high leg urban (rural?) legends

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I have not worked much with three phase systems and have heard many conflicting answers about their characteristics.

1) true or false, high leg deltas are typically found in rural areas an are rare in urban areas.

2) the high leg to ground voltage is:
a. 208 v
b. 277 v
c. varies from one system to another

What characteristics of the connection method or primaries result in a high leg?

Do utilities typically have a maximum size single phase service? What im getting at is what if you have say a large apartment complex (no three phase loads) whose load requirements exceed that of a single phase service and a three phase delta with a high leg is what is available. Does this ever happen and if so what would typically be done in this situation? Thanks! ~Ethan
 

Ed MacLaren

Senior Member
Re: high leg urban (rural?) legends

This may help with the answer to Question 2.

Ed

4Wdelta2.gif
 

jtester

Senior Member
Location
Las Cruces N.M.
Re: high leg urban (rural?) legends

electrofelon

High leg deltas used to be found many places. In areas served by some utilities, I find them in towns of 10,000 or more. Where I come from that is urban.

There would be very little benefit from using a high leg delta to increase the capacity of single phase services. The only loads on the power transformers could be 240 volt single phase loads, and all of your 120 volt load would have to go on the lighting transformer.

Jim T
 
Re: high leg urban (rural?) legends

Thanks for the diagram ed, makes perfect sense. But wait, my mentor always told me that not all deltas have a high leg, that is why i said 'high leg delta', was he wrong? From ed's diagram i dont see how there could not be a high leg.
 
Re: high leg urban (rural?) legends

....I think i got it: So by saying 'high leg delta', one would be refering to a 4 wire delta, that is a delta where one of the transformers has a Center tap for 120v loads. So by saying 'a delta with no high leg', one means that there is no c.t. connection thus only 240 load can be suppllied. There is no such thing as a 4-wire delta without a high leg. Is that right?
 

Ed MacLaren

Senior Member
Re: high leg urban (rural?) legends

So by saying 'high leg delta', one would be refering to a 4 wire delta, that is a delta where one of the transformers has a Center tap for 120v loads
Right.
Where I live, in Eastern Canada, our POCO hasn't provided 4-wire delta service for many years, and there aren't many buildings remaining with that kind of system.

As a result, as the years went by, fewer and fewer of the younger electricians were aware of the high leg, and the damage that they could cause by inadvertantly connecting 120 volt loads to the high leg bus.

So, if any work is done in these buildings, our Inspection Dept. now requires the loads to be segregated into separate single phase and three phase panels.

Ed

Trans14.gif
 

charlie

Senior Member
Location
Indianapolis
Re: high leg urban (rural?) legends

Ethan, your questions will get different answers depending on the person and in what part of the country (or which country) they work. Mine will be from an electric utility viewpoint.

1) true or false, high leg deltas are typically found in rural areas an are rare in urban areas.

False, they are normally used for smaller 3? loads mixed with 1? loads. Jim gave a great answer to this question.

2) the high leg to ground voltage is:
a. 208 v
b. 277 v
c. varies from one system to another

Assuming the delta is 240 volts, the answer is a. and Ed did a wonderful job with the graphics and explanation.

What characteristics of the connection method or primaries result in a high leg?

Ethan, you do indeed have it. It is permitted to have a three wire ungrounded delta, a three wire grounded delta (one phase is grounded), a four wire delta (which we are discussing with a center tapped transformer), and a four wire open delta (same as the connection we are discussing except one transformer is missing).

Do utilities typically have a maximum size single phase service?

Yes, it is generally based on the maximum size of the transformers they have. We have a maximum size of 800 amperes and 1,600 amperes for a single family home.

What I'm getting at is what if you have say a large apartment complex (no three phase loads) whose load requirements exceed that of a single phase service and a three phase delta with a high leg is what is available.

Normally, an apartment project is spread out and can be fed from multiple 1? transformers. This is preferred and Jim answered this very well.

Does this ever happen [yes] and if so what would typically be done in this situation?

If the building can not be served with multiple transformers and the load is too great, we would first go to 208Y/120 V, 3?, 4w for the building and the individual tenant would be served with 120/208, 1?, 3w. We have a building downtown where we have 480Y/277 going to dry type transformers, then 208Y/120 V, 3?, 4w for the meter center, then 120/208, 1?, 3w for each tenant. In larger cities, medium and high voltage is actually taken up into the buildings. :D
 

lady sparks lover

Senior Member
Re: high leg urban (rural?) legends

I've worked with several high leg applications, and for the most part it was in the city. Things could change, this is how it is right now.

I would say 208V, a, because I have not seen many other systems that have a high leg that are not 240/120V.

High legs I used were used to save the client money, because to put up a 208V transformer in a neighborhood with 240V/1ph would be expenisve to the client. About 3000 dollars expensive...however 208V would be more versatile! IMHO!


Lady :)
 
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