Reverse wiring step-down transformer

[winslowfam]

Hi...we are reviewing the distribution system of a new client. They have a 208VAC 3-ph, 4-wire service. Some of their loads, though, require 480VAC...so, they have an occasional standard step-down 480V-120/208V transformer wired in reverse to get the 480V they need for these loads out of their existing 208V service. I have read a few articles online and understand the basic risks of reverse wiring a stepdown transformer...one comment from these articles, though, is that it might not meet NEC or local code restrictions. Does anyone know of any such restrictions, code-wise?

 

[texie]

Hi...we are reviewing the distribution system of a new client. They have a 208VAC 3-ph, 4-wire service. Some of their loads, though, require 480VAC...so, they have an occasional standard step-down 480V-120/208V transformer wired in reverse to get the 480V they need for these loads out of their existing 208V service. I have read a few articles online and understand the basic risks of reverse wiring a stepdown transformer...one comment from these articles, though, is that it might not meet NEC or local code restrictions. Does anyone know of any such restrictions, code-wise?

I'm not a fan of this, but if done correctly, it is compliant. As you probably know, do not bond X0 on what is now the primary. The big down side though is you now have delta 480 secondary which has to be corner grounded or operated as an ungrounded system, both of which have drawbacks. And of course you have no neutral in case you need it.
I never understood why we see this so often given that a proper 208 delta X 480/277Y transformer is fairly readily available.
I might add that for the 2014 NEC, Art 450.11 will now require marking on the transformer to indicate reverse use is acceptable and must comply with their instructions.

 

[templdl]

I'm not a fan of this, but if done correctly, it is compliant. As you probably know, do not bond X0 on what is now the primary. The big down side though is you now have delta 480 secondary which has to be corner grounded or operated as an ungrounded system, both of which have drawbacks. And of course you have no neutral in case you need it.
I never understood why we see this so often given that a proper 208 delta X 480/277Y transformer is fairly readily available.
I might add that for the 2014 NEC, Art 450.11 will now require marking on the transformer to indicate reverse use is acceptable and must comply with their instructions.

Ditto. Quite often the installer doesn't think the application through on how they are going to address a delta secondary and often fail to understand why the X0 on the primary should be left unconnected entirely.