Trying to get 120/240v with no high leg

[Zalkon]

I did my apprenticeship in Oakland Ca. Worked there for 11 years. It was all new construction. Every 120/240v panel was just that, no 208v phase to phase or high leg. I moved to Wa. and all I find is 120/208v or 120/240v with a high leg. No one here has ever seen such a thing. A former co-worker I asked agrees with me that he remembers the same thing. But I've called distributers and they all think I'm crazy. I am familiar with the math (square root of 3 stuff)
So the basic question is what am I missing? I know we used square d transformers and they came preconfigured for what we needed. So are there any sparkies in Ca that can shed some light on this for me? Does it have to do with the taps on the transformer? Please help, I'm going crazy.

 

[Little Bill]

You either have a 240V/120V single phase with a center tap or a 240V/120V center tapped 3Ø Delta, the center tapped Delta will have a high leg of around 208V to ground, this is not a usable leg for any 120V loads. Or you have a 208V/120V single phase from a 208V/120V 3Ø.
The straight 240V/120V single Ø is what most residencies use. A lot of apartment buildings will use a 208V/120V single Ø from a 3Ø Wye system.
What exactly are you wanting to know?

 

[electrofelon]

Alas, it is not possible in this universe to have a 120/240 three phase system without a high leg. Also unfortunately, all the evidence points toward the universe expanding forever and NOT at some point ceasing to expand and ultimately shrinking back down resulting in another big bang that perhaps could result in a universe with different laws of physics that would then allow a 120/240 three phase without a high leg. I am not happy about it either but its the universe we got.

 

[texie]

Are you asking if you can get 240/120 3 phase with no high leg? That would not be mathematically possible.

 

[tom baker]

High leg, wild leg, B-**** leg are commonly used to get three phase and single phase from a bank of transformers, example for farms. There are two, the large one is for the 3 phase and the small one gets the single phase. The primary is an open delta, ie two hots and a neutral. To get 120/208 the POCO would have to run a third primary.
My POCO no longer offers a wild leg.
Make sure you understand where the POCO wants the wild leg in the meter, in our gear its B phase and is orange

 

[Carultch]

So are there any sparkies in Ca that can shed some light on this for me? Does it have to do with the taps on the transformer? Please help, I'm going crazy.

It's not about transformer taps, it is about the geometry of triangles, that represent the relationships among the phases and neutral.

It helps to draw a triangle to scale to see what is going on. The triangle ultimately represents the voltages are in "phase space". A brief explanation: the arrows from the origin to each point around the triangle indicate phase-to-neutral voltages. Projection onto the horizontal axis (real number axis) is the voltage in real time. The angular position around the circle is the history/forecast of what voltage was/will be. Smart$ has an icon that demonstrates this beautifully.

For 120/240V high leg systems, draw a triangle that is 24 centimeters on all sides, with its bottom edge horizontal. Label the points clockwise A, B, and C, starting at the bottom left. Split the side between A and C in half, and mark that point N. Measure from A to N, or C to N, and you will get 12 cm for each of these distances. Now measure from N to B. Instead of getting 12 cm, you will get 20.8 cm. This is why it is the high leg, because that 20.8 centimeters, represents 208V, instead of 120V. As you can see, the high leg system is 240V phase-to-phase, and only for phases A and C is it 120V to neutral, while phase B is a special phase that is 208V to neutral.

Now consider the 120/208V three phase wye system. Define a point as your origin, and label it N. Draw a line from N to 12 cm above it. Rotate 120 degees, and draw another 12 cm line from N. Rotate another 120 degrees, and draw a third 12 cm line from N. This will look like an inverted letter Y, hence the name "WYE system". The 12 cm measurement represents each phase being 120V nominal to neutral. Again, label the points around the edge A, B, and C. Now form a triangle by connecting points A, B, and C. Measure between any pair of these points, and you'll get 20.8 cm. This represents the nominal voltage being 208V phase-to-phase. No matter how you arrange your ruler, you will never see 24 cm anywhere on this triangle. This is why the WYE system that is 120V to neutral, is limited to 208V.

 

[Carultch]

Make sure you understand where the POCO wants the wild leg in the meter, in our gear its B phase and is orange

That's a good point too. At one point, some utilities had a standard for the high leg on the C-phase, and their meters were specifically built for a C-phase high leg. Then there was a change in the NEC to standardize to the high leg on the B-phase, specifically requiring it on the B-phase in panelboards. Inertia being a powerful "force", the utilities who previously had it on the C-phase didn't agree to follow this change. Depending on the meter technology, it may not even matter, as modern electronic meters measure everything they need to know, regardless of which phase you choose. But old-fashioned mechanical meters weren't as flexible.

In a disconnect, it is your choice which phase to use. To keep it simple, I'd recommend the B-phase for everything except where POCO requires otherwise. An old joke about this inconsistency: "you put it on the C-phase in the meter, the B-phase in the panelboard, and the A-phase in the disconnect".

 

[jaggedben]

I did my apprenticeship in Oakland Ca. Worked there for 11 years. It was all new construction. Every 120/240v panel was just that, no 208v phase to phase or high leg. I moved to Wa. and all I find is 120/208v or 120/240v with a high leg. No one here has ever seen such a thing. A former co-worker I asked agrees with me that he remembers the same thing. But I've called distributers and they all think I'm crazy. I am familiar with the math (square root of 3 stuff)
So the basic question is what am I missing? I know we used square d transformers and they came preconfigured for what we needed. So are there any sparkies in Ca that can shed some light on this for me? Does it have to do with the taps on the transformer? Please help, I'm going crazy.

When you moved did you switch from doing mostly residential (single phase) to mostly commercial (three phase)? Cuz that might kinda explain it.

Or maybe it's just that PG&E has less of a problem supplying single phase 120/240 services to larger buildings, as compared to the utilities there in WA.

 

[hillbilly1]

With delta and open delta three phase systems that are 120/240, they are easy to spot. One transformer out of the two or three will be larger. That larger transformer serves the 120 volt single phase loads, while the other one or two are the additional legs of the three phase loads.

 

[drcampbell]

I've never seen one, but I can envision a 3-phase, 6-pole, 7-wire 120/240 transformer, perhaps for a high-rise multifamily building that doesn't have any three-phase loads, but which has a total load that's too much for an unbalanced single-phase utility feed.

 

[winnie]

I've never seen one, but I can envision a 3-phase, 6-pole, 7-wire 120/240 transformer, perhaps for a high-rise multifamily building that doesn't have any three-phase loads, but which has a total load that's too much for an unbalanced single-phase utility feed.
View attachment 2556900

I suggested something similar for harmonic mitigation. Of course while it could be wired with all 240V single phase panels, you also have 208V and 120V available L-L if you select the correct phases.

I believe such coil arrangements are used for high power rectifier systems.

Jon

 

[drcampbell]

They're also used for low-power rectifier systems. Ford alternators, in particular.

 

[Hv&Lv]

High leg, wild leg, B-**** leg are commonly used to get three phase and single phase from a bank of transformers, example for farms. There are two, the large one is for the 3 phase and the small one gets the single phase. The primary is an open delta, ie two hots and a neutral. To get 120/208 the POCO would have to run a third primary.
My POCO no longer offers a wild leg.
Make sure you understand where the POCO wants the wild leg in the meter, in our gear its B phase and is orange

The size of the transformers is determined by the loads.
Large single phase loads and a small three phase load will get a large center tapped pot and a small wild leg pot.
generally if the three phase load is large enough for the large pot to be carrying the three phase and the smaller one for single phase, it’s better to close the delta at that point and add the third pot. That way you get 100% rather than 86%

For the primary side an open delta requires three phase wires 120 apart.
An open wye requires two wires and a neutral

 

[Jraef]

In Oakland you were on PG&E. They do still offer 240V 3 wire 3 phase as an option on special request, but you get no 120V from that unless you add another single phase transformer to get from 240V to 120/240V. So either you were mistaken about what your 3 phase system was, or you were working on single phase services.

 

[Besoeker3]

I've never seen one, but I can envision a 3-phase, 6-pole, 7-wire 120/240 transformer, perhaps for a high-rise multifamily building that doesn't have any three-phase loads, but which has a total load that's too much for an unbalanced single-phase utility feed.
View attachment 2556900

I have commonly used that arrangement, usually for low voltage Hexaphase rectifiers. Four banks, each 10kA to get 40kA. I'm inclined to think it may not be quite what the original poster envisaged,...................

 

[norcal]

In Oakland you were on PG&E. They do still offer 240V 3 wire 3 phase as an option on special request, but you get no 120V from that unless you add another single phase transformer to get from 240V to 120/240V. So either you were mistaken about what your 3 phase system was, or you were working on single phase services.

I remember seeing in Pacific Graft & Extortion's Green book, that 240V 3Ø 3-wire, & 480V 3Ø 3-wire , was only available to existing customers.

 

[Zalkon]

When you moved did you switch from doing mostly residential (single phase) to mostly commercial (three phase)? Cuz that might kinda explain it.

Or maybe it's just that PG&E has less of a problem supplying single phase 120/240 services to larger buildings, as compared to the utilities there in WA.

I did commercial light industrial. From my third year on I was the guy to install the electrical rooms. I must be going crazy cause I swear I never saw 208 anywhere in california. I understand the math, I did 5 years of apprenticeship , and CLEARLY didn't comprehend everything.

 

[kwired]

It's not about transformer taps, it is about the geometry of triangles, that represent the relationships among the phases and neutral.

It helps to draw a triangle to scale to see what is going on. The triangle ultimately represents the voltages are in "phase space". A brief explanation: the arrows from the origin to each point around the triangle indicate phase-to-neutral voltages. Projection onto the horizontal axis (real number axis) is the voltage in real time. The angular position around the circle is the history/forecast of what voltage was/will be. Smart$ has an icon that demonstrates this beautifully.

For 120/240V high leg systems, draw a triangle that is 24 centimeters on all sides, with its bottom edge horizontal. Label the points clockwise A, B, and C, starting at the bottom left. Split the side between A and C in half, and mark that point N. Measure from A to N, or C to N, and you will get 12 cm for each of these distances. Now measure from N to B. Instead of getting 12 cm, you will get 20.8 cm. This is why it is the high leg, because that 20.8 centimeters, represents 208V, instead of 120V. As you can see, the high leg system is 240V phase-to-phase, and only for phases A and C is it 120V to neutral, while phase B is a special phase that is 208V to neutral.

Now consider the 120/208V three phase wye system. Define a point as your origin, and label it N. Draw a line from N to 12 cm above it. Rotate 120 degees, and draw another 12 cm line from N. Rotate another 120 degrees, and draw a third 12 cm line from N. This will look like an inverted letter Y, hence the name "WYE system". The 12 cm measurement represents each phase being 120V nominal to neutral. Again, label the points around the edge A, B, and C. Now form a triangle by connecting points A, B, and C. Measure between any pair of these points, and you'll get 20.8 cm. This represents the nominal voltage being 208V phase-to-phase. No matter how you arrange your ruler, you will never see 24 cm anywhere on this triangle. This is why the WYE system that is 120V to neutral, is limited to 208V.

I like this, is mostly what I wanted to reply with. Will add if you build a wye system that is 240 phase to phase it will be about 138.5 volts from each phase to neutral. A little too high to utilize as nominal 120 most cases.

 

[Zalkon]

Thank you all for your input. Clearly my memory is faulty. From my 3rd year on I was the guy to build the electrical rooms. 277/480v pnl to the xfrmr to the 120/240v pnl always brown, orange yellow. black red blue . it must have been 120/208v because we used the entire panel with single pole breakers. I need to go get one of those coats where you hug yourself all day

 

[kwired]

With delta and open delta three phase systems that are 120/240, they are easy to spot. One transformer out of the two or three will be larger. That larger transformer serves the 120 volt single phase loads, while the other one or two are the additional legs of the three phase loads.

Have a lot of 240 volt full delta with all three pots the same size on farms where motor loads are the majority of the load.

They still do open delta for maybe 50 hp or less when it is in remote area and cost less to run two phases and neutral for primary. A few existing 75 and 100 HP irrigation wells on open delta, but really won't ever see a new one anymore, they want to run all three primary lines for those and will usually be 480/277Y.