240 volts vs. 208 volts in a residential building

|> 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.

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.

relationship

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.)

>

We obviously were thinking of different type of apartments. I was considering the suburbia type with lots of area on three floors. You're considering the more urban tower type. This would be completely out of the range of anything I have experience with.

I've lived in a complex which had a little single phase pad for each building, and in a different complex which had a big three phase pad for several buildings together. I have no idea what the design reasons were. I suspect the former had 120/240 and the latter had 120/208 in each apartment. I am trying to get a grasp on the why of decisions like these. One thing I want to know is the power company influence on it.

Typically these are decisions made by the electrical engineer. As you have observed, there are several approaches to powering a campus. The large transformer could also be 480 volt or higher, with individual transformers in each building. There could be non-electrical factors also, such as available space for transformers, new construction vs. remodeling, etc.

In my experience, the utility will do whatever you ask for, within the limits of what they have available. They do review your load application, but I have never had them reject one. They might charge you to bring a particular voltage to the area if it isn't within a certain distance already, but they will generally do it. For large industrial facilities, they get more involved because of the loading on their system.

Ben Miller

So if I were to build a large apartment building, even though the power company would be putting in their meters, they would have no issue with each of those meter sockets being single phase for all tenants to get

120/240 from single phase dry transformers on each floor, even though all other buildings in the same city would be 2 legged 120/208?

There are many subtle factors that might affect this decision. If the building is an elevator building, 3 phase might be desirable (if it is available), even though 240V single phase elevators are also available. Having just one outdoor 3-phase transformer connected with a wye secondary that can supply the entire building might be the best, cheapest, most economical way to go, if the load can be reasonable balanced, central hot-water, common ventilation, etc.

As I stated in a previous thread, being a tenant in such a building can be a pain if you personally have to deal with the installation of

208 volt ranges, dryers, hot-water heaters, etc. When I did live in such a building, most of my neighbors didn't have a clue why it took 1 and 15 minutes to dry a load of laundry. This was considered a luxury condo building too. A comparable rental building would be more likely to have a common-area landlord provided laundry room. This is the custom in the US for this type of housing in many major cities.

If it is a low rise multi-unit multi-building setup, it is quiet common in my current suburban area to go with the outdoor single/phase pad-mounted transformers providing 120/240V. In this case, there are no elevators or heavy HVAC loads and no common area service other than low demand building lighting circuits. Everybody gets a meter and 150 or 200A service. Everyone gets there own electric range and hot water heater. The downside? Cheap construction and inexpensive-to-install, expensive to run radiant electric heaters that the tenant pays to operate are commonly provided.

In a place like Chicago, there are a lot of high density, medium-rise apartments of and no room for outdoor, individual pad mount transformers. The transformers for these buildings are sometimes placed in underground vaults or basements rooms and they sometimes explode with an eruption of big black smoke. They are very expensive to replace and sometimes result in extended outages.

Beachcomber

| In a place like Chicago, there are a lot of high density, medium-rise | apartments of and no room for outdoor, individual pad mount | transformers. The transformers for these buildings are sometimes | placed in underground vaults or basements rooms and they sometimes | explode with an eruption of big black smoke. They are very expensive | to replace and sometimes result in extended outages.

Of all the news stories I've seen of transformers in basements and under the streets exploding, etc, I'd say nearly half seem to be in Chicago. One I remember many years ago was at a very large single building dorm at Illinois State in Bloomington. The transformer couldn't handle the increasing electrical use of students, and one year, right as students were all moving in and peaking demand because classes hadn't yet begun, it blew. I read somewhere they had to actually dig the ground to build an access to the basement to install the replacement, which was larger. I heard the place was dark for a couple weeks. Whoever figured the loads when the building was built had no idea of the future. Just how much they should have provided for in growth may have been overshadowed or overruled by state cost considerations. I'm sure they found a way to charge all the students for the costs, and then charge the taxpayers at the same time for the same costs.

College dorms are a classic example of cheap wiring that gets quickly overloaded. There are many such stories. The culprits a few years ago were refrigerators and cheap 1000 watt halogen lamps. Everyone brought one in because they provided a good reading light and didn't cost too much. The dorms wiring systems got overloaded and many colleges banned these types of lamps. A few years ago the Home Depot stores were offering a turn-in-your-1000-watt halogen-lamp and buy a fluorescent at a discount promotion.

Beachcomber

In some cases the utility owns the transformer. They have their own load factors that they use to size the distribution equipment, and they don't always know when the loads have increased from the initial sizing. ComEd came under a lot of fire in the Chicago area a few years ago after an investigation found widespread lack of infrastructure maintenance.

Ben Miller

On Sun, 03 Dec 2006 05:48:42 GMT Beachcomber wrote: | On 3 Dec 2006 04:22:59 GMT, snipped-for-privacy@ipal.net wrote: | |>On Sun, 03 Dec 2006 02:57:38 GMT Beachcomber wrote: |>

|>| In a place like Chicago, there are a lot of high density, medium-rise |>| apartments of and no room for outdoor, individual pad mount |>| transformers. The transformers for these buildings are sometimes |>| placed in underground vaults or basements rooms and they sometimes |>| explode with an eruption of big black smoke. They are very expensive |>| to replace and sometimes result in extended outages. |>

|>Of all the news stories I've seen of transformers in basements and under |>the streets exploding, etc, I'd say nearly half seem to be in Chicago. |>One I remember many years ago was at a very large single building dorm |>at Illinois State in Bloomington. The transformer couldn't handle the |>increasing electrical use of students, and one year, right as students |>were all moving in and peaking demand because classes hadn't yet begun, |>it blew. I read somewhere they had to actually dig the ground to build |>an access to the basement to install the replacement, which was larger. |>I heard the place was dark for a couple weeks. Whoever figured the loads |>when the building was built had no idea of the future. Just how much |>they should have provided for in growth may have been overshadowed or |>overruled by state cost considerations. I'm sure they found a way to |>charge all the students for the costs, and then charge the taxpayers at |>the same time for the same costs. |>

| | College dorms are a classic example of cheap wiring that gets quickly | overloaded. There are many such stories. The culprits a few years | ago were refrigerators and cheap 1000 watt halogen lamps. Everyone | brought one in because they provided a good reading light and didn't | cost too much. The dorms wiring systems got overloaded and many | colleges banned these types of lamps. A few years ago the Home Depot | stores were offering a turn-in-your-1000-watt halogen-lamp and buy a | fluorescent at a discount promotion.

Where did all those 1000 watt halogen lamps end up? I'll take a dozen :-)

Why do maintenance when you can make the profit sheet look better and give the investors a larger dividend so the directors that won't fire you at will can stay voted in, thus keeping your job and locking in that extra

10,000,000 share boosted option at retirement.

I suspect it's a case of whether the utility was trying to tie into a

120/208 network or not. In many older cities, they would put a few substations in a medium rise building and tie the building network 120/208 into the 120/208 network under the streets.

The whole network system is designed to provide reliable power back when medium voltage transformers and cables weren't reliable. Now with the reliability of equipment, it's no longer needed.

In newer areas, there isn't a network, it's all radial distribution from each transformer. In that case it's likely they would be OK with 120/240 transformers.

There are several people warehouses (what I call the medium rise and high rise apartments) going in nearby. You have piqued my curiosity. I do know a utility guy that works for the supplying utility, I'll ask him about the service. I'm suspecting it's dry type transformers every couple of floors and we don't have a network in the city, it's all radial distribution.

| | I suspect it's a case of whether the utility was trying to tie into a | 120/208 network or not. In many older cities, they would put a few | substations in a medium rise building and tie the building network 120/208 | into the 120/208 network under the streets. | | The whole network system is designed to provide reliable power back when | medium voltage transformers and cables weren't reliable. Now with the | reliability of equipment, it's no longer needed. | | In newer areas, there isn't a network, it's all radial distribution from | each transformer. In that case it's likely they would be OK with 120/240 | transformers. | | There are several people warehouses (what I call the medium rise and high | rise apartments) going in nearby. You have piqued my curiosity. I do know | a utility guy that works for the supplying utility, I'll ask him about the | service. I'm suspecting it's dry type transformers every couple of floors | and we don't have a network in the city, it's all radial distribution.

Someone posted, perhaps here, a while back about some kind of issue with a residence customer of a utility in a network area in some city. But the network was, for some reason, 480Y/277. They ended up with 480 single phase going into the house and a dry transformer stepping that doen to the usual 120/240. I would think if it's a 480Y/277 network, they would have to do that, or something like it, for everyone. But I also recall in the post that it was a building that had zero exterior space for a transformer outside. At least that would help reduce the available fault current on the lower voltage side.

I also read in tariff document somewhere that Portland Oregon's downtown network is 216Y/125. Maybe that's just because some of the wiring is a bit long and that gives them a reserve to stay within the needed voltage range (they might end up seeing 200Y/115 at the far end).

By this you mean that multiple transformers have secondaries paralleled and the length between the farthest ends of a single 120/208V system can be quite substantial?

On Tue, 5 Dec 2006 14:48:30 +0000 (UTC) Michael Moroney wrote: | "Matthew Beasley" writes: | |>I suspect it's a case of whether the utility was trying to tie into a |>120/208 network or not. In many older cities, they would put a few |>substations in a medium rise building and tie the building network 120/208 |>into the 120/208 network under the streets. | |>The whole network system is designed to provide reliable power back when |>medium voltage transformers and cables weren't reliable. Now with the |>reliability of equipment, it's no longer needed. | | By this you mean that multiple transformers have secondaries paralleled | and the length between the farthest ends of a single 120/208V system can | be quite substantial?

I see two reasons for how things are wired in high density areas. Beyond the issue of the reliability that brought about the parallelism there was the lack of adequate single space just anywhere designed to put in a MV to LV station. Where space was available would not necessarily be where the capacity was needed. Connecting it all together with reliable equipment today could still be essential in city downtown areas.

Of course the largest buildings could have MV delivered through the core to upper floors with a MV to LV step down room every few floors. Anyone know what the voltage and current levels were going into the former World Trade Center towers (a likely high end example)?

Yes, from one end to the other the 120/208V network would be several miles.

Each street had 120/208V feeders running along it and the feeders were cross-connected at every intersection. The cross connects were protected by 'network protectors' which are short circuit protection only fuses. Customers would be fed through taps at each street, also protected by network protectors. All secondary work is done live.

Primary to secondary transformers would be placed where there was room. The transformers had 'network relays' on the secondary that would disconnect the transformer on reverse power flow. The network relay would reconnect power when the transformer output was re-energized. There would be multiple feeders to each area, and one of them could be de-energized without disrupting a customer. This allowed work on the transformers or primary feeders to be done without disrupting customers as long as it wasn't at peak demand.

Once buildings started growing the voltage drop and required feeder size to serve the building became a problem. This became even worse as electric elevators came into use because the elevator motors were on the roof, far from the service. The solution was to place a substation on the roof, and interconnect it with the network. Usually at least two feeders would be brought up and two transformers would be on the roof to allow continued operation with one feeder out. Because the secondary conductors were not short circuit protected, substantial fireproof ducts were required going up to the roof. All of the feeders and transformers are owned by the utility, and usually the transformer vaults are restricted to utility worker only access. Services tap off the feeders at each floor, with metering through current transformers. This allowed all of the meters to be centrally located. It also allowed one meter to record for multiple services on multiple floors if one tenant occupied more than one floor. As buildings began to grow taller, mid building substations became common.

Newer buildings began to switch over to 277/480V when 277 volt fluorescent lighting became available. This allowed the bulk power consumers of the lighting, elevators and HVAC to all be powered from 277/480V with only the wall outlets and other sundry devices to be powered through local stepdown transformers. To supply the 277/480V, 'spot' networks were overlaid the existing network. The spot networks usually fed a smaller area around the tallest buildings in a downtown area. The spot networks usually had their own feeders. Just like the 120/208 networks, the 277/480 networks usually ran up a building with substations on multiple floors.

I stumbled across the figure 13.8 kV at one time, on a webpage somewhere, but I have no idea where. I suppose you could estimate the usage as being the same for normal office space per sq. ft., but then you'd have to guesstimate the load from all those elevators, plus the transmitters in the one tower.

The former Digital Equipment "mill" had MV running around the complex, but the transformers it fed were on pads outside the buildings. There seemed to be two parallel circuits feeding each of them.

This reminds me: The smaller substations mostly seem to be fed from two circuits. Certainly this is done for redundancy, the lights can be kept on while one circuit is repaired or upgraded. But how are they 'normally' used, are they usually paralleled? For example, near me there is one substation fed by 115kV I think (not sure), which I'll call 'A'. From A there are two 69kV circuits feeding 'B', two 69kV circuits to 'C', and two circuits leave 'C', running parallel for a while, but they split and one goes to 'D' and the other to 'E'. There is a circuit between 'D' and 'E'. How would the loads normally be connected? Would 'B' normally have half its load on each circuit or would the two be connected together there? Would the line between D and E normally be dead? (D and E are small, one has one 69-13.8 transformer, the other has two. D and E are about equidistant from C)

The feeds would normally be paralleled and both in use. What you describe for C-D-E-C is a loop and is commonly used. A City may have one or more loops with local substations being fed from the loop. There is no point or cost savings in having one section of line out of service unless it is faulty or routine maintenance is needed.