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Old 06-01-2013, 11:08 AM   #1
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Using 208v "B" phase in open delta?


Curious if anyone has done this. I have three phase 4-wire open delta to my building. I have some computer servers that can utilize 208V single phase. Wondering if I could use a single pole 208v breaker (http://www.tripplite.com/en/products...xtModelID=4543) to run the equipment.

Is this allowed and would it work?

Reason I ask is that while I have 3 phase 600 amp run to the outside of the building, the inside service panels are 2 200 ample single phase panels as I have no 3 phase loads. Both panels are reaching their 200 amp max capacity. I was wondering since the "B" phase is currently not being used if I could switch out one of the single phase 200 amp panels for a three phase 200 amp and the reload that panel with my single phase loads as before. However, I could then take advantage of the "B" phase for 208v loads.

Clear as mud??

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Old 06-01-2013, 11:51 AM   #2
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Using 208v "B" phase in open delta?


If you have an open delta, you do not have 208 volts.
you have 240 v phase to phase, and the high leg 180(plus or minus) to ground

208 is always 2 legs and not single pole.

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Old 06-01-2013, 11:57 AM   #3
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Using 208v "B" phase in open delta?


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Originally Posted by jbfan View Post
If you have an open delta, you do not have 208 volts.
you have 240 v phase to phase, and the high leg 180(plus or minus) to ground

208 is always 2 legs and not single pole.
In an open delta? I went out to the meter box and put a volt meter on all three legs plus netural. Here is the readout for L-N:

Phase A -> Neutral = 120V
Phase B -> Neutral = 208 V
Phase C -> Neutral = 120V

Here is the readout for L-L:
Phase A -> B = 240V
Phase A -> C = 240V
Phase B -> C = 240V
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Old 06-01-2013, 12:32 PM   #4
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Using 208v "B" phase in open delta?


There is a pretty good description of a two transformer, high leg delta service (I believe this is identical to what you are calling an open delta service) in Wikipedia, see http://en.wikipedia.org/wiki/High-leg_delta.

The article clearly states that in such a service, you would measure 240V phase to phase for all three phases, and two of the legs would measure 120V phase to ground, while the third leg would measure 120V x SQRT(3) = 208V phase to ground. This sounds exactly like what you have.

I do not know if it is permissible to put in a 208V high leg to ground breaker, perhaps one of the electricians on this forum has personal experience with this question. It would certainly make sense, else why would such a service be available? Presumably one of the advantages of this type of service is that you get three available voltages, 240V three phase, 120V single phase, and 208V single phase.
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Old 06-01-2013, 12:41 PM   #5
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Using 208v "B" phase in open delta?


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Originally Posted by jbfan View Post
If you have an open delta, you do not have 208 volts.
you have 240 v phase to phase, and the high leg 180(plus or minus) to ground

208 is always 2 legs and not single pole.
High leg to ground is 208.
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Old 06-01-2013, 12:41 PM   #6
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Using 208v "B" phase in open delta?


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Originally Posted by Daniel Holzman View Post
There is a pretty good description of a two transformer, high leg delta service (I believe this is identical to what you are calling an open delta service) in Wikipedia, see http://en.wikipedia.org/wiki/High-leg_delta.

The article clearly states that in such a service, you would measure 240V phase to phase for all three phases, and two of the legs would measure 120V phase to ground, while the third leg would measure 120V x SQRT(3) = 208V phase to ground. This sounds exactly like what you have.

I do not know if it is permissible to put in a 208V high leg to ground breaker, perhaps one of the electricians on this forum has personal experience with this question. It would certainly make sense, else why would such a service be available? Presumably one of the advantages of this type of service is that you get three available voltages, 240V three phase, 120V single phase, and 208V single phase.
This is exactly what I have. I guess I had the terminology wrong. High Leg delta I should have called it. Yes, the pole only has two transformers on it. Three wires come out of one transformer to my building while a single comes out of the other.

I have an electrician coming out Monday so I plan on asking him the same questions. I'm just trying to solicit as many opinions as possible.
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Old 06-01-2013, 12:46 PM   #7
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Using 208v "B" phase in open delta?


As a matter of good practice, the high leg should not be used for single phase loads. I'm not aware of a code issue with this, though. There may be a problem with this. If your equipment is made for 208V, it is almost certainly intended for two hot legs, each 120V to ground, with 208V between them - two phases of a 120/208V service. It is not intended for one leg to be grounded and the other to be 208V to ground. The phase-to-ground voltage is higher than it's expecting, even though the phase-to-phase voltage is correct. Are you sure the equipment won't run on 240V? Most 208V single phase equipment is rated 208-240V, so it's usable on different service types.
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Old 06-01-2013, 01:05 PM   #8
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Using 208v "B" phase in open delta?


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As a matter of good practice, the high leg should not be used for single phase loads. I'm not aware of a code issue with this, though. There may be a problem with this. If your equipment is made for 208V, it is almost certainly intended for two hot legs, each 120V to ground, with 208V between them - two phases of a 120/208V service. It is not intended for one leg to be grounded and the other to be 208V to ground. The phase-to-ground voltage is higher than it's expecting, even though the phase-to-phase voltage is correct. Are you sure the equipment won't run on 240V? Most 208V single phase equipment is rated 208-240V, so it's usable on different service types.
It's an active PFC power supply that is intended for single phase service. It can accept 110V - 250V but single phase only. So I was "thinking" that I could take advantage of the unused "B" phase at 208. The main problem is that I have only found one single pole 208 breaker which is made by Cutler Hammer. I prefer to have Square D and I don't believe they make a breaker like this.

http://www.tripplite.com/en/products...xtModelID=4543

I currently run these PSU at 240. However, I am needing to add several more of them but my two 200 amp panels are basically close to max capacity (not space wise but Amp wise) hence the idea of using the unused phase.

I could be wrong and it won't work. I figured I would ask around before I did anything. Even if this is idea doesn't pan out I will still probably go ahead and have one of the 200 amp single phase panels ripped out and replaced with a 3 phase panel. It is a shame I have 3 phase service yet can't take advantage of it currently with this setup.

Last edited by Biggen; 06-01-2013 at 01:09 PM.
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Old 06-01-2013, 01:10 PM   #9
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Using 208v "B" phase in open delta?


In a typical 240 volt high leg (120/240 volt delta) 3 phase service, the high leg is not intended to supply as many amperes as each of the other two legs. So if you draw a significant 208 volt high leg to neutral load, you could overload the high leg without realizing it.
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Old 06-01-2013, 01:13 PM   #10
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Using 208v "B" phase in open delta?


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In a typical 240 volt high leg (120/240 volt delta) 3 phase service, the high leg is not intended to supply as many amperes as each of the other two legs. So if you draw a significant 208 volt high leg to neutral load, you could overload the high leg without realizing it.
Ahh, ok. It would be approx. 50-60 amps at 208.

I need to give my Poco engineering division a call and see what they say.
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Old 06-01-2013, 02:55 PM   #11
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Using 208v "B" phase in open delta?


So this thread raises a question in my mind. If you want to use the 208V high leg to neutral, do you need a single pole or two pole breaker? This is effectively a single phase service, since you are connecting phase to neutral, however it may be necessary under code to interrupt the neutral as well as the hot leg, which would require a two pole breaker.
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Old 06-01-2013, 05:05 PM   #12
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Using 208v "B" phase in open delta?


I guess the most obvious question is what is the high leg for if you can't use it for single phase? Is it just the "nature of the beast" that it exists and can't be used?

I have bee doing a lot of reading on Wye and Delta designs. Most everyone recommends to balance single phase loads across all the phases. However, if you have a high leg delta service you can't balance the loads across the three phases. Only two can be used.

I do like the fact that with my high leg delta I can run 240V single phase loads at 240V. The Wye design would only run them at 208V. Guess pros and cons with each design.
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Old 06-01-2013, 10:11 PM   #13
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Using 208v "B" phase in open delta?


The other issue it will come up with High leg system is the breaker slection on wild leg due the line / netural format is not a common mode at all so if you have to use two pole breaker it will have to be soild rated not the slashed ( / ) rating what the common two pole breakers are.

The soild rated breakers are not super common and I really doubt any big box store will stock this. most case electrique supply centre will have this or order one for ya.

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Old 06-02-2013, 06:15 PM   #14
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Using 208v "B" phase in open delta?


In the panel you would have to use a single breaker rated for at least 208 volts (actual rating would probably be 240 or 250 volts).

It would snap into the panel in the same fashion as a breaker for a 120 volt circuit.

You would not be able to switch the other leg going to the 208 volt appliance using a panel breaker because a double breaker would connect with hot fins underneath and not have a terminal to connect to the neutral bus bar.

I do not see any reason for interrupting the neutral other than a code requirement for a specific city. When the single breaker is flipped off interrupting the 208 volt to neutral single phase, the de-energized appliance is equally safe compared with a 120 volt appliance whose breaker is flipped off. Neutral in the latter is supposed to be insulated from (not bonded to) exposed metal parts.

Interrupting both legs of a "normally connected" 240 or 208 volt load in a system that also has 120 volts hot to neutral is done because both legs are hot, not because the appliance uses more than 200 volts.
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Last edited by AllanJ; 06-02-2013 at 06:24 PM.
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Old 06-02-2013, 08:19 PM   #15
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Using 208v "B" phase in open delta?


Quote:
Originally Posted by jbfan View Post
If you have an open delta, you do not have 208 volts.
you have 240 v phase to phase, and the high leg 180(plus or minus) to ground
208 is always 2 legs and not single pole.
Ummm... I believe a high-leg delta is 208v high-leg phase to ground?

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