How to tell single from 3 phase

[Greg Fretwell]

Richard, 2 phase certainly does exist but most people will never see it. It is 2 phases, 90 degrees out with 4 poles.

Jeff Wye is 3 phases with all the transformers having the secondaries tied to ground on one end, delta has them conected in a ring (actually a triangle) with the ground either at the center tap of one of the secondaries or on a "corner". Delta is generally 240v between phases, wye is 208.
In a center tapped delta you have 120v to ground on 2 ungrounded coinductors and 208 on the "wild/orange/red" leg. This is very common in light industrial where most loads are 120/240 and they have a small need for 3p.
This usually only uses 2 transformers.
The center tapped transformer is a big one with the wild leg using a smaller one. I have seen 3p, center tapped delta with 3 transformers but it is pretty rare around here. If you are ever at "Hogs Breath" in Key West look at the transformers feeding that building (and several others) from the raw bar. You will see a 3p delta with 3 transformers right over the bandstand.

[whandley]

http://en.wikipedia.org/wiki/Three-phase_electric_power

[Roy D. Cooke, Sr]

Originally Posted by Greg Fretwell

Richard, 2 phase certainly does exist but most people will never see it. It is 2 phases, 90 degrees out with 4 poles.

Jeff Wye is 3 phases with all the transformers having the secondaries tied to ground on one end, delta has them conected in a ring (actually a triangle) with the ground either at the center tap of one of the secondaries or on a "corner". Delta is generally 240v between phases, wye is 208.
In a center tapped delta you have 120v to ground on 2 ungrounded coinductors and 208 on the "wild/orange/red" leg. This is very common in light industrial where most loads are 120/240 and they have a small need for 3p.
This usually only uses 2 transformers.
The center tapped transformer is a big one with the wild leg using a smaller one. I have seen 3p, center tapped delta with 3 transformers but it is pretty rare around here. If you are ever at "Hogs Breath" in Key West look at the transformers feeding that building (and several others) from the raw bar. You will see a 3p delta with 3 transformers right over the bandstand.

Thanks Greg more on two phase .......... Cookie
http://en.wikipedia.org/wiki/Two_phase
There was even some experiments on five phase.
http://www.electricityforum.com/3-ph...ectricity.html

[George P. Wells, CMI]

Originally Posted by pdickerson

George, you are spot on. But most home inspectors will never see some of the more interesting power configurations that are found in commercial and industrial installations around the country. The missing information in the original post is what kind of building this is. If this is an appartment building or condos (most of us are probably assuming so), than we are almost certainly looking at 3 phase. If this is a manufacturing facility, well then who the heck knows.

Paul,

Thanks. Kenneth's original question was "How do you tell if a building has 3 phase electric service?" There simply isn’t enough information provided by the picture to make the determination. If I were a gambler, I’d put my money on it being thee-phase. The service in the photo is more likely to be three-phase four-wire service than either a single-phase four-wire or a two-phase four wire. If the building is small commercial or multifamily, the odds are also in favor of it being Wye, not Delta. Even though the odds are in favor of the service being three-phase, the odds are not good enough that I’d want to put much money on the bet.

It is true that most home inspectors will never see the different installations that exist. There are many inspectors, however, who are interested in doing commercial inspections. I don’t know whether Kenneth merely had a casual curiosity or this is a building he was actually inspecting. Either way, I was only trying to give him a reasonable and accurate answer to his question.

I’ve had customers buy buildings only to get into the building and incur big unforeseen expenses because they did not have a qualified inspector look at the electrical system.

[George P. Wells, CMI]

Originally Posted by rcooke

There was even some experiments on five phase.
http://www.electricityforum.com/3-ph...ectricity.html

Six-phase is also popular with utility companies.

[George P. Wells, CMI]

Originally Posted by rbennett

Two phase does not exist

rlb

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.