Large Residential

[Do You Like Ham]

Hypothetical scenario here.. say one is designing a large multi-family residential building, and the building wanted to have all units at 240/120 1ph 3w, and the utility is provided at, say, 12kV 3ph.

What is the 'typical' way to do this?
I'm imagining that this would be done by having, potentially, three transformers... primary side essentially connected like a delta configuration.... on the secondary, is it sensible to have a mid-tap on each of the three transformers?

Something like this:

 

[Joethemechanic]

Yeah, not like you could make it a delta secondary. Be better if they could use 120/208 but residential appliances usually don't have a 208 option

 

[Do You Like Ham]

that was sort of my thought... it's like a delta, but all three mid-tapped.
Curious what folks have done in this case.. if there are other options.

 

[Joethemechanic]

How far do the secondary conductors from the transformers have to run?

 

[electrofelon]

that was sort of my thought... it's like a delta, but all three mid-tapped.
Curious what folks have done in this case.. if there are other options.

Most likely the utility is not going to supply a primary service so you won't be selecting transformers.

Personally I would try to get a 120/240 single phase service, but that's going to Max out on what utility will provide pretty quick so then it would be 120/208Y

 

[Do You Like Ham]

@Joethemechanic - this is completely hypothetical... just trying to learn a bit about distribution in large scale residential. E.g., could be an apartment tower of 50 floors, I suppose... so, maybe from ground to the top level units might be 500'.

@electrofelon - are you suggesting that in such a large building, it's not likely to be 240/120, and instead, units would require 208v capable appliances?

 

[electrofelon]

@Joethemechanic - this is completely hypothetical... just trying to learn a bit about distribution in large scale residential. E.g., could be an apartment tower of 50 floors, I suppose... so, maybe from ground to the top level units might be 500'.

@electrofelon - are you suggesting that in such a large building, it's not likely to be 240/120, and instead, units would require 208v capable appliances?

Well there is no such thing as 208 volt capable appliances. Your standard appliances work fine on 208, and as far as I know no one makes 208 specific appliances.

Okay, I wasn't sure what you meant by "large residential". If you're talking really really big like a skyscraper then yeah you may well take primary service, send it up the building and step it down in various places. @infinity has done some large projects, can't remember how he said they did it maybe he will jump in. I think on some of them the power company would only supply 208 so they actually stepped that up to medium voltage to distribute it if I remember correctly.

 

[Strathead]

Hypothetical scenario here.. say one is designing a large multi-family residential building, and the building wanted to have all units at 240/120 1ph 3w, and the utility is provided at, say, 12kV 3ph.

What is the 'typical' way to do this?
I'm imagining that this would be done by having, potentially, three transformers... primary side essentially connected like a delta configuration.... on the secondary, is it sensible to have a mid-tap on each of the three transformers?

Something like this:

View attachment 2565777

There are people here who understand residential better, but a couple of observations. The Utility usually provide 12KV Wye, a resi 120/240 transformer is usually connected between one leg and the neutral. Not between two high phases. I don't know what utilities are willing to do, but obviously you can theoretically get multiple single phase 120/240 services. The max size and quantity can only be answered by your utility.

 

[Do You Like Ham]

@Strathead
Like this?

 

[norcal]

Well there is no such thing as 208 volt capable appliances. Your standard appliances work fine on 208, and as far as I know no one makes 208 specific appliances.

Okay, I wasn't sure what you meant by "large residential". If you're talking really really big like a skyscraper then yeah you may well take primary service, send it up the building and step it down in various places. @infinity has done some large projects, can't remember how he said they did it maybe he will jump in. I think on some of them the power company would only supply 208 so they actually stepped that up to medium voltage to distribute it if I remember correctly.

Many have dual ratings, my range is rated 9.5 KW @ 120/208V, & 12.5 KW @ 120/240V.

 

[electrofelon]

Many have dual ratings, my range is rated 9.5 KW @ 120/208V, & 12.5 KW @ 120/240V.

Right that's what I meant. I meant that they pretty much all have dual ratings and there isn't a specific one you buy just for 208

 

[wwhitney]

No clue on what is commonly done, but the transformer arrangement in the OP would effectively be creating a 7-wire 120V "hexaphase" system. That is, if you measure the L-L voltage across all pairs of the 6 ungrounded secondary conductors from the 3 single phase transformers, you will get voltages of 120V, 208V, and 240V. I believe you could create a single 3-phase transformer that would do the same thing, but I would not expect it to be readily commercially available.

Cheers, Wayne

 

[Joethemechanic]

If you are talking about a building that big, you are going to have all kinds of common building systems to run. Elevators, HVAC, booster pumps for domestic water and fire sprinkler systems, etc. So you are going to want to have 480v three phase for that stuff

 

[Strathead]

@Strathead
Like this?
View attachment 2565780

yes

 

[Do You Like Ham]

@wwhitney interesting.. hadn't dawned on me that someone would attempt to connect between meter banks.

@Joethemechanic - ah, I wrestled w/ that one a bit... one assumption I had would be that each unit would be individually metered.. and to do that would require just one service(?)... so, I was assuming that common building systems (and perhaps some first floor light commercial tenants) were 208/120 3ph 4w. The original case I was contemplating wasn't so big (I think the example was more like 5 floors)... with a total of like 16 residential tenants and 4 commercial... so a total of 20 meters. So, as a bit of thought experiment, was wondering if quite a bit larger.

But the general question remains... assuming a building has a 3ph utility, how is the individually metered 240/120v 3w for each tenant derived.

 

[Do You Like Ham]

yes

interesting... that's essentially a wye transformer configuration on the primary... isn't it? I didn't think is that was common?

 

[electrofelon]

But the general question remains... assuming a building has a 3ph utility, how is the individually metered 240/120v 3w for each tenant derived.

It's not. You would serve individual units with 120/208 single phase.

 

[Do You Like Ham]

@electrofelon - would the units tend to be individually metered by utility? if not, is this where something like this comes into play where the utility bills the building owner, and the owner sub-meters and/or uses a 3rd party to do so?

 

[electrofelon]

@electrofelon - would the units tend to be individually metered by utility? if not, is this where something like this comes into play where the utility bills the building owner, and the owner sub-meters and/or uses a 3rd party to do so?

Could be either. Sub metering is common in some areas and may be illegal in others