ECN Forum Forums General Topics of the Trade General Discussion Area Delta & 1Ph Load

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?

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.

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Charlie Eldridge, Indianapolis, Utility Power Guy

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.

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.

Thanks for the replies.

I misunderstood the question. You know, that is the first mistake I have made this year.

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.

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Charlie Eldridge, Indianapolis, Utility Power Guy

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.

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

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, I would be interested in the reference in the Code that prohibits that installation, I have never been able to find the reference.

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Charlie Eldridge, Indianapolis, Utility Power Guy

The single poles are rated that way because of people who use two of them to obtain 240v.

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