240 volts vs. 208 volts in a residential building

Which of the following methods of wiring a large residential building would be preferred between these two choices (there certainly are other
choices, but I'm focusing on these two right now).
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.
2.
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.
For both of these cases, assume the total load is within the capacity of available transformers, or that multiple transformers could be used to supply each floor when there is a greater load than one transformer can supply. Also assume that special building-wide loads such as elevators and centralized HVAC can be powered by 480Y/277 directly if applicable, or by a voltage system derived from 480Y/277 as needed.
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.
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)?
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On 29 Nov 2006 22:48:47 GMT, snipped-for-privacy@ipal.net wrote:

208 sucks in a residential building unless the landlord owns the laundry, provides HVAC and all the heat producing appliances are fossil fuel. Consumer grade dryers, ranges, water heaters and A/C units are designed for 240. You can 208 volt equipment but it costs more and the output of 240v heaters at 208 is significantly less.
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On Wed, 29 Nov 2006 20:00:50 -0500, snipped-for-privacy@aol.com wrote:

I agree that 208 sucks and is to be avoided if possible, but you can purchase electric range heater elements and dryer conversion kits for 208V. It's a pain in the A though if residents don't understand and buy or bring the wrong replacement equipment. A 240V dryer running on 208V, in my experience will run a lot longer to dry the clothes properly.
Of course if you have gas ranges and gas dryers, it doesn't matter.
The larger (portable) air conditioners are rated for 240V and you may have trouble finding one that works OK on 208v. Fixed AC (compressor units are usually rated for 240v but sometimes wired on 208V. circuits. Then there might be elevator motors, sump pumps, air handlers, vent fans and other motors to consider.
Beachcomber
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| On Wed, 29 Nov 2006 20:00:50 -0500, snipped-for-privacy@aol.com wrote: |
|> |>>Which of the following methods of wiring a large residential building |>>would be preferred between these two choices |> |>208 sucks in a residential building unless the landlord owns the |>laundry, provides HVAC and all the heat producing appliances are |>fossil fuel. |>Consumer grade dryers, ranges, water heaters and A/C units are |>designed for 240. You can 208 volt equipment but it costs more and |>the output of 240v heaters at 208 is significantly less. | | | I agree that 208 sucks and is to be avoided if possible, but you can | purchase electric range heater elements and dryer conversion kits for | 208V. It's a pain in the A though if residents don't understand and | buy or bring the wrong replacement equipment. A 240V dryer running on | 208V, in my experience will run a lot longer to dry the clothes | properly.
Not all ranges have 208 volt elements available. Some do. Most don't as far as I've seen. The more expensive ones seems to be the ones that do, but you're just tossing out a fine 240 volt element after special ordering a 208 volt element.
| Of course if you have gas ranges and gas dryers, it doesn't matter. | | The larger (portable) air conditioners are rated for 240V and you may | have trouble finding one that works OK on 208v. Fixed AC (compressor | units are usually rated for 240v but sometimes wired on 208V. | circuits. Then there might be elevator motors, sump pumps, air | handlers, vent fans and other motors to consider.
I have seen some for 208 volts. But not very many. And those were commercial units for motels.
IMHO, three phase should be limited to 480 volts in the USA.
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On Wed, 29 Nov 2006 20:00:50 -0500 snipped-for-privacy@aol.com wrote:
| On 29 Nov 2006 22:48:47 GMT, snipped-for-privacy@ipal.net wrote: | |>Which of the following methods of wiring a large residential building |>would be preferred between these two choices | | 208 sucks in a residential building unless the landlord owns the | laundry, provides HVAC and all the heat producing appliances are | fossil fuel.
Exactly.
| Consumer grade dryers, ranges, water heaters and A/C units are | designed for 240. You can 208 volt equipment but it costs more and | the output of 240v heaters at 208 is significantly less.
There are 208 volt elements for many things, but not all. That itself is clear indication that 208 volts for 240 volt stuff sucks.
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Personally, I would prefer single phase 120/240 over a "2 out of 3" 120/208 service.
There is, maybe, a better selection of 240 volt over 208 "stuff" including office equipment.
OTOH if someone "needed" 120/208 3 phase with the help of a two transformers he could "reconstitute" is from the 3 wire 120/208.
I
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| Personally, I would prefer single phase 120/240 over a "2 out of 3" 120/208 | service. | | There is, maybe, a better selection of 240 volt over 208 "stuff" including | office equipment. | | OTOH if someone "needed" 120/208 3 phase with the help of a two transformers | he could "reconstitute" is from the 3 wire 120/208.
You probably mean:
A * \ / \ N C / B
But it still doesn't solve the 240 volt problem.
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wrote:

120/208
including
transformers
Quite correct. I just wanted to give the ONLY reason to give the units 120/208 over 120/240.
I'm definitely on the 120/240 side.
Old Tom Edison got it right the first time!
BTW: That is a VERY good ASCII representation of the way of generatig the 3rd phase.
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| | wrote: |> |> | Personally, I would prefer single phase 120/240 over a "2 out of 3" | 120/208 |> | service. |> | |> | There is, maybe, a better selection of 240 volt over 208 "stuff" | including |> | office equipment. |> | |> | OTOH if someone "needed" 120/208 3 phase with the help of a two | transformers |> | he could "reconstitute" is from the 3 wire 120/208. |> |> You probably mean: |> |> A * |> \ / \ |> N C |> / |> B |> |> But it still doesn't solve the 240 volt problem. | | Quite correct. I just wanted to give the ONLY reason to give the units | 120/208 over 120/240. | | I'm definitely on the 120/240 side. | | Old Tom Edison got it right the first time! | | BTW: That is a VERY good ASCII representation of the way of generatig the | 3rd phase.
Well, Tom had part of it right. I'm still fence sitting on the DC thing. AC is certainly more practical right now because everything is designed around it ... just as so many things are designed around 240 volts instead of 208 volts.
I wonder what the power company would think if I put big single phase loads on that derived C-N connection :-)
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loads
They would prefer it over drawing 240 volts on one phase only.
That's the "other reason" the 120/208 is popular: large loads at least draw from 2 phases. Seems like "around here" the power company just doesn't like "weird" 3 phase arrangements anymore. New service is Y or single phase. It may have something to do with deregulation: the local distribution company might have to pay a penalty for "unbalanced" draws from suppliers. Before de-reg it was all in the same company.

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|> I wonder what the power company would think if I put big single phase | loads |> on that derived C-N connection :-) | | They would prefer it over drawing 240 volts on one phase only. | | That's the "other reason" the 120/208 is popular: large loads at least draw | from 2 phases. Seems like "around here" the power company just doesn't | like "weird" 3 phase arrangements anymore. New service is Y or single | phase. It may have something to do with deregulation: the local | distribution company might have to pay a penalty for "unbalanced" draws from | suppliers. Before de-reg it was all in the same company.
If the one load is given a voltage in the C-N phase angle, but the only connections are A and B, how does this affect the current phase angles?
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Off hand, they would be 60 degrees off from the AB and 30 off from AN.
I guess the "next question" is WTF happens with reactive or harmonic loads.
I don't know. Also, I'm too lazy to figure it out right now but maybe the urge will increase during the day.
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|> If the one load is given a voltage in the C-N phase angle, but the only |> connections are A and B, how does this affect the current phase angles? | | Off hand, they would be 60 degrees off from the AB and 30 off from AN. | | I guess the "next question" is WTF happens with reactive or harmonic loads. | | I don't know. Also, I'm too lazy to figure it out right now but maybe the | urge will increase during the day.
In this case we have:
A * A \ / \ \ N C pretending to be N---C (120 per leg) / / B B
It would seem to me that N-C or C-N is 90 degrees off from A-B. N-C would be 120 degrees off from A-N or B-N (each in an opposite way). The power factor seen in the current on A or B with respect to A-N and B-N voltage would by my thinking be 0.5. So for 100 amps on N-C there would be 100 amps on N, and also 100 amps on each of A and B.
Someone else was suggesting that if a load has a current which is N degrees offset from its voltage (e.g. reactive if N!=0), and that voltage came from 2 or more windings with mixed phasor angles (e.g. either of the case of how to derive 3 phases from 2 120 degree phases, or the angled buck-boost case I described elsewhere with a B/B winding that was 30 degrees offset from the winding it was bosting) that the current would somehow change as it goes through the core to the primary winding such that the N degrees relationship between voltage and current would be preserved in the primary as seen by the source supply. That didn't see right to me, but I wasn't absolutely sure if there was something else going on.
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OK. In the case of the "derrived" C, in general (even with a 100% resistive load) the current through the transformers will not be in phase with the voltage across the windings. So, the AB currents will be 60 degrees out of phase of the AN or BN voltage.

degrees
from
how
the
relationship
the
Nope!
Transformers do a very good job by making the primary current equal to the secondary current (with the allowance for the current needed to magnetize the core and, of course, the turns ratio.)

There may be "something" else going on upstream. For example, a transformer powering the 120/208 3 wire arrangement would "see" a power factor less than 1.0.
This kind of stuff is tedious. Since I don't get paid to look at such things I don't worry much about what an unbalanced load "looks" like at the end of, say, 200 miles of transmission line. I hope it's not too nasty but I just don't know. Maybe one of the "Power Pros" on this list will enlighten us.
(Slight OT: some time ago one of the "power pros" tried to enlighten me on why you can't transmit AC power more than a certain distance. The lesson didn't completely sink in but maybe I will try again as a mental exercise.)

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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.

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).

Often done. Not a problem.

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.

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
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Benjamin D. Miller, PE
B. MILLER ENGINEERING
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| |> Which of the following methods of wiring a large residential building |> would be preferred between these two choices (there certainly are other |> choices, but I'm focusing on these two right now). |> |> 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.
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I major gripe I have with many of the apartment complex developers is the choice of resistance heat. The owner pays for the equipment, but the tenant pays the power bill. That usually leads to the cheapest equipment and maintenance cost but highest operating cost choice of electric heat. I'm a fairly hands off the market person, but I'm almost willing to support legislation on more efficient heat for apartments.
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|> 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. | | I major gripe I have with many of the apartment complex developers is the | choice of resistance heat. The owner pays for the equipment, but the tenant | pays the power bill. That usually leads to the cheapest equipment and | maintenance cost but highest operating cost choice of electric heat. I'm a | fairly hands off the market person, but I'm almost willing to support | legislation on more efficient heat for apartments.
Don't forget to also include a requirement for an abundance of high R factor insulation in the exterior walls, and even between apartments (FYI, I set my thermostat to 15C in winter, which is well below what most people set it at, so I'm effectively taking heat from neighbors that are usually at 20C).
Those willing to be less intrusive on how to construct the buildings could instead simply mandate disclosure of things like the total insulation value or the heat loss rate for the unit. But most consumers won't understand these figures.
How many of you "smart" engineers know how much your credit card companies are ripping you off for? Businesses make money off of consumer ignorance and actively fight legislation to inform consumers, even when most of them wouldn't even understand the information.
I'm all for mandating savings ... except I'll keep (and pay for) incandescent lights in my kitchen. As long as I'm the one paying for the energy, I should have that right. If I build an apartment building, I would be expected to put energy saving lights in, however.
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I lived in two apartment complexes prior to buying my first home. Both were larger complexes and both had 120/240 distribution. The utility provided multiple services from single phase pad mount transformers. I'm assuming that given the size of the complex, the primary was three phase with the transformers distributed among the phases.
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I guess I didn't finish my point, so here goes:
A distribution transformer has about the same cost and efficiency going from primary voltage to either 480V or 240V. If you add the extra intermediate voltage of 480V, there is the extra cost of transformers and the extra loss of transformers to go from 480V to 240V. This is offset by the lower losses and lower cost in SOME of the distribution equipment, mostly the wires. 480V panels tend to be more expensive than 240V for the same power at the lower power end of things - like an apartment would be. In an industrial / commercial application, much of the load is at 480V. The lighting, elevators and HVAC can all be run at 277Y/480V. Only a small fraction of the load ends up being double transformed. I have seen buildings designed with both 480V and 208V service from the utility. I expect that the loss with this type of service would be the lowest. In an apartment complex, only a small amount of the load could be utilized at 277Y/480V. Most would be double transformed, so the extra transformer loss would be more significant.
The point I made above is that a complex could be feed by a multiple of three single phase services. That would give 120/240 to everyone. That would give the utility reasonably balanced load.
One more thing to consider is the dislike many utilities have for delta connected transformers. I talked to a utility engineer since our least thread on this subject about the common 480V to 120Y/208V in commercial and industrial buildings. He indicated that he would actually prefer that they were wired Y-Y, but realizes that won't happen. He said that they don't amount to too much of a problem because they are such a small fraction of the load - usually 25% or less. In a shorted phase or ferroresonance incident, the primary breaker of the transformer often trips because it's so much smaller than the large transformer feeding the service. The other common line to line loads are motors. In a phase loss or power loss situation, they usually trip off line so they are not a problem. Motors also don't cause ferroresonance like a transformer does.
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