3 phase Vrs 1 Phase Power

In terms of utility bill, would a three phase power cost more than a single phase? Considering all loads equal.

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Kissi Asiedu wrote:

Energy is energy. So it should cost the utility the same at the generator. Three phase motors are somewhat more efficient, so they will cost you somewhat less to run. But, until recently, having a three phase motor (or other load) meant having a three phase service (more service equipment, more expensive) and the utility will have to provide three phase lines to the customer (higher capital costs).
Recently, with the advent of cheap solid state controls, it has become possible to build appliances with three phase motors, driven by variable frequency drives, supplied by a single phase supply. So now we can have the best of all worlds.
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Kissi Asiedu wrote:

It depends on too many things to say. In my area three phase is not even an option for residences but you can get it in some agricultural communities and I've heard that in parts of Europe residential 3 phase is common. Here you only find it in commercial and industrial sites.
Three phase requires more complex gear to supply but the rate structure is up to the utility.
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An Edison system already has three full sized conductors. One more conductor, is all that is necessary for three phase.
Bill
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Salmon Egg wrote:

One more conductor means two additional windings on the transformer, sometimes two entire transformers, a more complex meter, more bus bars in the service panel, one more pole on all the breakers. It's quite a bit more complex.
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It is true that in Europe residential 3 phase is common. It is common for example in Norhern Europe. I live in Finland and here residential 3 phase is very common, the default on any new installation.

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Each EU country seems to have a single phase limit, i.e. the max current you are allowed on a single phase supply before you have to take a 3-phase supply (for which there is no limit).
In the UK, the single phase limit is 100A, which is enough for most homes, so 3-phase into the home isn't very common. You can ask for it though as it runs past in the street, and it will be forced on you if you would otherwise need to upgrade a single phase supply above 100A. In some other EU countries, the single phase limit is as low as 20A, in which case just about everyone has a 3-phase supply.
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Standard service in North America is 200A 240V split single phase. Larger houses occasionally have 400A with twin panels, particularly if they were built in an area without natural gas for heat, and I've seen as large as 600A with 3 panels in a very large house, all of that single phase. It would be nice if 3 phase were standard but at least now inverter drives for motors have become so cheap that we can have most of the advantages of 3 phase power regardless of what comes into the place.
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For comparision in Finland the standard for normal house or bigger aparment seems to be 3x25A. The three phase voltage here is 230/400V (230V from neutral to phase, 400V between phases).
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Tomi Holger Engdahl wrote:

Are all three phases available for loads like motors, or are the branch circuits just one phase? It's always interesting to see how this stuff is done in other places, a friend in England sent me a bunch of pictures of the electrical system in his house and I was surprised to see how different it is from what we do here.
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The three phases go the the main distribution panel. Then branch circuits are wired from that as needed. Normally there are three phase branch circuits and single phase branch circuits. Three phase branch circuits are used for heavy loads (stove, sauna, water heater) and motor loads (more common in industrial applications than homes). Single phase branch circuits are the most common for most normal uses like normal electrical outlets, lights etc...

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Oh, you can get three phases in just about ALL "communities." At least along the major roads or wherever the power company locates its feeders.
Likewise, 3 phases are available for half or so of most important roads.
But the "side roads" only have 3 phases until the next road branch. Then, typically, 2 phases go down the more important branch, and one phase goes down the spur.
The same thing is true is more suburban communities and even the distribution system is moderate density cities. LARGE apartment buildings often have 3 phases with each unit getting 2 phases (120/208 service). "Garden" apartments typically get a single phase for each section (120/240 service.)
Important/large new developments (hospitals, larger schools, shopping centers) can justify new 3 phase wiring to the location regardless of the prior situation.

The utility business is separating the power generation business from the distribution business. Even some of the distribtion business is separated with one company running the really high voltage ties and another handlilng lower voltage distribution.
Any particular "utility" company is likely moving toward not owning both power and distribution in the same market area. It makes it easier to negotiate rates.
But where two distribution companies interchange or power generation companies meet distribution companies, everyone in the deal perfers nicely "balanced" loads. Thus, there is a push to get larger customers to present a balanced load so that the utility doesn't have to mix and match customers to what phases they are connnected to and in what order.
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In the area where I live, almost everyone lives on "side roads" that branch off the main arterial, nobody wants their driveway to connect to a busy through-way. Houses are clustered in these neighborhoods, each of which is supplied by a single phase branching off down at the main road. I've never seen or heard of a residential 3 phase installation here.

That is common, the apartment complexes are downtown where everything else is 3 phase too. The single family homes are in the hills and valleys surrounding and between towns.
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--------- Essentially going back about 30-60 years when this was normal- advantages to both utility and customer arose due to the utility being able to predict future demand reasonably well I.e. deregulation in the power industry, a relatively recent phenomena has, in most situations, increased cost to the consumer because it encourages short term economic planning because of uncertainty of future loads with multiple suppliers in a region. ------------------

Nothing has changed in this area. large customers will have enough load to take advantage of the actual simplicity of 3 phase and it is also to the customer's benefit to have balanced loads. In addition, large customers generally have larger motor loads and single phase motors simply don't cut the mustard compared to polyphase machines in terms of simplicity, performance and also with regard to harmonics when electronic control is used.
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There is somewhat additional investment in providing three phase. Single phase motors will not be as efficient as three phase motors. In small installation, it is probably simple and cheaper to just use single phase motor, especially for units under one horsepower.
Bill
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wrote:

----- And miss all that comes in between birth and old age (where I am now)?
Your probable estimate is right. The economics involved, along with the history of supply development as well as mass manufacturing has meant that fractional single phase HP motors are cheaper than 3 phase motors. Try buying a 3 phase 1/4HP motor. If you can find it, it will be more expensive than a single phase equivalent
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snipped-for-privacy@shawcross.ca wrote:

I couldn't find a 1/4HP, but I've bought two 1/3HP 3 phase motors recently and they were dirt cheap, of course they were surplus.
I suspect fractional HP 3 phase motors will get a lot more common over the next decade or so as the inverters to drive them from single phase have become so cheap and are getting cheaper still all the time.
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On Tue, 08 Jun 2010 09:48:49 -0700, James Sweet

The flip side of that is the computer industry used to use a lot of fractional HP 3 phase motors (probably where yours came from) and now they don't so it may actually make them get harder to find.
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I doubt they'll get too hard to find, they're showing up in consumer white goods now. The biggest competition they have is the permanent magnet brushless motor, another 3 phase machine that can be used in similar applications. Switched reluctance motors are also fairly common, harder to drive but as with the others, the electronics keep getting better and cheaper.
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wrote:

------------------------------- That may be but it is likely just as economical to go to brushless DC in this range as is the case with many applications that now exist- electronics no more expensive- possibly cheaper in that there is no need for frequency dependent voltage control and likely little difference in manufacturing costs. The comparison that would then be valid is that between brushless DC and inverter driven induction machines in terms of performance and cost.
Interesting times. Cheers,
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