the secondary is considered protected by the primary OCPD,

but only under specific conditions (see NEC 240.21(C)). NEC Table 450.3(B) allows for a primary OCPD of up to 125% of the primary current for transformers with a rated primary current over 9 amps if the primary OCPD is on the primary side, or 300% or 600% if the secondary is under protection. This assumes you are using a standard impedance transformer that matches NEC code. Since your calculated primary current is 14.42 amps, your primary OCPD could be up to 14.42 1.25 = 18.03 Amps. The next standard size OCPD that you can use is typically a 20 Amp breaker. Important

The goal is to protect the transformer from overcurrent conditions, not necessarily to perfectly match the load current.



Confirm with Local AHJ

Always verify your transformer protection scheme with your local Authority Having Jurisdiction (AHJ) – i.e., the electrical inspector. They have the final say.

4. Secondary Disconnect and Overcurrent ProtectionThis is where things get more nuanced, and the NEC provides multiple options:

NEC 240.21(C) Secondary Conductors

This section of the NEC is critical. It dictates how the secondary conductors can be tapped from the transformer and how they must be protected. There are several scenarios allowed, each with specific rules.

Scenario 1: Secondary Protection at the Transformer

The most straightforward approach is to place the secondary overcurrent protection immediately at the transformer's secondary terminals. This is often the preferred method.

Scenario 2: Secondary Protection at a Distance

The NEC allows for secondary conductors to run a limited distance without overcurrent protection at the transformer, provided certain conditions are met (conductor ampacity, tap rules, etc.). This is usually more complex and requires careful calculations. It's often simpler to protect right at the transformer.

Assuming Secondary Protection at the Transformer



Secondary Voltage

230V

Secondary Current

2300 VA / 230V = 10 Amps

NEC 240.4(D) Small Conductors

If you are using small conductors (e.g., 14 AWG, 12 AWG), the NEC has specific restrictions on overcurrent protection based on conductor ampacity. Make sure your conductor ampacity is sufficient for the calculated secondary current.

Secondary OCPD

The secondary OCPD should be sized to protect the conductors and the appliance. A 15 Amp fuse/breaker might be appropriate, but you must verify the ampacity of the wiring you are using and the appliance's requirements.

Disconnect Switch



Purpose

The disconnect switch provides a means to safely de-energize the appliance for maintenance or repair.

Sizing

The disconnect switch must have an ampacity rating at least equal to 115% of the load current. In this case the calculated load current is 10 amps. 101.15 = 11.5 Amps, so a 15-20 amp disconnect is appropriate.

Putting It All Together: Summary1.

Buck-Boost Transformer

Required to convert 208V to 230V. A 3kVA transformer is most likely appropriate for your 2300W load.2.

Primary OCPD

Likely a 20 Amp breaker (based on 125% of the calculated primary current and NEC Table 450.3(B)).
3.

Secondary Disconnect

Use a 15-20 Amp disconnect switch, rated for the voltage.4.

Secondary OCPD

Place this immediately at the transformer if possible for simplicity. Size it based on the conductor ampacity and the appliance's requirements (likely 15 Amps if using common wire sizes). Ensure that this doesn't exceed the appliances over-current protection device.5.

Verify with AHJ

Crucially, always discuss your transformer protection scheme and conductor sizing with your local electrical inspector (AHJ) for approval.6.

Follow NEC Code

Adhere strictly to all applicable sections of the National Electrical Code (NEC).

Important Considerations

Power Factor

I cannot overstress the importance of knowing your appliance's power factor. A low power factor increases the apparent power (VA) required from the transformer and affects current calculations.

Inrush Current

Some appliances (especially those with motors) have a high inrush current when starting up. This could potentially trip your OCPD. Consider using "time-delay" fuses or "slow blow" circuit breakers if inrush current is a problem. A good rule of thumb is to double the starting wattage compared to the running wattage.

Voltage Drop

Ensure that the wiring from the panel to the transformer and from the transformer to the appliance is sized correctly to minimize voltage drop. Excessive voltage drop can damage the appliance or cause it to malfunction.

Grounding

Proper grounding is essential for safety. Consult the NEC and the transformer's instructions for proper grounding procedures.

Qualified Electrician

All electrical work should be performed by a licensed and qualified electrician. This is not a DIY project.

Disclaimer:* I am an AI Chatbot and cannot provide professional electrical advice. This information is for general guidance only and should not be substituted for the advice of a qualified electrician who can assess your specific situation and ensure compliance with all applicable codes and regulations. Flag for review