Autotransformer

[journeyman0217]

I have never really dealt with autotransformers and came across something I thought was odd but maybe it's not... Scenario: autotransformer with a primary input of 240x480 and secondary output of 120/240 single phase. The transformer is being feed by 240 on the secondary side, and the load is coming off the primary side (480v circuit). Is this a common practice to reverse feed an autotransformer and be code compliant?

 

[hillbilly1]

Are you sure it’s an auto transformer? Sounds like a standard step up step down isolation transformer. And yes, you can backfeed it, but will have a higher inrush if not designed to be backfed.

 

[jim dungar]

A true auto transformer does not have a primary or secondary. They have high voltage and low voltage terminals, so they can be fed in either direction.

 

[journeyman0217]

 

[journeyman0217]

Whats that mean when it says, "for autotransformer connections..."

 

[hillbilly1]

Whats that mean when it says, "for autotransformer connections..."

It can be connected as an auto transformer, but I don’t know what usable voltage it would be. That’s quite a jump. May be just boiler plate instructions.

 

[jim dungar]

Whats that mean when it says, "for autotransformer connections..."

That is an isolation transformer whose windings may be connected into an auto transformer when you follow the referenced literature.

Typically the literature will be for voltage changes of less than 20%.

 

[journeyman0217]

Ah...thank you for confirming. So, why would someone have installed this transformer knowing what the primary and secondary voltages were? Is it a cost thing? and I didn't see anywhere that it was designed for reverse feed.

 

[jim dungar]

Ah...thank you for confirming. So, why would someone have installed this transformer knowing what the primary and secondary voltages were? Is it a cost thing? and I didn't see anywhere that it was designed for reverse feed.

Reverse feeding isolation transformers is a common practice. In the past years some manufacturers have stopped using the terms primary and secondary, which allows them to be in compliance with the NEC requirements for manufacturer approval.

Under what NEC edition was your installation done?

Was this an auto transformer to go from 480V to 600V? Or simply a setup from 240 to 480V.

 

[MTW]

Ah...thank you for confirming. So, why would someone have installed this transformer knowing what the primary and secondary voltages were? Is it a cost thing? and I didn't see anywhere that it was designed for reverse feed.

Here is some links to learn more about transformers their uses and connections for different uses. Older information but good to know.

Federal Pacific Transformer University - Federal Pacific

www.federalpacific.com

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