Connecting Transformers in Series

[NicolasK]

I want to connect the secondary side of two 120/240V transformers in series to get 480V. The transformers are 10 kVA units (additive polarity). The H1 bushings will be tied to the same primary phase, each with its own fused protection (cutout). H2 bushings will be tied to the system neutral. On the secondary side, tie X3 of transformer A to X1 of transformer B. Then I should have 480 V across the remaining X3/X1. For the service wire ground, can I tie it directly back to the system neutral(ground)?

Assuming I use identical transformers(voltage rating, imp., etc.), will this work? Anything I need to watch out for?

Thanks.

 

[tinfoil]

Are you sure that your 120/240V bushings are fully insulated for 480V L-G?

 

[NicolasK]

The secondary side is one hole lugs. I'm checking with the manuf. of the rating.

 

[ScottyUK]

I might be totally out of line for the NEC, but consider tying the centre tap to ground. Then the highest you can get is 240V line to ground. Obviously you then have two hot conductors, but you didn't specify the purpose of this supply.



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[NicolasK]

I need to supply 480V to some parking lot lights.

 

[busbar]

 
Series secondaries do not seem to be prohibited in IEEE C57.12.70-2000, but not directly permitted either. If the transformers have 30kVBIL secondaries, it sounds OK. If applicable, secondary-side metering, overcurrent protection and grounding should be considered.

 

[steve066]

What if one fuse blows. Then you will have only have 240 across the lights. And the second transformer will backfeed the primary and the open fuse.

Why not just get a 120 to 480v transformer, or fixtures with a 240v ballast?

 

[busbar]

 
steve066, I think you have a valid point, but would a center-grounded 240/480V 3-wire configuration be so different than part of a 3ø grounded-wye—grounded-wye bank with one primary fuse open? From an operational standpoint, the two transformers could be considered a ‘transformer bank,’ but a single-phase one.