480V, 3 phase delta to 240V with high leg question

[Volt-Amps]

Hello, electrical gurus!
Im working on an apartment building. The electrician wrote that he wants a transformer on each floor to be 480V, 3 phase primary to 120/240V secondary with high leg because that's the equipment they have available , he wrote "Transformer will produce three legs 120V not grounded leg 240 direct".

1. Could you please clarify what that means?
2. What is the downside to this configuration, if any?
3. How does that affect the number of wires on the primary and secondary sides of the transformer.
4. The secondary power from the transformer will go to feed a meter bank on each floor. I want to bring 3-phase power to the meter bank but 120/240V single phase to each dwelling.

Thanks for your help

 

[Eddie702]

Sounds to me if your feeding apartments that you would want 480 3 phase primary to 120/208 volt secondary. I don't think you want 120/240 Delta high leg.

 

[hillbilly1]

Yeah, the high leg would be almost unusable in an apartment building. About all that could use it would be the elevator (if present) and if they have a central heating and air plant.

 

[tortuga]

to 120/240V secondary with high leg because that's the equipment they have available

If the apartments must have 120/240V and for whatever reason 120/208 wont cut it, I go with a Scott-T transformer (two single phase transformers one with 86% tap) on every other floor, it creates two single phase 120/240 feeders from the 480 3-phase.

 

[augie47]

I have no idea on a translation of his wording
As others have stated, the most common arrangement would be a 480-208Y/120 transformer which would be suitable in most situations.
A "standard" 480-120/240 transformer will have a "high leg" and often a reduced neutral capacity and is not the best suited for this application.
As tortuga noted, a Scott-T transformer would give you the desired 120/240 but they are not commonplace and replacements if needed might take a while

 

[LarryFine]

Could you please clarify what that means?

I read it as they want 120/240v, not corner-grounded.

What is the downside to this configuration, if any?

The high leg is mostly wasted with 1ph loading.

How does that affect the number of wires on the primary and secondary sides of the transformer.

It means all 3ph equipment and feeders.

The secondary power from the transformer will go to feed a meter bank on each floor. I want to bring 3-phase power to the meter bank but 120/240V single phase to each dwelling.

Then I agree with the 208Y/120 3ph secondaries and 120/208 1ph for each unit.

An alternative is multiple 1ph 480v to 120/240 transformers and 1ph equipment.

 

[jim dungar]

Hello, electrical gurus!
Im working on an apartment building. The electrician wrote that he wants a transformer on each floor to be 480V, 3 phase primary to 120/240V secondary with high leg because that's the equipment they have available , he wrote "Transformer will produce three legs 120V not grounded leg 240 direct".

1. Could you please clarify what that means?
2. What is the downside to this configuration, if any?
3. How does that affect the number of wires on the primary and secondary sides of the transformer.
4. The secondary power from the transformer will go to feed a meter bank on each floor. I want to bring 3-phase power to the meter bank but 120/240V single phase to each dwelling.

Thanks for your help

It is not physically possible to produce (3) 120V line-neutral/ground and (3) 240V line-line from a single transformer.

You need an RFI.
Your electrician needs to learn transformer configurations.

 

[Volt-Amps]

We have 3 phase 480V. We will also have 208Y/120V 3phase for building loads but for the apartment units, the above is what he wrote. Of course I didn't want to seem like I dont know what that is, hence my question in this post.
I did my calculations and everything based on 208Y/120 for the apartment before he asked me to change it.

 

[LarryFine]

Then you need clarification.

 

[Volt-Amps]

Thank you all for your help.
The hardest part of my career was and still is not having a mentor to teach me electrical design. I find myself relying on this forum to understand things I dont understand, online videos, the code books (which sometimes confuse me more than helping on some issues), and learning from my own mistakes, which almost always comes with embarrassment.

 

[LarryFine]

. . . learning from my own mistakes, which almost always comes with embarrassment.

That's a small price to pay for education.

 

[winnie]

120/240V single phase is _slightly_ preferable for a dwelling than 120/208V. But for large buildings it is preferable to present a balanced three phase load to the utility.

The electrician might be thinking that a high leg delta transformer will somehow balance the three phase loading.

I agree with the others, a high leg delta is a bad choice for this application. Most high leg deltas are designed for three phase 240V loading with incidental 120/240V loading. You can in principal make a high leg delta which will function correctly with mostly 120/240V single phase loading, but now you are talking about an uncommon beast.

I concur with using 120/208; but if the electrician is really pushing to get 120/240 then IMHO the simplest approach is to use single phase transformers and put different floors on different 480V legs to balance the loading.

If you want fancy, the 'scott-T transformer' mentioned in post #4 is one possible choice; the other option is 'hexaphase'.

-Jonathan

 

[electrofelon]

. The electrician wrote that he wants a transformer on each floor to be 480V, 3 phase primary to 120/240V secondary with high leg because that's the equipment they have available

I dont really understand this, and sounds like a red flag to me honestly.

 

[Volt-Amps]

Thank you. I will review the thread and make use of all the useful comments.
One thing I know from code search is that diversity of electric ovens, electric dryers are conditional based on the load being distributed on 3phase source tables 254 and 255. Not sure how to apply this with a high leg scenario. Granted that the electric range, dryer and condenser are all 230V so those can go on the B phase, but still unsure. Also, this may lead to larger equipment since most of the load will be on 2 out of the 3 phases

 

[Volt-Amps]

I dont really understand this, and sounds like a red flag to me honestly.

I was just communicating with the architect, and had the very same question. HE said he believes the contractor does not want to provide 150A panels, he wants to go down to 125A. I believe this is indeed the case to save on cost

 

[hillbilly1]

Thank you. I will review the thread and make use of all the useful comments.
One thing I know from code search is that diversity of electric ovens, electric dryers are conditional based on the load being distributed on 3phase source tables 254 and 255. Not sure how to apply this with a high leg scenario. Granted that the electric range, dryer and condenser are all 230V so those can go on the B phase, but still unsure. Also, this may lead to larger equipment since most of the load will be on 2 out of the 3 phases

Actually no, ranges and dryers are 120/240, so a misplaced B phase will fry them, but compressors and water heaters (240 volt) would be ok. But all of those will work on 120/208

 

[qcroanoke]

Actually no, ranges and dryers are 120/240, so a misplaced B phase will fry them, but compressors and water heaters (240 volt) would be ok. But all of those will work on 120/208

If he's looking to use the high leg and a neutral to power water heaters and compressors I'm pretty sure he can't do that.

 

[LarryFine]

Granted that the electric range, dryer and condenser are all 230V so those can go on the B phase, but still unsure.

That's only true for 2-wire L-L loads that do not have a neutral connection.

Also, this may lead to larger equipment since most of the load will be on 2 out of the 3 phases

Another reason to try to avoid the high-leg scenario.

 

[hillbilly1]

If he's looking to use the high leg and a neutral to power water heaters and compressors I'm pretty sure he can't do that.

Yes, but another phase and high leg will work fine for straight 240 volt loads.

 

[qcroanoke]

Yes, but another phase and high leg will work fine for straight 240 volt loads.

Agreed. But it kinda sounds like this electrician might be a little shady......