Hi, thanks for reading my question. I'm an electrician with some basic knowledge of electricity but this is puzzling me. In the USA, a single transformer on a pole means split phase. Three transformers on a pole means 3 phase. What does two transformers mean? Is it split phase with a high leg?Thanks
That is an excellent question. A single phase transformer is referred to as a split phase because it is center tapped. What that means is that the transformer as a whole is 240V with a wire tapping the coil in the center and grounded to create two separate 120V legs. The single phase transformer is essentially split into two pieces which is where the split phase terminology is derived. Three phase banks are three separate transformers working 120 degrees out of phase which is an ideal balanced setting for three phase commercial load. You can almost create true three phase using two transformers. The phase separation isn't ideal, but it does work when three phase service isn't available or a single phase service is needed in addition to a three phase service off the same bank of transformers. The term high leg is derived from the non-ideal phase orientation which creates a higher single phase voltage on one leg. BTW am I the only one tired of seeing one or two know it all people answer all the engineering questions even when they obviously haven't got a clue?
This is about the practices of your local power company. One transformer could be for 3 phases (see reference for a sample). Two transformers could be two split phase transformers or one split phase and one 3 phase. You can get a better idea by counting the number of wires going into the transformer.
In my area there are sometimes 3 or four transformers on a pole. This is a residential area though and a lot of the houses use electrical resistance heating. Looking at the transformers on the nearest pole I see see two wires output and a ground cable coming from each transformer. The transformer coming to my home is single phase 240 volt ( 120V on each hot line). The transformers are rated at 50 kva, so I'm assuming four 50 kva transformers was probably cheaper and more flexible than using one 200 kva transformer.
