Wild leg and compressors

[rbkurt]

A walk-in cooler has a compressor that requires 208/230/60/1 circuitry. The service to the building has a wild leg. In order to keep more of the non-wild leg slots open, I would like to run the compressor off of the wild leg and a grounded conductor. A few of my colleagues think that this is crazy, stating that you must have "2 hot wires." But voltage is merely a difference in potential, and it would seem to me that 208 V - 0 V should be as legitimate as 120 V - (-120 V). Is there something I'm missing here?

 

[e57]

Although you could do it - doesn't mean you should do it. (Hmmm... where's that drawing of a high-leg delta? Imagine one...) The circuit path to neutral is kinda conveluted... You're getting that voltage from 1.5 sets of windings in either direction, or both alternately. What you end up with is a higher impedance circuit. And depending on where the noodle tap is, and how well the transformer is balanced, you could get some wildly fluctuating voltage... The OCP +/or motor may not be able to handle that...

If you want more 120 slots - stop now, and toss in a sub-panel. They're cheaper than a new motor.

 

[infinity]

Where will you get a single pole CB rated for 208 volts?

 

[barbeer]

rbkurt,

Just because the voltage has an alternative lifestyle does not mean it is wild! :smile:

I have installed many 208-240v loads and used the "high leg" as one of the ungrounded conductors. The energy is used as a complete circuit, not as two separate legs of voltage. I believe it will operate flawlessly! The thing you should take care with however is make sure there are no control circuits fed from the high side and only motor or resistive loads.

 

[LarryFine]

RB, you sholuld definitely NOT connect any load from high-leg to neutral. This motor is rated to be run from line-to-line voltage, so you can use the high leg. You will need a breaker rated at 240v, not 120/240.

 

[hardworkingstiff]

You're getting that voltage from 1.5 sets of windings in either direction, or both alternately. What you end up with is a higher impedance circuit. .

I've heard this before, and see it on a diagram. What I've seen is X1 and X3 of the lighting transformer make up the 120/240-volt part of the service (with X2 grounded). Then X3 of the lighting transformer is connected to X1 of the 3rd phase transformer, X2 of the 3rd phase transformer is ungrounded, and X3 is the "wild" leg.

In my simple mind, if I hook up a 240-volt circuit to X1 of the lighting transformer and X3 of the 3rd phase transformer, it would wind up using the windings of both transformers (a 240-volt 2-pole circuit). How is this acceptable and using the X2 of the lighting transformer and X3 of the 3rd leg transformer not acceptable (other than the breaker rating issue)?

 

[rattus]

My one cent:

My one cent:

Poor power factor in windings supplying the 208V is reason enough to limit its use, if you use it at all, to small 208V loads. PF in the centered tapped winding is zero for a resistive 208V load. its is 87% in the other windings.

 

[hardworkingstiff]

Poor power factor in windings supplying the 208V is reason enough to limit its use, if you use it at all, to small 208V loads. PF in the centered tapped winding is zero for a resistive 208V load. its is 87% in the other windings.

Rattus,

If you connect two different 240-volt single-phase circuits, one to the "wild" leg and A phase and one to the "wild" leg and C phase of the 3-phase delta, will both circuits act the same as far as power factors and transformer loading?

 

[jtester]

Using a wild leg to ground creates problems because the voltage is not controlled by a coil,there is no one coil between the wild leg and the neutral-ground. It is controlled by the voltage drop across two coils not in series. There is more than normal voltage drop in a circuit like that than usual. It is similar to using the open leg of an open delta. You are very limited in load that will work.

Jim T

 

[hardworkingstiff]

Using a wild leg to ground creates problems because the voltage is not controlled by a coil,there is no one coil between the wild leg and the neutral-ground.

Why would that make a difference? In a 208-volt single-phase circuit, you have two secondary coils and two primary coils between the two conductors.

It is controlled by the voltage drop across two coils not in series.

This is the same as the 208-volt single-phase circuit?

It is similar to using the open leg of an open delta. You are very limited in load that will work.

Jim T

Agreed.

 

[rattus]

Rattus,

If you connect two different 240-volt single-phase circuits, one to the "wild" leg and A phase and one to the "wild" leg and C phase of the 3-phase delta, will both circuits act the same as far as power factors and transformer loading?

Stiff, if I understand your question, the answer is yes providing that the loads are identical.

 

[hardworkingstiff]

Stiff, if I understand your question, the answer is yes providing that the loads are identical.

Thanks for the response rattus. The reason I'm asking these questions is that when I look at a schematic of the two transformers that make up the delta 3-phase, I noticed that one of the circuits will pull amperage through both transformer secondary coils and the other will only pull amperage through the transformer that develops the high leg. This appears to work this way because X1 of one transformer and X3 of the other transformer are tied together.

I guess that all this does is to increase the heat generated (at the transformers) by the circuit that goes through both transformers.

My simple mind can't under stand why it is OK to wire a circuit that goes through both secondary coils of the two transformers, but it's not Ok to wire a circuit that will go through one of the secondary coils and 1/2 of the secondary coil in the 2nd transformer.

 

[jtester]

Why would that make a difference? In a 208-volt single-phase circuit, you have two secondary coils and two primary coils between the two conductors.

Lou

In the 208 volt single phase circuit, the grounded neutral plays an important part. It anchors the voltage at that point vs an open delta or a high leg to ground, which doesn't have an "anchor", but a phase that floats around.

Jim T

 

[rattus]

Thanks for the response rattus. The reason I'm asking these questions is that when I look at a schematic of the two transformers that make up the delta 3-phase, I noticed that one of the circuits will pull amperage through both transformer secondary coils and the other will only pull amperage through the transformer that develops the high leg. This appears to work this way because X1 of one transformer and X3 of the other transformer are tied together.

I guess that all this does is to increase the heat generated (at the transformers) by the circuit that goes through both transformers.

My simple mind can't under stand why it is OK to wire a circuit that goes through both secondary coils of the two transformers, but it's not Ok to wire a circuit that will go through one of the secondary coils and 1/2 of the secondary coil in the 2nd transformer.

Stiff, we are talking of a 4-wire, wild leg delta service, right? Three secondaries, AB, BC, and AC with a CT? If you apply a load between B and the CT, current is provided by all three secondaries. If the PF in the load is 100%, it is 0.0% in both halves of AC and 87% in AB and BC. The POCO doesn't like zero PFs. Need a diagram?

If we apply 120V and 240V resistive loads, the PFs in the secondaries will be 100%.

Remember, the wild leg voltage, Vb-ct, is 90 degrees out from Vac. That is the fly in the ointment.

In my opinion, the wild leg just happens when we add the CT and serves no useful purpose. Some of you pros may correct me here

 

[e57]

How is this acceptable and using the X2 of the lighting transformer and X3 of the 3rd leg transformer not acceptable (other than the breaker rating issue)?

Here's another reason, and more to the point. And I think some are going to debate this one... Ever think of why it is called a "wild" leg? If you make this unintended and now parallel connection (C~N) of voltage and current between the grounded point of the split winding (A, N ,C) you change the voltages and current flow of the other two points (A+B, A+N, B+N, B+C, A+C) to a degree depending on the load between N and C. "WILD" voltage and current flowing all over the place..... That relative potential (voltage) of ~208-N is an unintended by-product of the transformer configuration, the voltage ( after voltage drop through the load) applied from C to the middle of A-B will change all of the voltages and current flow.

In terms of a high-leg 120/240 delta - just think of it as a straight 240 delta - that you can also have some 120 with... Not some 208 too!:smile:

 

[hardworkingstiff]

Thanks for posting a drawing e57 (I really need to learn how to do that).

As I understand what PoCo's deliver for a 120/240 3-phase delta service would be like this drawing except the windings from A to C (or B to C) would not exist. Also, since the NEC requires the high leg to be in the center of the panel, I have been calling what this drawing shows as C, B.

So, using this drawings labeling, if the windings from A to C are not there (like I believe most PoCo's deliver), it seems that wiring a 240-volt load between A and C is just as hard on the transformers as wiring from C to N. Wiring a 240-volt load from B to C would be a non-issue. Am I on the right track, or am I still in the fog?

Thanks for everyones patience on this with me.

 

[jim dungar]

As I understand what PoCo's deliver for a 120/240 3-phase delta service would be like this drawing except the windings from A to C (or B to C) would not exist. Also, since the NEC requires the high leg to be in the center of the panel, I have been calling what this drawing shows as C, B.

There are two types of wild leg arrangements. They have different advantages. Different POCOs may favor one over the other.
First is the closed delta (although no one uses the wrd closed). This has three transformers cans, and is pretty stable. This one is common where the three phase load is large compared to the single phase This is the one Rattus has been describing.
Second is the open-delta. This only has two transformers, and is kind of unstable. These are very often used with small three phase loading relative to the single phase. This is the one that you are describing

 

[LarryFine]

And, just to clarify, you can indeed treat the open side of the open Delta as if it were a complete Delta, and obtain 240v from it. Granted, it's not as low impedance a source as it would be with three transformers.

In theory, if you were to open a corner connection of a full Delta system, you should measure 0 volts across the gap. The ability to maintain this is a function of the impedance of the system, just as with the open Delta.

The open Delta probably originated when 3-phase was added to existing single-phase systems in order to provide the maximum increase of power delivery with a minimum of additional hardware.

 

[e57]

Hardworkingstiff...

This is is the one you are thinking of.

The images are from another forum here

If you see either of them, they can be easily confused with eachother as both can be done with two transformer cans (pole-pigs) depending on the connections and split windings etc. And there are some wierd ones. I say this because I have seen closed deltas with two cans, one large slpit winding for A-C, C-B, and a smaller can for A,N,B set up delta/delta. The local POCO is notorious for putting up whatever they have at the moment.

According to my old Electricians Handbook (which is bible sized ) the open delta is a method of providing power while removing a damaged transformer.... Same book (Most of which is originally written in antiquity it seems) also states that the split winding that provides the 120 legs was for providing a source for 120 lighting. Worded as it were some sort of convenience....

And locally in the more industrial areas I get to see a wide variety of transformer set-ups. Mostly closed deltas, most are high-C phased. (Not sure when the code decided to make it B?) I also have seen a rare few open deltas like the one above, and a handful of corner grounded deltas.

A several years ago I worked in a building with 4 seperate services. (All mounted on the same set of poles out front - I would love to get a pic - but they are all underground now, and the building is gone....) It had a 208 wye, 480 delta, 240 delta, and 120DC complete with rectifiers all mounted on a platform between four poles fed from two different utilities, SF Hetch Hetchy, and PG&E. It was a monster.

 

[hardworkingstiff]

Yup e57, that's what I was thinking about. I don't remember ever seeing a closed delta service. Most of the deltas I've seen have been in commercial buildings with a small 3-phase load, I guess that's why.

The code section for B phase being the high-leg is 408.3(E). I remember in the 70's, it was C phase.