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Three Phase Transformer Wiring, Delta and Wye
A three phase pad-mount is usually wired delta on the primary or grounded wye, and wye on the secondary. The notation 12470 GY/7200 means a grounded wye primary at 12470 volts line to line and 7200 volts line to neutral. 480Y/277 on the secondary means the same thing at the low side.
Reading the voltage notation
12470 GY/7200 means a grounded wye primary. 12470 volts measured line to line, 7200 volts line to neutral. The two numbers are related by the square root of three.
480Y/277 is the same convention on the secondary. 480 volts between any two phases, 277 volts from any phase to neutral. This is the standard US commercial service.
Delta and wye
A delta winding connects the three phases end to end in a triangle with no neutral. A wye winding connects one end of each phase to a common neutral point. Wye gives you a neutral and therefore a phase to neutral voltage, which is why secondaries are almost always wye on commercial work where single phase loads exist.
The common arrangement
Most three phase pad-mounts on US distribution are grounded wye primary to grounded wye secondary, or delta primary to grounded wye secondary. Which one matters for how ground faults behave, and the serving utility will usually dictate it.
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