Step-up and Step-down Transformers

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Derek S's picture
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Joined: 2016-11-24

I wanted to start a discussion on wiring transformers in reverse.

What are some important considerations when you wire a step-up transformer in reverse to make it a step-down transformer? And what are the important considerations when you wire a step-down transformer in reverse to make it a step-up transformer? Is this against Code?

I look forward to hearing all the brilliant minds comment on this!

mc5w's picture
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Joined: 2013-10-09

Larger transformers can be used in reverse because the "primary" taps are exact at no load. For control transformers, the secondary is more along the lines of 130V or 27V to compensate for voltage drop during the inrush period of a motor controller coil or control relay.

Article 690 allows the use of a transformer in 2-way service in which case the side that is energized from the utility needs a "mission critical" circuit breaker that has a magnetic trip 2,000% of the overload trip so that it will tolerate inrush current and provide overload protection. For instance, Xantrex recommends using 277V primary 120x120/240V secondary transformers in the wye-wye configuration for connecting 120V line interactive inverters to 277Y480V.

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