Transformers

[Big Guns]

My question is if you have a 480 volt step down xformer to 208 can you wire it back words by feeding the secondary at 208 and get 480 out , if so do you need the netural on the 208 side.

 

[augie47]

My question is if you have a 480 volt step down xformer to 208 can you wire it back words by feeding the secondary at 208 and get 480 out , .

Yes, but here are some comments from a transf. manufacturer:

7. Can Acme Transformers be reverse connected?
ACME dry-type distribution transformers can be reverse
connected without a loss of KVA rating, but there are certain
limitations. Transformers rated 1 KVA and larger single phase,
3 KVA and larger three phase can be reverse connected without
any adverse effects or loss in KVA capacity. The reason for this
limitation in KVA size is, the turns ratio is the same as the voltage
ratio. Example: A transformer with a 480 volt input, 240 volt
output— can have the output connected to a 240 volt source​

and thereby become the primary or input to the transformer,

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then the original 480 volt primary winding will become the
output or 480 volt secondary. On transformers rated below
1 KVA single phase, there is a turns ratio compensation on the
low voltage winding. This means the low voltage winding has
a greater voltage than the nameplate voltage indicates at no
load. For example, a small single phase transformer having a
nameplate voltage of 480 volts primary and 240 volts secondary,
would actually have a no load voltage of approximately 250 volts,
and a full load voltage of 240 volts. If the 240 volt winding were
connected to a 240 volt source, then the output voltage would
consequently be approximately 460 volts at no load and
approximately 442 volts at full load. As the KVA becomes
smaller, the compensation is greater—resulting in lower output​

voltages.

When one attempts to use these transformers in
reverse, the transformer will not be harmed; however, the​

output voltage will be lower than is indicated by the nameplate

if so do you need the netural on the 208 side.

No and to make such a connection will damage the transformer.

 

[IMM_Doctor]

Absoluetly NOT

Absoluetly NOT

When using a standard step-down transformer in reverse.

DO NOT connect XO on the low voltage side.

Supply only 3 wire 208 volts to the low voltage side. You will get 480 volts 3 phase 3 wire only from the high voltage side.

You most likely will wnd up corner grounding the output. But you must ensure that all connected devices on the high voltage side have a straight 480 vac rating and not a 480/277v rating. There will be two phases that are each 480 volts to ground, and one phase that is now a grounded conductor.

 

[jim dungar]

I have yet to find a major manufacturer, of general purpose transformers, that does not use language similar to the ACME instructions quoted by Augie47.

 

[DavidA]

When using a standard step-down transformer in reverse.

You most likely will wnd up corner grounding the output. But you must ensure that all connected devices on the high voltage side have a straight 480 vac rating and not a 480/277v rating. There will be two phases that are each 480 volts to ground, and one phase that is now a grounded conductor.

Does this mean tying one of the H# lugs to the case and ground?

 

[pete m.]

Does this mean tying one of the H# lugs to the case and ground?

If you wish to end up with a corner grounded system.

Pete

 

[DavidA]

What situation would necessitate making the delta corner grounded?

 

[LarryFine]

Does this mean tying one of the H# lugs to the case and ground?

Yes, but not only. It should be wired like any grounded conductor, white or gray insulation/marking, not fused unless OCP opens all conductors, etc.