Sizing 3 phase transformers with one or two phases pulling much more current than the other(s)

[KyleFowler]

I have a hypothetical question that I have tried to research an answer to but have been unsuccessful in finding an answer. Below I picked 9kva because it was a small round number for clarity.

What if you had a 9kva three phase transformer with a 480v delta primary and a 208/120v wye secondary, could you pull the full 25 amps its rated for from one phase to neutral? Or could you pull 25 amps from just two of the three phases? I understand none of this is good practice but I'm just talking simple hypotheticals.

I understand that in single phase transformers wired in series with a secondary neutral tap each leg to neutral can only provide half of the wattage. Does the same sort of principle apply to 3 phase except it's only a third? This old ECM web page seems to suggest this without outright saying so although it's hard to tell because some of the links and pictures are broken.

StackPath

Thanks in advance!

 

[Carultch]

You are asking, what you have to do when sizing transformers for imbalanced loads. Note that this response only applies for what you need to do for a customer-owned transformer that is governed by the NEC. I.e. not a utility-owned transformer. Utilities have different sizing standards for equipment in their scope.

Take the worst-case-scenario current among the phases. And assume the remaining phases are just like it. Then determine the corresponding KVA.

So for a 9 kVA transformer, with a 120/208V wye secondary, this would mean the maximum operating load (100% non-continuous + 100% continuous) would be 3 kVA at 120V, which is 25A. You could have up to 25A on all three phases, you could have up to 25A on two of them, or you could have up to 25A on just one phase. The other phases can be much less than 25A if needed, even if phase A is 25A. What you would want to avoid, is the possibility of having 75A on just one phase, even if it is guaranteed that the other two windings will have 0A at any time this happens.

In the transformer windings, there will be some ability to share the thermal mass among the three windings, but the assembly has only been tested on the assumption that the phases will not exceed the full KVA as if it were balanced among the phases. The windings themselves might share thermal mass, and allow the unloaded windings to absorb heat generated by the overloaded winding, but the sections leading between the terminals and windings are not going to be able to share their thermal mass, and are only rated to withstand a current that is consistent with their share of the KVA.

 

[LarryFine]

The simple answer is that each third of a 3ph transformer "maxes out" at one-third of the transformer's rating.

So, yes, in that regard, it works like the center-tapped 1ph transformer.

 

[Besoeker3]

The simple answer is that each third of a 3ph transformer "maxes out" at one-third of the transformer's rating.

So, yes, in that regard, it works like the center-tapped 1ph transformer.

I don't understand why it works like a 1-phase centre tapped unit?
Rather than a simple 3-phase unit?

 

[LarryFine]

I don't understand why it works like a 1-phase centre tapped unit?
Rather than a simple 3-phase unit?

I meant that a multiple-secondary transformer's capacity is distributed between (or among) the secondaries, and can not be allocated to just one winding. You can only attain the full rated capacity when each and every winding is used and equally loaded.

In other words, a single secondary of a 3ph transformer can only supply one third of the unit's total capacity; likewise, a single secondary of a dual-secondary 1ph unit (or half of a center-tapped secondary) can only supply one half of the unit's capacity.

 

[kwired]

I have a hypothetical question that I have tried to research an answer to but have been unsuccessful in finding an answer. Below I picked 9kva because it was a small round number for clarity.

What if you had a 9kva three phase transformer with a 480v delta primary and a 208/120v wye secondary, could you pull the full 25 amps its rated for from one phase to neutral? Or could you pull 25 amps from just two of the three phases? I understand none of this is good practice but I'm just talking simple hypotheticals.

I understand that in single phase transformers wired in series with a secondary neutral tap each leg to neutral can only provide half of the wattage. Does the same sort of principle apply to 3 phase except it's only a third? This old ECM web page seems to suggest this without outright saying so although it's hard to tell because some of the links and pictures are broken.

StackPath

Thanks in advance!

Line to neutral loads in a wye secondary yes, you can load each phase to 25 amps in your case.

If you have line to line loads however things change.

Say you had a 25 amp load from A to B and another 25 amp load from B to C and clamped a meter on the supply conductors you would read A: 25 amps, B: 43.3 amps, C 25 amps. B coil of the transformer would be overloaded.

If a delta secondary however with same thing I just mentioned the three supply conductors would have same amp readings, but the A to B winding sees 25 amps and the B to C winding sees 25 amps. If a full delta there is no load current on the A to C winding, might be some circulating current from within the delta though.

 

[KyleFowler]

thanks everyone for the fantastic answers, I won't lie I was nervous asking what felt like a dumb question but I'm glad I did because I learned even more than I had hoped for. This forum is a wonderful resource I tell every electrician I know about so they can learn too. I am grateful that you guys dedicate so much time to educate us. I hope one day by asking enough dumb questions to be as smart as some of you guys.

Merry Christmas!