You're on the right track with your thinking. Here's a breakdown of why you can't simply tap half the split-phase and why a buck-boost transformer is a common solution

Why You Can't Just Tap Half the Split-Phase for 120V
Split-Phase Basics

A 480V/240V split-phase transformer provides 240V by taking the voltage between the two "hot" legs (let's call them L1 and L2). Each leg is 120V to neutral. The problem is you want the neutral to be referenced to ground for safety in a 120V system. Taking just one "hot" leg (L1 or L2) from a 480V/240V split phase transformer and connecting it to a neutral that is not properly grounded will not yield a safe and usable 120V system.

Phase Relationship

Even if you could somehow isolate and use one leg, the phase relationship between that leg and a separate 120V neutral would be completely undefined. You'd likely have unpredictable voltage and potential for damaging currents.

Utility Regulations

Utilities generally frown upon or outright prohibit pulling an unbalanced load (just one leg) from their secondary windings. This can cause issues on their system and could lead to problems for other customers.

Why a Buck-Boost Transformer is a Good Solution Isolation and Transformation

A buck-boost transformer gives you the isolation and voltage transformation you need. It allows you to take the existing 240V and safely create a 240V/120V secondary.

Standard Configuration

They are designed for this type of voltage reduction and are available in various sizes to match your load requirements.

Safety and Code Compliance

When installed correctly, a buck-boost transformer provides a properly grounded 240V/120V system that meets safety standards and electrical codes.

Alternatives and Considerations

While a buck-boost transformer is often the easiest and most reliable option, here are a few other things to consider, although they may not be practical in your situation: New Dedicated Transformer

This is the best option, but often the most expensive. Running a new service drop and installing a dedicated 480V/240V/120V transformer is the cleanest solution. It provides a balanced service and avoids any potential interactions with the existing lighting circuits. This also ensures you have adequate capacity and that your new service is fully compliant with all codes.

Check Utility Policies

Always talk to your local utility company. They may have specific requirements or restrictions on tapping into their existing infrastructure. They might also be able to offer alternative solutions. They will need to approve any connection to their equipment.*

Load Calculation

Carefully calculate your load requirements for the new 240V/120V service. This will ensure that you select the appropriate size of buck-boost transformer or dedicated transformer. Undersizing can lead to voltage drops and overheating.

In Summary

Using a 240V/120V buck-boost transformer to step down the 240V from the existing split-phase transformer is generally the most practical and code-compliant way to provide a 240V/120V service. Always consult with a qualified electrician and your local utility company to ensure a safe and compliant installation. Flag for review