High leg

[roger]

Correct, but if you use a wye connected transfomer your voltage would be 208 phase to phase, not 240,

Once again we have a communication problem, I never said 208 was not usable did I? I said the high leg was unusable refering to it's use at 208 volts

that is the advantage of the delta connected transformer, you would have to use buck/boost transformers to work with the rated voltage on the German equipment. A lot of convience store compressors are 240 volt single phase, running them on 208 shortens their life.

And that has nothing to do with the high leg, you are referring to 240 volts phase to phase, the third point of a center grounded delta is just another phase unless it's connected to the grounded conductor.

Roger

 

[hillbilly1]

Once again we have a communication problem, I never said 208 was not usable did I? I said the high leg was unusable refering to it's use at 208 volts

And that has nothing to do with the high leg, you are referring to 240 volts phase to phase, the third point of a center grounded delta is just another phase unless it's connected to the grounded conductor.

Roger

OK, you are talking about using it by itself, without the other phases! I got you now, Yes, by itself, it is useless.

 

[roger]

OK, you are talking about using it by itself, without the other phases! I got you now, Yes, by itself, it is useless.

Now we're on the same page.

Roger

 

[hillbilly1]

Now we're on the same page.

Roger

I did come up with a use for it though.......It's fun watching someone who does not know about high legs put a breaker in.....funny, every third space is open?....... Hey I can throw a breaker in there for this receptacle!:grin:

 

[kwired]

Why have a high leg? What was the original intention of the design?

Around my area these systems exist in the alley behind small business buildings where one transformer may serve several buildings. many of them have single phase services but occasionally a small restaurant, bar or even retail store may have limited three phase loads such as air conditioning, coolers, or commercial cooking equipment. Many old services can be pretty interesting how the third phase was added to existing equipment and usually not up to code now or when it was installed.

There are also open delta services in the rural areas that serve single loads, or small facilities, this usually is done to save the POCO money on their equipment. They use two transformers instead of three and if they need to install a lot of primary line to reach a limited load they like to do this by using two phase lines and the neutral instead of all four lines on the primary. Sometimes you will even see single phase primary coming from one direction that serves other single phase customers and a single phase (from a different phase) coming from an other direction that also may serve other customers come together at one pole to build an open delta transformer bank.

This is done with 240 or 480 services. On a 480 4 wire delta there is a "high leg" just like on a 240 system the voltage is approximately 415 to ground, on the "high leg" and 240 to ground on the other two phases.

 

[mivey]

Now we're on the same page.

Roger

I thought I read something a year or so ago about some UPS systems making use of the high leg. I thought it had something to do with making use of a voltage that nothing else would be on. Ever read anything like that?

Might have just been some internet fodder because I don't recall having found anything from any UPS manufacturer on it.

 

[hurk27]

The 120/240v part is exactly like the single-phase service in your house, and you can use it freely like that

Is it not true that a 4-wire delta neutral loads should be limited to only 5% of the total service?
or I'm I mixing up another type of service?

 

[jim dungar]

Is it not true that a 4-wire delta neutral loads should be limited to only 5% of the total service?
or I'm I mixing up another type of service?

No, the 5% limit is only with some (most?) three phase dry type transformers with a three-legged core (i.e. standard transformer designs). Transformer banks made up of individual units (either two or three) and some new single core designs do not have this limit.

 

[LarryFine]

Sometimes you will even see single phase primary coming from one direction that serves other single phase customers and a single phase (from a different phase) coming from an other direction that also may serve other customers come together at one pole to build an open delta transformer bank.

Hey, that's not fair! We can't do that! grin

I wonder if they have to put signs at each disconnect locating the others?

 

[hurk27]

No, the 5% limit is only with some (most?) three phase dry type transformers with a three-legged core (i.e. standard transformer designs). Transformer banks made up of individual units (either two or three) and some new single core designs do not have this limit.

I was wondering, it seemed to be somewhere in my memory.

The POCO here doesn't allow 4-wire deltas any more, but we still have a few in the area, but I have noticed that the center taped tank is much larger than the others, I think one I saw had 2 25kva tanks and one center tap 75 kva, but I always thought the problem was the ballance on the primary side was the reason for this requirment?:-?

 

[LarryFine]

The POCO here doesn't allow 4-wire deltas any more, but we still have a few in the area, but I have noticed that the center taped tank is much larger than the others, I think one I saw had 2 25kva tanks and one center tap 75 kva, but I always thought the problem was the ballance on the primary side was the reason for this requirment?:-?

An open Delta is available here for new installations, but only up to a single 200a service, and they won't upgrade an existing one beyond that.

The larger center transformer is sized for the larger expected proportion of 120/240v single-phase loads vs. the 3-phase portion.

 

[hurk27]

An open Delta is available here for new installations, but only up to a single 200a service, and they won't upgrade an existing one beyond that.

The larger center transformer is sized for the larger expected proportion of 120/240v single-phase loads vs. the 3-phase portion.

Weird Page 4 wouldn't show until I pressed post, so I copied it to the last post of yours lol, it worked just dont click on the little post find arrow, it will take you to the wrong post lol

ok on subject:
That make sense, especially if the building was up graded from single phase, and had a large single phase load, but just added a few three phase loads.

I'm seeing these installations many years after they were installed, and most of these buildings have changed so many hands, so I have no way of knowing what they had originally, but that does add up, thanks

 

[quogueelectric]

That seems odd, I would expect 240, 240 and 0 to ground or if it's ungrounded delta who knows what you will read to ground.

It is a 4 wire delta centertaped one winding. It looks like a xmas tree in the schematic grounding the centertaped neutral.

 

[quogueelectric]

Please give an example.

Roger

Gas station or service station. Lots of 3 phase loads very little singe phase needed.

 

[LarryFine]

Weird Page 4 wouldn't show until I pressed post, ...

Same here; I couldn't view my last post, and it's happened before. I click on the little '4' and it brings me to page 3 again.

ok on subject:
blah, blah, blah, thanks

You're welcome.

 

[roger]

Gas station or service station. Lots of 3 phase loads very little singe phase needed.

Where did I ask for an example of where three phase loads are used?

Roger

 

[kwired]

I was wondering, it seemed to be somewhere in my memory.

The POCO here doesn't allow 4-wire deltas any more, but we still have a few in the area, but I have noticed that the center taped tank is much larger than the others, I think one I saw had 2 25kva tanks and one center tap 75 kva, but I always thought the problem was the ballance on the primary side was the reason for this requirment?:-?

the larger center tapped tank is the one with your usual 120/240 single phase load connected to it. It is larger because it has more load connected to it (usually). I see systems with lots of 3 phase load and little neutral load sometimes and on these systems all three transformers are the same size.

 

[kwired]

I was wondering, it seemed to be somewhere in my memory.

The POCO here doesn't allow 4-wire deltas any more, but we still have a few in the area, but I have noticed that the center taped tank is much larger than the others, I think one I saw had 2 25kva tanks and one center tap 75 kva, but I always thought the problem was the ballance on the primary side was the reason for this requirment?:-?

most POCO here have done away with 480 volt corner grounded systems in favor of 4 wire delta systems for safety reasons. Although I don't think there is much difference in safety for properly trained people the untrained users (mostly farmers here) are the ones being protected (to some degree).

A long time ago I had one farmer tell me about how one of the fused supplying his irrigation equipment kept blowing, so he switched the fuse with the slug (from the grounded phase) in the disconnect. Then he had a fire on the POCO transformer bank someplace. He then said he switched the fuse back before POCO came to check it out.

If you want to see code violations come to some farms around here that are not required to have inspections. As long as something does what you want it to do, (on when switch is on) it is good. I cant tell you how many breaker panels with missing covers I have seen (even ones located outdoors).

 

[mivey]

Where did I ask for an example of where three phase loads are used?

Roger

You asked about the high leg. I would not call it unusable from a theoretical standpoint. You would have to find the right single phase breaker.

I would not think some bank configurations would work well if the single phase load on the high leg was significantly large. You might have extra neutral current but I haven't done the math. I'm sure there would be a difference for an open delta vs a closed delta.

As for an example: I have never seen the high leg used for single phase load and would like to know if anyone has (legitimately).

PS: Maybe extra neutral current is not the right phrase, but it could get higher than the phase currents.

 

[roger]

You asked about the high leg. I would not call it unusable from a theoretical standpoint. You would have to find the right single phase breaker.

Correct, I asked for an example of where the high leg is used to supply a 208 (not 240) volt load.

I would not think some bank configurations would work well if the single phase load on the high leg was significantly large.

Nor would I

You might have extra neutral current but I haven't done the math. I'm sure there would be a difference for an open delta vs a closed delta.

I don't think open verses closed really matters until we know of a specific case we would use the high leg and neutral to supply 208.

As for an example: I have never seen the high leg used for single phase load and would like to know if anyone has (legitimately).

Same here and "legitimately" is the key word for the example.

PS: Maybe extra neutral current is not the right phrase, but it could get higher than the phase currents.

We could use oversized conductors to compensate. :grin:


Roger