confusing voltage measurement

[pipedown]

I was trying to use what I thought was a 220V receptacle at work but the voltage was only 208V. When measuring, I see 120V between each hot and neutral but only 208 V between the hot leads.

Am I really looking at two legs of a three phase Wye connected branch circuit as opposed to a 220V single phase supply (common in residential). Is this a common practice in commercial buildings? Should the round twist lock style plug be telling me something? (nothing like a range or dryer plug)

Does that sound right or do I need to open the panel and trace it back to figure it out. These receptacles are leftover from the previous tenant so their original purpose is lost to me. They are protected by a regular two pole breaker (with tie bar).

[lee houston]

Yes lee h

twa

[Doug Miller]

That's exactly what you'd expect to see at a 208V 3ph receptacle...

That would be my first guess...

Yes.

Not necessarily. The NEMA L6-series locking plugs are used on 208V and 240V circuits.

Simply looking at the panel may be enough to explain the situation. Is it a

3-phase panel? Hint: if you don't know how to tell the difference, maybe you should leave this alone.

[Beachcomber]

The company's electric bill should tell you if you are being supplied with 3-phase service. This is typical in commercial buildings that have a lot of medium to heavy-duty motors such as for elevators, larger-size air conditioners, etc. 120/208 volts is standard for a Y connected, three-phase secondary.

This is also a common voltage in mult-dwelling residential units (condos and apartments) as one economical three-phase transformer can supply all of the power, lighting, and electro-motive needs of the building.

If the service entrance outdoors is overhead and you see 2 or 3 transformers on the pole, you can almost be certain that this is so. If you have a pad (ground) mounted transfomer, odds are that it is a 3 phase transformer with this voltage configuration.

For single phase circuits....

As opposed to the more commonly used 240V, 208 V. is sometimes a problem with electric clothes dryers, electric ranges (often special heating elements can be obtained for this voltage by special order). whirlpool spas and air conditioners (many require 240V).

So the tradeoff is that you get 3-phase power with convenient 120V phase-to-ground voltage, all with just one 3 phase transformer (or 2 or 3 single phase transfomers) but with the slightly less desirable

208 Volt phase-to-phase voltage...

instead of the 240 volts available as the higher standard voltage in the split phase 3 wire 120/240 single phase system.

Beachcomber