Upgrading Center tapped Delta Service to Wye grounded

mull982

Senior Member
We are looking at that a project that currently has an ungrounded Center-Tapped Delta service entrance to provide 120/240V/208V service to a facility to feed 120V single phase loads and 240V 3-Phase loads (not sure how many actual 208V loads if there are even any). There are also have 480V 3-Phase loads that are fed via step-up transformers to 3-Phase panels.

The customer is now upgrading their service entrance to a 480V wye-grounded secondary which will now feed the 480V loads directly. For the existing 120/240V loads we will need to install (2) 500kVA delta center-tapped transformers to connect to the existing 120/240V panels in order to continue to feed the existing 120V 1-Phase loads and 240V 3-Phase loads.

I had initially thought about perhaps converting this the 120/240V loads over to a more traditional 120/208V however they are several motors that are only rated for 240V that could perhaps be negatively impacted from operating at lower voltage. There are some motors that have a 208-240V rating however there appear to just be too many listed specifically for 240V to justify changing to 120208V system.

I was curious to hear thoughts from others weather the change to a 120/208V system should be further investigated or given the number of existing motors and plant personnel's familiarity with the existing system to just leave it as a 120/240V system?

Any things that need to be considered with providing the new center-tapped transformers to re-feed existing 120/240V panels?
 
I actually like 120/240 three phase. The thing that bugs me about 208 wye is the low voltage (even 240 is too low IMO - if we could start over). Obviously the negatives of the high leg system are (potentially) running out of 120 busbars in panelboards and balancing. I guess another thing we hear about is many VFD's not liking Delta.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If the motors in question are 10HP and under, and they are not IEC motors, I would not worry about feeding them with 208V, especially if I have control of the 208V distribution in terms of distances from the transformers etc. NEMA design 230V rated motors are good down to 207V, so the problem with 208V is usually the VD between the source and load. Control that and you avoid the potential issues. For the few that end up below 207V at the motor, add small buck/boost transformers.

Read the "Cowern Papers" if you haven't already, it's the definitive treatise on applying motors. In the following link, the pertinent info starts with the section called "Motors" on page 49 and continues on through page 50 discussing this issue..
https://www.baldor.com/mvc/DownloadCenter/Files/9AKK107303
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I'm thinking the OP means he has a grounded system-not ungrounded. An ungrounded system of this type would not be allowed by NEC or furnished by the POCO.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If you have significant 120V loads, you should consider that the neutral current from unbalanced 120V loads on a center-tapped delta is typically limited to the equivalent of 5% of rated transformer KVA. So on a 500 KVA unit that would be 208A.
In the FAQ at the link below Schneider claims to have units that support up to 30% but I haven't found that on their spec sheets.


https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.schneider-electric.us/en/faqs/FA123842/&ved=2ahUKEwjUzvuymdbkAhXQ1lkKHa9QC8gQFjAAegQIARAB&usg=AOvVaw0wxAB03sPXNWdxsKZR-2DP​​​​​​
 
Last edited:

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If you have significant 120V loads, you should consider that the neutral current from unbalanced 120V loads on a center-tapped delta is typically limited to the equivalent of 5% of rated transformer KVA. So on a 500 KVA unit that would be 208A.
In the FAQ at the link below Schneider claims to have units that support up to 30% but I haven't found that on their spec sheets.


https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.schneider-electric.us/en/faqs/FA123842/&ved=2ahUKEwjUzvuymdbkAhXQ1lkKHa9QC8gQFjAAegQIARAB&usg=AOvVaw0wxAB03sPXNWdxsKZR-2DP​​​​​​
Most utilities will enforce the 5% limit when you apply for the service. They don't like the unbalanced systems either.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I'm thinking the OP means he has a grounded system-not ungrounded. An ungrounded system of this type would not be allowed by NEC or furnished by the POCO.
Good point, although I think when he posted that it was "120/240/208" we all understood that as a center tapped system it was therefore grounded, so I for one was ignoring the "ungrounded" portion of his posting.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
In my opinion, converting existing 120/240v 1p panels to 208/120 would be a large job, require installing and wiring new panels and shifting 1/3 of the loads to the third ph, and dealing with MWBCs. I would keep the 1ph panels as they are, and go with the keep-the-center-tapped setup.

You should be able to minimize load current imbalances by using the two transformers to place the more-heavily-loaded secondary phases on different primary phases.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
You should also check to see how many of the 240V motor loads are actually dual voltage motors that could be moved to the 480V side of things.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
Most utilities will enforce the 5% limit when you apply for the service. They don't like the unbalanced systems either.
If being supplied by open delta to begin with - it is already unbalanced and most likely was intended to be so.

Every install is going to have it's differences, but I'd be trying to convert as many of the three phase motors (especially those over 5 or 10 HP) to 480 volts as is practical. OEM equipment motors might be fixed voltage, most general purpose NEMA motors under 75 HP are usually dual voltage.
 
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