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#136774 - 05/05/03 05:20 AM Rated voltages in the USA?
swiss Offline
Junior Member

Registered: 04/30/03
Posts: 4
Loc: Zurich
I've got a question about the rated voltages in the USA. I know that there is normally a voltage about 120VAC/60Hz (single phase). Is there another common single phase voltage? How about 250VAC/60Hz? Is it common? Do I need a separate transformator to generate this voltage?

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#136775 - 05/05/03 06:06 AM Re: Rated voltages in the USA?
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden

You are our first member from Switzerland. To answer your question: Yes there is a higher single phase voltage, 240V (60Hz). Unlike the European 230/400V system the 240V is in the USA 120/240V. It is usually called a center tapped single phase system. To get 240V single phase you simply connect the load between the two "hot" ("phase") wires. Available in almost all American homes.

For industrial use there is also a 277/480V system. Works just like our 230/400V system. (Note: Canada uses 347/600V instead)

It can be noted that apart from the frequency, the American 240V (almost) falls within the European 230V specifications. (Min 220V max 254V compared to 207-253V)

#136776 - 05/05/03 06:11 AM Re: Rated voltages in the USA?
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
I better point out that getting 240V in an American home is about as difficult as getting 400V in a Swiss home. Usually only the cooker, dryer and a few other heavy appliances use it. You won't find 240V sockets in ordinary rooms.

#136777 - 05/05/03 08:42 AM Re: Rated voltages in the USA?
SvenNYC Offline

Registered: 08/19/02
Posts: 1685
Loc: New York City
Some air conditioners use 240-volts (15 and 20-amp dedicated circuits) and I occasionally see houses and apartments with these outlets installed. Usually though they're in commercial establishments though.

The 15-amp/240 volt American socket looks like this:

240volt /20 amp:

The 20 amp socket can accept 15-amp 240 volt caps.

There's also this 30-amp configuration (looks like the 15-amp but it's much larger) - probably used for air conditioners in stores:

I've also seen this dual-voltage outlet (120/240 volts 20 amps) in some places

[This message has been edited by SvenNYC (edited 05-05-2003).]

#136778 - 05/05/03 02:53 PM Re: Rated voltages in the USA?
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Paydirt! I stumbled across a {sort of “official” AND on-line} chart with the common lower US voltages and their acceptable ranges. The big kahoona is ANSI C84.1-1995, but you have to pay money for a copy of that one. page 6.

[Sven, your bottom "crowfoot" picture is officially rated “125/250-volt non-grounding," but for window air-conditioning circuits the third blade was once common for grounding 230V metal appliance frames.]

For the sake of other readers, Sven’s top two pics are the current “code” versions, termed 250-volt [not 125/250] grounding style. Note that the “T-slot” opening in the second photo is to allow insertion of 15- or 20-ampere plugs. {The plug pins are not T-shaped, but a flat blade either positioned vertically for 20 amp, and horizontally for 15 amp.}

#136779 - 05/05/03 06:12 PM Re: Rated voltages in the USA?
djk Offline

Registered: 12/26/02
Posts: 1269
Loc: Ireland
The 30 amp version looks remarkably like BS 1363 turned upside down with a rounded ground pin.

The crow foot design would be leathal if you plugged an australian applience in?

125/250 = 2L + 1N ???

#136780 - 05/05/03 09:01 PM Re: Rated voltages in the USA?
Hutch Offline

Registered: 05/27/02
Posts: 383
Loc: South Oxfordshire, UK
Doesn't quite fit. I've tried it

#136781 - 05/05/03 10:52 PM Re: Rated voltages in the USA?
frenchelectrican Offline


Registered: 02/06/03
Posts: 938
Loc: Wi/ Paris France { France for ...
also i will add here about usa voltage on single phase is 120/208 that is off from the three phase network and few home and alot of large apartments use that voltage and it is very common to find it in commercal appcation my shop have mixed bag of voltages both single and three phase whatever you got it i have it in my shop from 120 all way to 600 ( the 600 volts is kicked up with transfomer NOT autotransfomer!) and most 240 volts single phase plugs can use on 208 volts also but somehow it will be marked down to beaware of 208 system and to find 208 appliances of course they do make it and alot of heating equempent do run on 208 and few other stuff it kinda pretty long list to type for me but a quick point here about 240 and 208 volts single phase they are line to line not line to nuetural just let you know about that and i am not talking about delta at all in case this is single phase from three phase wye system

merci marc

ps if get confused i will add the link to the tech centere to show the digram
Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

#136782 - 05/06/03 01:21 AM Re: Rated voltages in the USA?
swiss Offline
Junior Member

Registered: 04/30/03
Posts: 4
Loc: Zurich
Thanks to everybody for your inputs.
208V and 240V are the same voltages if i understood that correctly!?!
Our company is developing micro computed tomography systems for laboratories (Universities and Hospitals).
The newest system has about 2.5kW. We are quit not sure how to connect the system.

#136783 - 05/06/03 04:14 AM Re: Rated voltages in the USA?
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Guten Tag Swiss, und Wilkommen!

The 208 and 240V ratings are not the same. Some appliances, such as baseboard heaters, may be dual-rated for use on either system. For some other devices, e.g. motors, the difference is large enough that it could be a problem. I think that our friends in the U.S. sometimes use small buck/boost transformers where a 240V device needs to run on 208V or vice versa.

The 208V rating is the nominal phase-to-phase voltage of a 120/208 wye system.

Most residential single-phase services in the U.S. are 3-wire, with the center-tap of the transformer secondary grounded and brought into the house as the neutral, giving 120V from either line to neutral and 240V between lines.

240V 3-phase delta arrangements also exist in commercial services.

One peculiarly American system is the "4-wire delta" arrangement. It starts as a basic 240V delta service, but you then add a ground to the center-point of one transformer winding and bring it in as a neutral. This provides 120V for small loads between two of the phases and neutral, while still leaving 240V between any two phases. The third phase ends up at 208V to ground.

As far as the practical aspect of wiring your power supplies is concerned, for a single-phase load of 2.5kW you are well within the rating of a 15A 240V receptacle/plug combination (the first image in Sven's post above).

Just remember that because the 240V American supply has both legs "hot", if you have switching, fusing, or circuit-breaker protection on the input, then you should make these devices double-pole.

By the way, the official NEMA designation of the 240V 15A plug is type 6-15. They're available from Leviton, Hubbell, or any of the other main U.S. manufacturers.

[This message has been edited by pauluk (edited 05-06-2003).]

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