AUTOTRANSFORMER CONNECTIONS

[fifty60]

I am wiring up a buck boost transformer (autotransformer). I see from NEC 450.4 that I only need to have OCPD's on the line side (is this still the "primary" for an autotransformer?) of the auto transformer. Looking at exhibit 450.6, this is clearly shown. I am adding the buck boost transformer to a compressor branch circuit. Does it matter where my contactors are placed in the circuit? Should they be placed on the line side of the transformer so that power is disconnected from the transformer or can they be placed on the load side so that the contactors only control the Compressor?

 

[augie47]

It would depend partly on the purpose of bucking/boosting the voltage. Is the compressor the only item involved ? What about control transformers, refrigerant sump heaters, condenser fans ??

 

[fifty60]

Only bucking the system 2 compressor. The condenser fan is on connected to System 1. The bucking transformer is bucking the voltage 24 Volts to the compressor only. There is nothing else on the branch circuit.

 

[augie47]

IMO, it would make no difference which side of the contactors. Others may know of a reason.

 

[JoeStillman]

The contactor should be connected to the load side of the transformer. That way the overloads can be selected for the actual motor current.

 

[GoldDigger]

The contactor should be connected to the load side of the transformer. That way the overloads can be selected for the actual motor current.

Also, insignificant though it may be, the contactor on the load side is not handling the inrush current of the autotransformer. (Probably below detection compared to the motor inrush.)
If you are actually working on the transformer, you would need to use a disconnect rather than the contactor anyway, unless the contactor could be positively locked out.