Supplied Voltage -- 208 vs. 240

wrote:


For new construction and/or power installation, you can have anything you are willing to pay for. My business is located in a light industrial area and the standard supply is 208/3. To get 480, we have a BIG transformer. The primary transformer feeding our building is rated 208/3 at 215 KVA. Our suite has three meters feeding power to us at 200 amp each. The rest of the building has a single 200 amp meter for each suite. The HID lights are fed off the same circuit, but are only single phase.
We are located in Central Oregon. That power design is typical of all but so called heavy industrial zoning.
Paul
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Really high voltages are used to generate, transmit and use in large equipment. Here, I will only cover the "low voltage end of the spectrum, because that is what you want to know about. And, oh yeh, we are talking about the USA, not Europe, etc..
208 volts is what you get across one phase of a "Wye" connected 3 phase system. This is usually a 4 wire (plus protective ground) system. The nice thing about it is that if you go from any phase lead to the neutral lead, you automatically get 120 volts. So, if you are in an industrial situation, where you have a lot of 3 phase running around, it is easy to get 120 volts for general appliances. A "Delta" configured 3 phase system has only 3 wires (plus protective ground. The voltage across any two phases is 240. There is no nuetral, so there is no way to get 120 volts without using a transformer.
That's industrial and power distribution stuff.
For most residential power, the power company may use moderate voltage delta or wye on the poles that go by your house. Often, they send only one phase of "whatever it is" to each part of a community. Out here in the country, it is 14,400 volts. In a suburb where I once lived, it was 7800 volts.
Now for the answer to your question (as I see it, anyway): The "usual" thing they do (residential) is to use a transformer on the pole to reduce the line to 240 volts. They put a center tap in the transformer and that becomes the neutral (white wire) in your house. You get 3 wires coming into your house: 2 "Hot wires", that are black and one neutral wire that is white. The voltage across the 2 hot wires is 240 and the voltage across either hot wire and the neutral wire is 120 volts.
So, unless you have 3 phase power somewhere in the building where you are, you are unlikely to have 208 volts available. I say it this way because, if you work in an NYC skyscraper, etc., anything is possible.
Last disclaimer: These voltages are all approximate. The last I remember was that "120" is really meant to be: 117 vac +/- 10%, or 105 volts to 129. The same tolerance would apply to the 208 and the 240.
I hope this helps, Pete Stanaitis
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Existential Angst wrote:

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This was brought up once b4. IIRC the story was voltage started out at 110, went to 115, then went to 117 and is now at 120 to get more power through the exsisting wire network.
See my reply to gunner, my power company goes by +/- 5%
Thank You, Randy
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We used to has some weird wild leg delta 240 3 phase here years ago, but it all has been replaced with 208 3 phase. Fargo, ND here. Seems to ne that 208 three phase is the current trend, unless you want 480 3 phase. Greg
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Existential Angst wrote:

120/208 Wye service is common in office and apartment buildings in the US, as you can run heavy appliances such as ovens, clothes dryers, air conditioners off the 208 (sometimes needs an autotransformer if they don't like 208 and require 230) and you get 120 V from line to neutral.
it is EXTREMELY, EXTREMELY uncommon in single-family residential communities. For instance, we have single-phase 7200 V distribution on my street, with the 7200 V 3-phase a block away. I know of two houses on my way to work that have 3-phase service. I can tell because they have 3 25 KVA pole pigs on their pole. These are BIIIIG houses, one was featured in the paper some years ago, and is 14000 sq ft. In many areas, the power company will flatly refuse to install 3-phase to a residence. I don't exactly know the reason for that, it may have to do with state utility tariff stuff and the requirement that 3-phase has peak demand metering and residential is net rate metering.
Jon
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