How to tell single from 3 phase

Richard,

Not only do two-phase exist, they seem to cause the most confusion. I’ve had experienced electricians ask me to identify what type of system it was that they were looking at because they could not understand the internal distribution. A quick glance at the transformers will usually answer the question.

Take a walk in Detroit, Chicago, Pittsburg, Cleveland, or New York and you will see lots of two-phase four-wire services. You will be able to recognize them by the fact that they often have two single-phase transformers. The transformers are usually mounted on a rack directly on the building. The two transformers of a two-phase system will often be different sizes (unlike three-phase systems which are usually matched sets). For example, you might see a 75kVA paired with a 100kVA. If the two are electrically connected to one another, you are probably looking at a two-phase system.

You won’t see many pole mounted two-phase services because pole mounted equipment is more likely to be owned by the utility company. The building owners are buying primary power from the utility company.

There is a very good reason that so many building owners have retained two-phase services. A two-phase service meets the requirements of having only one service to a building while, in effect, providing two single-phase services. Also, is much less expensive to buy primary power than secondary power. Unfortunately, two-phase services often create unforeseen problems.

Imagine that you want to open a small machine shop. Your equipment is 3θ 240/480V and you find a building on the East side of Detroit. You have an inspector look at your small 4000 square foot 150 year old building. The inspector tells you that you have a three-phase service so you think you are good to go. So, what do you do when you move into the building and your electrician does not understand the distribution?

This is the point where my phone would ring. In fact, I had exactly this situation with a small company that rebuilds racecar motors. The owner didn’t know what to do. His electrician didn’t know what do. I ended up reconfiguring the building’s internal power distribution to be two single-phase systems with a common disconnect and phase converters for his 3θ 240/480V equipment. He kept his two-phase service.