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#44144 10/28/04 10:03 PM
Joined: Jan 2002
Posts: 324
A
Member
The open delta thread brought this question to mind.

Is it wise to land one leg of a double pole breakers on the B phase for common loads such as water heaters, motors or HID lighting as long as your breaker has a 240 volt rating?

Or are double poles only rated in slash ratings?

#44145 10/28/04 10:12 PM
Joined: Apr 2004
Posts: 201
Member
No. The voltage furnished by the electric utility is 120/240 volts, 3 phase, 4 wire and does not include the 208 volts that you are wanting to use. If you use it, the voltage is sometimes a bit screwy and we don't care about that voltage. Some electric utilities even prohibit loads being connected in that manner. [Linked Image]

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Charlie Eldridge, Indianapolis, Utility Power Guy


Charlie Eldridge, Indianapolis Utility Power Guy
#44146 10/28/04 10:41 PM
Joined: Jan 2004
Posts: 615
J
Member
huh?

what if the voltage you are looking for is 240V, then a 2-pole breaker on A & B phase would be 240V. What is the problem with that? I am not sure I follow. Charlie, are you talking about using a single pole breaker on the high phase ("wild leg") that is prohibitted?

I would say it wise to use a 2-pole breaker that way, it leaves you more 120V spaces open for your single pole breakers.

#44147 10/28/04 10:58 PM
Joined: Aug 2003
Posts: 1,374
R
Moderator
If you have a corner grounded delta and have a two pole breaker that lands on phase B, it must be straight rated (240) and not slash rated (120/240).

This is due to the fact that a ground fault on phase A or phase C is 120 volt, but a ground fault on phase B is 208, which is in excess of the individual pole rating of the slash rated breaker. See 240.85

To answer the second part of the question, no, not all 2 pole breakers are straight rated. In fact, most are not. Last time I saw, a straight rated breaker costes about 5 times as much as a slash rated.


Ryan Jackson,
Salt Lake City
#44148 10/28/04 11:38 PM
Joined: Jan 2002
Posts: 324
A
Member
Thanks for the replies.

#44149 10/29/04 06:45 AM
Joined: Apr 2004
Posts: 201
Member
I misunderstood the question. You know, that is the first mistake I have made this year. [Linked Image]

I thought the question was to go from the grounded conductor to the high phase with the circuit to get 208 volts and that is the way I answered it. Sorry. [Linked Image]

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Charlie Eldridge, Indianapolis, Utility Power Guy


Charlie Eldridge, Indianapolis Utility Power Guy
#44150 10/29/04 07:56 AM
Joined: Jan 2004
Posts: 615
J
Member
Ryan, are sure that's how the slash rating is to be read, I have always understood the slash rating to expand the use of these breakers, not limit them. Where can I find more info about slash rating, and why is a single pole QO slash rated? (as I type MW branch circuit comes to mind.)

Also a phase to phase short would be a 240V short with a more likely lower impedance path. How is it a breaker could be designed to handle that, yet somehow a lower voltage and likely lower energy fault to ground be beyond the breaker's capacity and rating?

no sweat Charlie, at that pace it should be your only mistake this year. [Linked Image]

[This message has been edited by Jps1006 (edited 10-29-2004).]

#44151 10/29/04 08:18 AM
Joined: Apr 2004
Posts: 201
Member
Ryan's correct for circuits that include the B phase, read 240.85. Of course the single phase circuits that only use the A and C phases can use the slash rated circuit breakers since they are using just a single phase transformer. Since we serve a lot of residential customers with open delta banks (we use the three phase to serve a small business that is next to the residential area), the residential customers would all have to use full rated circuit breakers if that wasn't permitted.

By the way,
Quote
Charlie, are you talking about using a single pole breaker on the high phase ("wild leg") that is prohibited?
I would be interested in the reference in the Code that prohibits that installation, I have never been able to find the reference. [Linked Image]

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Charlie Eldridge, Indianapolis, Utility Power Guy


Charlie Eldridge, Indianapolis Utility Power Guy
#44152 10/29/04 08:23 AM
Joined: Oct 2001
Posts: 172
W
Member
The single poles are rated that way because of people who use two of them to obtain 240v.

#44153 10/29/04 09:03 AM
Joined: Aug 2003
Posts: 1,374
R
Moderator
Jps: If you have a minute, download this presentation from Bussmann. It is about 2.5 Megabytes, but is well worth it, even if you have a dial up connection. You might want to right click the link and select "save target".

http://www.bussmann.com/services/training/docs/240-85Pres.ppt


Ryan Jackson,
Salt Lake City

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