We have several delta/delta 150 KVA transformers 480 volt primary and 480 volt secondary on the job to install. These are 3 wire with no neutral. The purpose is to isolate motor loads from the UPS systems. Does the secondary need to be grounded i.e. does the code require us to make the transformer a "corner grounded delta"? (none of the secondary coils are center tapped)
Delta/Delta iso xformers
- Thread starter sparkies
- Start date
- Status
- Location
- Massachusetts
You are not required to ground it, check out 2005 - 250.20(B).
Your secondary side does not fit 250.20(B)(1), (2) or (3) so it is not required to be grounded.
However 250.21 requires you install ground detectors if you choose not to ground the system.
Your secondary side does not fit 250.20(B)(1), (2) or (3) so it is not required to be grounded.
However 250.21 requires you install ground detectors if you choose not to ground the system.
brian john
Senior Member
- Location
- Leesburg, VA
Wonder why they bought these 480/480 I just tested several sites with isolation transformers feeding lighting inverters they were all 480-480/277. Just a thing with me I like the wye grounding.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What do the designers of this system propose that you do about secondary grounding? I would think that if they're going through the trouble of designing something with isolation transformers then they would certainly have an opinion about whether or not the secondaries should be grounded.
There are many people who believe that just because they don't need a neutral that a delta secondary is what they should use. They don't give a thought as to of and how it should be grounded.
With a delta if left ungrounded will they put a ground detection system on it? Are they aware that if left ungrounded that if one line goes to ground and there is a second line that has a arcing fault to ground the it is common to have greater than a 480v arcing fault. If you do have a ground detection system what action should be taken if the is a ground detected.
If you corner ground the delta you will have 480v to ground form the ungrounded conductors.
Then there is the 'Y'. Ground the X0 and you have 277v to ground.
I the many years that I have sold transformers more often than not the user doesn't have a clue as the advantages and disadvantages of a delta or wye secondary and specifies the delta for the wrong reason.
Then there is the phase shift with delta wye that you don't have with a delta-delta or wye-wye. This it's interesting when a Y-Y is requested and the cost of the transformer goes up 25-30%.
With a delta if left ungrounded will they put a ground detection system on it? Are they aware that if left ungrounded that if one line goes to ground and there is a second line that has a arcing fault to ground the it is common to have greater than a 480v arcing fault. If you do have a ground detection system what action should be taken if the is a ground detected.
If you corner ground the delta you will have 480v to ground form the ungrounded conductors.
Then there is the 'Y'. Ground the X0 and you have 277v to ground.
I the many years that I have sold transformers more often than not the user doesn't have a clue as the advantages and disadvantages of a delta or wye secondary and specifies the delta for the wrong reason.
Then there is the phase shift with delta wye that you don't have with a delta-delta or wye-wye. This it's interesting when a Y-Y is requested and the cost of the transformer goes up 25-30%.
There is no requirement to reference the secondary to ground, however the hurdle is how to provide secondary over current protection. You have two options.
- Ground the Xo, then just run phase conductors to the load equipment with an EGC
- Install ground fault detection.
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