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#65022 - 04/23/06 01:35 AM 208, 240, & 277 volt questions  
aldav53  Offline
Joined: Aug 2001
Posts: 545
Chandler, AZ USA
Why is 120/208 volt 3 phase used mostly in commercial buildings, and not 120/240v like residential single phase homes? (My question is not about the single or 3 phase, but about 208 vs 240v). What is the 3 phase 208v wild leg usually used for in commercial panels?
Also since romex (NM) is not allowed in commercial buildings, has anyone ever seen an application where 277/480v was used, using NM? NM insulation is good for 600v.

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#65023 - 04/23/06 06:46 AM Re: 208, 240, & 277 volt questions  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
3 phase is used because it pushes a large motor so very well. Also, for heavy loads, 3 phase gets you 1.73 times more power for 1 additional wire, thereby saving dollars on the cost of the conductors. Residential loads are not heavy and they have few large motors, so it is not economical to run 3 phase to homes.


#65024 - 04/23/06 06:59 AM Re: 208, 240, & 277 volt questions  
walrus  Offline
Joined: Jul 2002
Posts: 680
Bangor Me. USA
Since when is NM not allowed in commercial buildings?

#65025 - 04/23/06 07:20 AM Re: 208, 240, & 277 volt questions  
Steve McKinney  Offline
Joined: Apr 2004
Posts: 79
Marion, Illinois
120/208 is used mostly because of not only having the luxury of 3 phase for motor applications, but it also avails you to 3 seperate 120 volt circuits from the transformer secondary. If you are using 120/240, you would only have two, because it is a center-tapped single phase. Thanks to the connection of the "grounded" conductor, which would be your common on your three phase "wye" secondary, you have the opportunity for more circuits on a much more balanced load. The more balanced the load, the less you are asking of your neutral. Hope this helps.

#65026 - 04/23/06 11:26 AM Re: 208, 240, & 277 volt questions  
aldav53  Offline
Joined: Aug 2001
Posts: 545
Chandler, AZ USA
I know why they use 3 phase instead of single phase. I was wondering why they use 208v instead of 240v?

From as long as I can remember about 30 years doing elec work, NM is not used in commercial buildings. (except some rare "against the code" handy man work).

The Golden Rule - "The man with the gold makes the rule"

#65027 - 04/23/06 12:33 PM Re: 208, 240, & 277 volt questions  
NORCAL  Offline
Joined: Sep 2002
Posts: 874
Not to be a smart aleck, but if its against code please cite the code section(s)prohibiting the practice of using NM cable in commercial buildings.

#65028 - 04/23/06 12:36 PM Re: 208, 240, & 277 volt questions  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
"What is the 3 phase 208v wild leg usually used for in commercial panels?"

You are more confused than you realize. Let's assume you do understand the 240/120v 1ph system: a single transformer secondary winding with a grounded center tap. The two hots are condicered 180 deg. opposed to each other.

You could do the same thing with two separate 120v-secondary transformers: join one end of each (in phase!) and the junction becomes the center tap, which we ground. That gives us two points that each provide 120v to ground, and 240v between them.

A Y-system is basically the same thing, except we have three 120v-to-ground secondaries, all joined at one end, the grounded neutral. The hots are only 208 between one another, because of the 120-deg. phase angle.

A center-tap-grounded Delta system is actually one transformer connected exactly as a single-phase 240/120v transformer, plus two more, also 240v hot-to-hot, connected as an equilateral triangle; the Delta.

The phases are normally identified as A, B, and C, with A and C as the two ends of the center-tapped secondary, and the third hot as B, aka the "wild leg". A and C are each 120v to ground, but B is 208 to ground, so not useable for line-to-neutral loads.

An "open Delta" system omits one of the two extra transformers. It still works, but there is a bit less "stability" in keeping the voltage steady with varying loads; in other words, a greater source impedance.

The Y-system is mostly used where there is mostly line-to-neutral loads. The Delta is best for high-current 3-phase loads, and the open Delta (generally on the way out of use) is where there is a combination of large 240/120v 1-ph loads, with a bit of 3-phase loads.

The corner-grounded Delta is just what it says; a triangular connection with one phase grounded. It's used where only line-to-line loads are used, and no secondary has the center tap connected to anything.

Larry Fine
Fine Electric Co.

#65029 - 04/23/06 12:38 PM Re: 208, 240, & 277 volt questions  
JBD  Offline
Joined: Jul 2001
Posts: 599
On wye systems:
208V line-line gives (3) 120V line-neutral circuits
240V line-line gives (3) 136V line-neutral circuits.

On delta systems:
240V line-line gives (2) 120V line-neutral and (1) 208V line-neutral circuits.

So if your intent is to have as many 120V cicuits as possible then you want 208Y/120.

#65030 - 04/23/06 02:24 PM Re: 208, 240, & 277 volt questions  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
Great thread.

Can never know enough about transformers, how they work, and what applications they are used in. However, I'm still a bit confused about phase angles. I just finished a course on AC principles, bought a Mike Holt book about the subject, and I'm still confused. I know more about it now than I did 3 months ago for sure. It's difficult for me to understand the relationship between inductance and capicitance, and to apply them.

Thanks for all the insight.

[This message has been edited by ShockMe77 (edited 04-23-2006).]

#65031 - 04/23/06 06:27 PM Re: 208, 240, & 277 volt questions  
Obsaleet  Offline
Joined: Apr 2003
Posts: 362
Larry, Nice job explaining those differences
may be using some of that, if I can with some of my guys. Thanks!


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