I was c ommisioned my my Dry Type Distribution Transformer (DTDT) product manager to review the coordination of these transformers with the primary OCPD breaker. Being a molded case circuit breaker application engineer I had all of the trip curves but getting my arms around the inrush values for the kva range of the transformers as well as taking into consideration the various temperature rises (150, 115, and 80degC) and K-factors added some spice to the study. My study relied heavily on the esimated inrush values as provide by a long tome tramsformer design engineer as testing of calulating the inrush data values was far from the top of his priority list. But, never the less he gave me some values that he would expect.
In general expect 150degC transformers to have an inrush of up to 23x FLA for DTDTs up to about 7-1/2kva, down to 8x for transformers in the 170kva range. My notes show that a 115degC would be about 20% higher, 25% higher for K4 designs, and an 80degC or k13 designs 80% higher.
Remember that ths is not an exact science but gives some "ball park" values as to when you can expect a possible problem.
The common 100kva 150degC 480-120/240v 1ph 3w transformer that I have reference to 208fla. If the pri OCPD was sized at 125% that would be 260a which is a nonstandard size. You are then allowed to go up to the next standard size which is 300a. Knowing the a inrush value for a 100kva transformer is in the ballpark 208 x 9-10 times or 2080a and that the magnetic trip of a 300a breaker is calibrated at 10x its rating, or 3000a +-20% or 2400-3600a (you can expect them to be commonly calibrated on the high side) as you can see 3000a is much greater than 2080a. A 115degC be 2496a which should not be an issue. When you get you to the 80degC then it may become an issue with an anticipated inrush of over 3328a.
When this inrush value gets on the high side if applied correctly using that 250% value as alowed in NEC art 450 can get you out of trouble if you have a nuisance trippin issue.
Expect the inrush for you 150kva transformers to be 9x or a bit less the pri. FLA. or 1800a + 20% and + 60% for the 115 and 80degC designs.
Remember that these are ballpark figures to be considered in as attempt to stay away from potential problems of to take into consideration should there be a nuisance tripping problem.
One of the biggest errors that I have seen is when the contractor sizes the pri. OCPD based on his anticipated load, sized the cable, and then applies the pri. OCPD which often is much less than pri. FLA rating of the transformer.
Or, where a panel limits the allowable size of a breaker such a 100a or at 225a where you cant install a 250a or 300a breaker with little or no condideration for the inrush of the transformer.
A 225a breaker applied with that 150degC, 1ph 100kva with 208 pri FLA my be pushing it a bit. The 225a breaker for the 150kva seems OK unless the transformer is 80degC. The you may anticipate a problem.
Dave
