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	Subject - transformer overcurrent protection Jaimie My question pertains to the tripping of a 100a branch and a 200a main breaker with an unloaded single phase 37.5kva 240 to 480 transformer .The initial surge when turning on the 100a breaker trips the 200a main. My first thought was that the breakers should be changed to a delay type breaker to maintain for the initial surge.My theory is pretty weak in this area .What am i missing ? Are there other solutions to this problem. Thanks ,Jaimie JimmyDee The initial surge on a transformer is similar to a motor starting. The only way you may get around it is to have the adjustable type breakers that are made for motors. I would think that the 200 would hold. Double check you hook up to make 100% sure you have it wired correctly. I've seen a transformer that was 480 high side only with voltage taps for 460, 440 and 420 or something like that and it was wired like a regular 4 wire primary with H3 and H4 tied together. That one took down a sub station. You certainly don't want to by pass the instantaneous protection if you have a problem with the transformer or the wiring of it. Jim Alfred You might also check your phase arrange is correct,boy,or yob, or oby. This could change current rotation inside the transformer. Always check to make sure this is correct before energizing. Thanks Alfred Johnson Electricman This may be a dumb question and I apologize if I have read your post wrong, But the 100 amp breaker? Is this the overcurrent device for the primary side of this transformer? devo Either your x-frmr is undersized or your breakers are oversized...nothing to do with the tripping but a 30KVA x-frmr works out to 83 Amps @ 208V and only 36 @ 480V according to my pocket chart from Boston Edison lucky1122 please correct me if I dont have this right. You installed in a commercial building a 37.5KVA transformer, with a 240V primary 2 wire feeder,that provides a separately derived system to supply a 480V single phase ,secondary system to supply loads at that voltage .A 37.5KVA transformer will draw 37.5X 1000 /240 on the primary side which is 156.25A a primary breaker would be sized at 125% of that which is 195.31 or the next size higher which would be a 200 A breaker . I'm assuming the 100A breaker is off the secondary side since that also is sized properly for that application. It would be permissable to increase the size of the Oc protection up to 250% on the primary side as long as the transformer is protected on the secondary side at 125% which it is. You haven't mentioned any info which I would find pertinent such as what kind of breaker is it on the primary side and what size conductors are you using. as was previously stated transformers when exited or energized can cause false tripping so the proper selection of overcurrent devices is key. lucky1122 I would also like to add this. Jimmy Dee's advice was well stated make sure if there are variable taps the connections are correct !!!! With a variable tap primary, assuming you use the same taps 480/460/440 you would only raise or lower the secondary voltage . You did say there was no load connected when the primary breaker tripped .