What type of transformer is required to step down the 240V delta service connection?

My customer is installing a 250 kW ground mounted solar installation in New Jersey with a service connection that requires a three phase 240 V with a high leg delta.  I am going to use a Xantrex GT250 three phase inverter with a 480 V output.  What type of transformer is required to step down the voltage to enable this service?   What exactly is a high leg 240 V 3 phase electrical service connection? 


Our friends at Wikipedia tell us that the advantages are:

This type of service is usually supplied using 240V line-to-line and 120V phase to neutral. In some ways, the high leg delta service provides the best of both worlds: a line-to-line voltage that is higher than the usual 208V that most three-phase services have, and a line-to-neutral voltage (on two of the phases) sufficient for connecting appliances and lighting. Thus, large pieces of equipment will draw less current than with 208V, requiring smaller wire and breaker sizes. Lights and appliances requiring 120V can be connected to phases 'A' and 'C' without requiring an additional step-down transformer.

Where the three-phase load is small relative to the total load, two individual transformers may be used instead of the three for a full delta or a three-phase transformer, thus providing a variety of voltages at reduced cost. This is called "open delta high-leg" and has a reduced capacity relative to a full delta.

But the disadvantages are:

Since one phase-to-neutral voltage (phase 'B') is higher than the others, no single phase loads can be connected to this phase. This eliminates the ability to use one third of the breakers in a panel for single-phase loads. Further, the lack of loading on the high leg will result in an unbalanced load.

But this doesn't really answer your direct question.  Any thoughts?

Take a  look at this drawing from ElectricianTalk.com - I can't find a better illustration on the web than the one offered by user "Oldman".

It's a simple way of having both large (High power 3-phase) loads at 240 VAC and smaller, convenience loads (120 VAC) on the same transformer.

This type of service is generally found in the Eastern and Central US, with 480 VAC 3Phase Wye services more common on the West coast. Here is an unverified breakdown gathered from the web and probably very close to fact:

120Y208 (4 wire); New standard for commercial and light industrial

208V Delta (3 wire); rare legacy

240V Delta (3 wire); mostly legacy but still some occasional new services

120/240V Delta with a center tap between A and C phases for 120V (4 wire "stinger leg"); still provided for small commercial users with >10% 120V 1 phase loads

440V Delta (3 wire); legacy

480V Delta (3 wire); mostly legacy but some new rural services are still done this way as Open Delta or Corner Grounded Delta

277Y480V (4 wire); new standard for industrial service

575V Delta (3 wire); rare legacy, but found in the South East in textile mills and the North West in some Lumber Mills.         From CR4


Here's a typical transformer you might use for a 250 kW inverter But really, step 1 of this process is to contact the engineering department at the inverter manufacturer and confirm the compatibility of their inverter with the High Leg Delta service. Most inverters over 25 kW can be ordered with custom output transformers or preconfigured to match the most cost-effective isolation transformer that will connect them to the high leg delta service.

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I suggest that you use two transformers to separate the 240v 3ph from the high leg (which is 2x 120v center tap from one phase). As illustrated in the diagram above.


The 240v 3ph high leg is a custom transformer with unknown de-rating, limited availability, unbalanced loading and in case of replacement (it not being an off-the-shelf item) Therefore, I suggest using two separate transformers.  You can use the standard   GT250-480 3ph - Going to external 480 to 240 3ph transformer + external 240 to 2x 120 is preferred solution. 



1. Xantrex GT250 - Output 480 3Phase

2. External 480 to 240 3Phase Transformer

3. External 240 to 2 x 120 Transformer