Transformer secondary protection.

[lquadros]

The transformers are normally 100% rated. What would be the reason of having 125% FLA for transormer secondary protection? Is it the same if we use breakers or fuses? Is it also dependant on type of breaker or fuses being used etc.? :?: Thanks in advance for your replies.

 

[RUWired]

lquadros,The 125% rating at 80% will give you 100 % capacity.If you protected at 100% you would be limited to 80% capacity.(continuously).Your conductors are sized not less than the 125% rating.
Rick

 

[lquadros]

what about small transformers

what about small transformers

Thanks for your reply RUWired.
According toTable 450.3(b), we can upsize the conductors upto 167% the secondary FLA, in case of smaller transformers (9A or less secondary current). How does this scenario work?

 

[RUWired]

For xfrmr secondary currents less than 9 amps, you can protect at 167%.That would be 15amps.The minimum size conductor would be 14 awg.IMO anything less than 15a is a non standard rating. I don't think anyone would want an 18 awg wire on their xfrmr, so they set the rating @ 167%.
Rick