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#100083 - 10/16/06 11:05 AM 3 phase vs single  
Reel-Break  Offline
Member
Joined: Apr 2002
Posts: 178
nc
When building a system when would you choose 3 ph over single or vice versa. We`re putting in some equipment into a textile plant where they have 3 phase 600 volt I`ve noticed some three phase 600 to 120/240 transformers and some single phase 600/120/240.We`re installing a small panel with mostly 120 volt loads for small equipment and some lighting. Thanks in advance.


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#100084 - 10/16/06 11:29 AM Re: 3 phase vs single  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
Generally speaking, use 3 phase whenever it is available. For the cost of one additional conductor, you increase the capacity of the panel by a factor of 1.73(over single phase).

But, if you consider the cost of the minimum size allowable panel (often 100 amps) versus the cost of a 3 phase 100 amp panel, AND, if the total load to be used can be handled by the single phase 100 amp panel, then that is the most economical choice. (As is the case in residential services.)

In your case, the industrial load may not be sufficient to warrant the additional cost. The question you need to ask is: Who is paying the bills, and do we want to plan for any future loads?


Earl

#100085 - 10/16/06 02:39 PM Re: 3 phase vs single  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Unless you need the 3 phase for some equipment there is no point in having a "120/240/" 3 phase system. I say that because you will not be able to use the high leg for anything on 120v. or 240v. single phase. If you have a 208/120 3 phase you could use all three legs.

And since I'm being picky the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.

Finger don't tipe rite. [Linked Image]



[This message has been edited by George Little (edited 10-16-2006).]


George Little

#100086 - 10/16/06 06:03 PM Re: 3 phase vs single  
ShockMe77  Offline
Member
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
Quote
the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.


See that? I learn something new everyday here.

Thanks!


#100087 - 10/16/06 06:16 PM Re: 3 phase vs single  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
The basic, and informal, method is:
On single phase list the low voltage first, like: 120/240 and 120/208

On three phase list the high voltage first, like: 480/277, 240/120, and 208/120

A letter Y is usually added for wye systems - 208Y/120 or 480Y/277.


#100088 - 10/21/06 05:58 AM Re: 3 phase vs single  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.


Any idea how far back this convention goes?


#100089 - 10/21/06 07:38 AM Re: 3 phase vs single  
Eddy Current  Offline
Member
Joined: Sep 2006
Posts: 111
Ontario,Canada
240/120 3 phase? I always thought you can only get 208/120 from 3 phase?


#100090 - 10/21/06 08:53 AM Re: 3 phase vs single  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Eddy, the 240/120 3 phase is a Delta Configuration of windings with one winding center tapped for the grounded conductor whereas the Wye configuration is tapped at the end of three windings.

This is why only two legs/phases of the Delta will give you 120 and all three phases of the Wye will give you 120.

Scott has some good information and drawings of these transformer configurations in the Technical area.

Roger


#100091 - 11/01/06 06:41 PM Re: 3 phase vs single  
EASports  Offline
Junior Member
Joined: Oct 2006
Posts: 4
Hutchinson, KS
"Unless you need the 3 phase for some equipment there is no point in having a "120/240/" 3 phase system. I say that because you will not be able to use the high leg for anything on 120v. or 240v. single phase."

This is partly true. With a high leg delta system you have 240V between any two phases, so you could use the high leg and one other to run a 240V single phase appliance (if the breaker is rated to handle the high leg voltage).

[This message has been edited by EASports (edited 11-01-2006).]


#100092 - 11/01/06 07:06 PM Re: 3 phase vs single  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
EASports- Be careful-
Quote
This is partly true. With a high leg delta system you have 240V between any two phases, so you could use the high leg and one other to run a 240V single phase appliance.

You need to look at 240.85, especially the latter part of the text. Unless that breaker you are referring to has a straight 240v. rating, you would not be able to use it on a single phase 240v. circuit using the high leg. Some breakers have a slash rating- 120/240 and they could not be used on the high leg of a 3Ø 240/120v panel if they are accross the high leg. Common violation.

[This message has been edited by George Little (edited 11-01-2006).]


George Little

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