|> The building is supplied by power at 480Y/277 volts, or at a higher |> voltage stepped down in a secured electrical room to 480Y/277. One or |> more large capacity 480Y/277 volts circuits feed through the vertical |> core of the building to an electrical room on each floor. On each |> floor, a dry-type transformer steps 480 volts down to service voltage |> for each customer. |>
|> 1. |>
|> Each floor is powered through a three phase transformer that steps the |> 480Y/277 volt subfeed down to 208Y/120 volts. Each tenant is supplied |> with just TWO phases of the three phase service, with the choice of |> phases approximately balanced. | | Nothing unique about this. It is common. In fact, I saw one today. The | distribution panels in the metering room on each floor are three-phase, each | tenant gets 208/120 single phase. I don't know what 240 volt loads are in | the units. If you use gas for the cooking and laundry, the entire issue of | 208 vs. 240 in the tenant units goes away. The only other load would be A/C, | and those are normally dual rated for 230/208 volt.
It is in fact A/C that I had my first real life experience with that taught me why 208 is bad when things are designed for 240. After several burnouts of the A/C blower motor, the service people finally had to special order a 208 volt motor. The motor will run on a lower than normal voltage, but during peak heat, our computer room drove it to nearly continuous operation. The ambient heat plus extra current heat apparently rose it above the heat some part of the motor just could not withstand for the time periods involved.
|> Each floor is powered through a single phase transformer that steps |> just two legs of the 480Y/277 volt subfeed from 480 volts to 240/120 |> volts. Each tenant is supplied with this normal single split phase |> voltage. The diversity of the floors are approximately balanced. |>
| | Distributing three phase is more efficient in terms of the wiring, panel, | and transformer cost. At the same voltage, you can carry 73% more power with | 33% more copper (25% more copper if you have an egc as well).
I don't see where you get those exact figures. I know three phase can be more efficient to distribute power, but those numbers don't seem right. By saying "33% more copper" that sounds like you are going from a 3-wire Edison style single phase to a 4-wire Wye derived three phase with the same size conductors. But that doesn't give 73% more power. Assuming the same L-N voltage, it's only 50% more. Assuming the same L-L voltage, it's only 57% more. And if you dismiss the neutral in both cases, which forces assuming the same L-L voltage, then you are increasing the copper by 50% to go to three phase, with the 57% power increase, for a net gain of 15.47% efficiency for the same amount of copper.
One can always increase the efficiency of "copper" by increasing the voltage. The conducting material doesn't really care. The insulating material sure does. Of course you have to weigh the comparitive costs and I do believe copper is pretty damned expensive. So is aluminum (which utility transmission and distribution tends to use).
|> As a variation of choice #2, where more than one transformer is needed |> for a building with large floors, these transformers can be balanced |> as reasonably possible over the core subfeed phases. Also assume that |> additional subfeed circuits can be separately wired if a single feed |> would be inadequate, up to as many feeds as needed, such as one feed |> separately to each floor. | |> I'm not specifying a particular size for this building. Instead, what |> I want to focus on is the practicality of supplying 120/240 volts for |> single phase tenants (and generally residential will be single phase) |> instead of 120/208 volts. | | Talk to some building owners about what they are willing to pay for extra | transformers and wiring just so their tenants can have 240/120 volts instead | of 208/120, and I think you will see the practical problem. I have clients | (owners) trying to squeeze every last penny out of the cost of the | electrical systems in these buildings. They just aren't going to buy into | the extra $$, when there really isn't any problem. If appliances were | failing every week, that would be different.
In most cases, appliances won't fail; they will just underperform. For example an electric water heater takes longer to recover on 208 volts than if on 240 volts. Penny pinching owners won't want to pay extra to get water heaters with special 208 volt elements.
This is also wasteful on energy costs. Heating elements are generally thermostatically controlled (watch the red glow going on and off with a glass-top range). At 208 volts (compared to 240 volts) they will be drawing 86.6% of the current but only providing 75% of the heat. This means they will be switched on 33% more (133% total) time. That means there is a 15.47% extra energy loss in the wiring.
So really, the penny pinching owners are shifting the costs to the tenants, even if both parties don't even realize it.
So home much more does a 75 kVA single phase transformer cost over that of a 75 kVA three phase transformer? My first scenario is to run each floor on just one of the phases, not be split up three ways. If 75 kVA is enough for the floor (4 apartments, as an example), this would work.
More likely, they would end up not running 480 volt feeds to transformers on each floor, especially if the building is small (say 3 to 6 floors). They'd put in a big 208Y/120 transformer coming into the building, maybe as a pad mount out back, and just run everything from that.
They could run full three phase to each apartment and put in a three phase panel. Then just hook appliances within at various phases and put each apartment in different rotation offsets to diversify things well. But they don't. It's actually _more_ expensive to do that, at least for the cost of a branch breaker panel (home comsumer demand and competititon has certainly driven down the price of single phase panels in the 100 to 200 amp range).
|> Another question: would this preference be any different if some or |> all of the tenants were light business use, such as lawyer offices, |> corporate branch sales offices, recruiters, etc, with no unusual |> electrical needs (but would have a small kitchen with normal cooking |> facilities for employee use such as lunch breaks)? | | Nope. Many of those buildings also use 208/120 four wire. It is all about | the cost of distributing the power through the building. There is no | compelling reason to add cost or complexity. | | Ben Miller
When I was around age 7 or 8 or so, I remember that my grandfather had a brownout problem. The power company was unable to deliver full voltage for a day or so. The voltage was something like 15% or 20% less. While lights did work, the electric stove just didn't even work at all. His freezer compressor (ran on what was supposed to be 120 volts) burned up.
One big issue in this scenario was that he actually had three phase power at presumably 208Y/120 volts. He got that because he powered his wood shop in the back of the house with it since several of his big machines specifically used three phase (and presumably were designed for 208 volts).
The problem with the stove, though, was that it was dealt a double blow in brownout effects. It was a normal home model presumably intended for
240 volts. It was being run on 208 volts. With the brownout at say 15% it was now getting only 177 volts.
The point here is this. Things designed for 240 volts might well work fine over the whole range of voltage they could get when connected at
240 volts, and work fine when getting the true nominal voltage when connected at 208 volts. But run them at the lower end of the voltage range when also connected at 208 volts, and this is pushing things to the extreme. If the 240 volt appliance can in fact operate OK all the way from 200 volts to 280 volts actual, putting it on what is nominally at eaither far end could be a problem when that supply voltage swings away from that nominal voltage the more extreme way.
If the appliance can operate at + or - 20% around its designed nominal voltage, and the utility supply can vary by + or - 10% long term and an additional + or - 10% short term, AND if you intentionally connect the appliance to only 86.6% of the voltage, then it really can end up being out of range at least sometimes ... when that 86.6% voltage ends up actually being really just 73% of the voltage.
How much of a range would an appliance need to support to be able to handle the FULL high swing from a 240 volt connection to the FULL LOW swing of a 208 volt connection.