Transformer Connections

[Dwight W]

Hello everyone.
Our drawings show a 225kva, 3 phase transformer 480 pri. / 240, 3 phase Delta sec. feeding a 600A, 240V, 3 phase panel. The contractor is having an issue getting a 240V rated 3 phase panel. He wants to take (3) three 240V single phase panels with MCB and hook them up to the transformer with one panel on A and B, one on B and C, and one on A and C.
In theory this will work. But in section 240.21 (2) and (3) under the tap rules, it states that the secondary conductors shall terminate to a single OCPD. Which means to me that we can provide a 600A fused disconnect to a trough and tap within the trough for the three panels, which meets code. I have been in the trade for 45 years and have never seen anyone take 3 single phase panel feeders to a three-phase transformer.

With that said, does the code allow us to take three sets of single-phase conductors and tap two legs for each panel?

 

[augie47]

240.21(C) will allow multiple secondary feeds from a transformer to multiple OCP devices.

 

[jim dungar]

What are the loads? 240V 2-wire circuits are not common, except for heating.

Yes, the transformer can have (3) separate taps each feeding individual single phase panels with main breakers.

 

[augie47]

240.21(C) will allow multiple secondary feeds from a transformer to multiple OCP devices.

(Assume he has no 3 phase loads)

 

[LarryFine]

Any 120v loads?

 

[don_resqcapt19]

If it is straight 240, you can't use slash rated breakers. Slash rated breakers are more common and cheaper than straight rated ones. Even if there are no 120 volt loads, I would make the secondaries 120/240.

 

[jim dungar]

If it is straight 240, you can't use slash rated breakers. Slash rated breakers are more common and cheaper than straight rated ones. Even if there are no 120 volt loads, I would make the secondaries 120/240.

You won't be able to if it is a single transformer built as a 3 phase delta. At best one of your panels might have a neutral if the transformer has a center tap on one winding.

 

[Dwight W]

Thank you for the response.
It is actually a welding lab in a community college. With 208V power. The school bought 240V single phase residential welders for students that will not run on 208V and verified by manufacture. We have taken multiple 3 phase panels to a single transformer but have not taken single phase panels to a 3 phase transformer, seems like using different phases it would be considered a double tap. We just wanted a second opinion.

Thanks

 

[augie47]

Be sure to keep 240.85 (slash rated) {as noted} and 250.21(B) (Ground detection) in mind.

 

[jim dungar]

Thank you for the response.
It is actually a welding lab in a community college. With 208V power. The school bought 240V single phase residential welders for students that will not run on 208V and verified by manufacture. We have taken multiple 3 phase panels to a single transformer but have not taken single phase panels to a 3 phase transformer, seems like using different phases it would be considered a double tap. We just wanted a second opinion.

Thanks

The actual transformer connection point is not considered a tap, per article 240, as it is fully rated.

Have you confirmed your welders are acceptable on 240V to ground systems? When I hear residential, I think of 120V to ground maximum.

 

[don_resqcapt19]

You won't be able to if it is a single transformer built as a 3 phase delta. At best one of your panels might have a neutral if the transformer has a center tap on one winding.

I was thinking 3 transformers and not 3 panels.