Inrush can be somewhat elusive in that a breaker that trips when a transformer is first energized may not trip the next time. It's not really an exact science but an educted guess in an attempt to stay away from an obvious problem.
Inrush is not something that is required to be tested for and, as such, very little if any data exists. Inrush is not something that can be calculated unless you had the coil winding information, the core design and the magnetic properties of the core steel and how the two are assembled together. The design engineer does have the ability to calculate what the expected inrush would likely be from his/her design information.
As a breaker and DTDT application engineer the DTDT product line manager asked me to do a study on the application of pri. MCCBs and DTDTs.
Working with a transformer design engineer he provided me with some typical inrush values for variety of transformers. He did not have calculated values but had enough data to provide what inrush values one would expect. Inrush is express as a multiple of the FLA of the xfmr. Tpicall as the transformer kva get greater the multiplier decreases. Here?s an example based upon 150degC:
X FLA KVA
23 To 7-1/2
19 To 11
14 To 14
11 To 51
10 To 75
9 To 145
8 To 170
These values are not definite value but values that will provide an overall picture of a trend.
As you can see the median appears to be 75kva a 10x the FLA. These values, however, were values from about 8 years ago. Since TP-1 was set in place these values probably increase to some degree.
Besides energy efficient transformers have a greater inrush, going to 115degC will increase the inrush and 80degC a bit more. If one thinks about it in each case the primary winding have to be change to either increase efficiency or reduce temperature rise which will increase inrush values.
K-factor also increases inrush values also.
115degC rise and K4 may increase inrush values 15-25% where 80degC and K13 design could be and much as 50% more.
Custom tranformer manufactures can design transfomers with reduced flux in order to reduce inrush. HV coils are normall placed on the outside and the LV on the inside. When this transformer is used as a step up an inrush value that would have been 10x on the HV windings will most likely be more because the LV winding is closed to the core. However, those coils can be reversed when designing step up transformers reducing inrush values on the LV winding. Anyway you can get the taps on the correct winding then because it's on the outside.
I have always advised that there is no real established values which most of us are looking for. If you use this transformer you must use this breaker of risk nuisance tripping, etc.
The only thing that I advise is to not back yourself in a corner by being forced to use a small frame breaker because that's the biggest breaker that can fit in the panel. Another good one is getting a good deal on a 75kva transformer and only have a 45kva load max, sizing the cable and MCCB based on the anticipated load.
The expected inrush of a 75kva transformer with a 480v, 3ph pri rated 90FLA would be 90x10=900a minimum. It most likely would be higher. If you a limited to a 100at breaker that breakers magnetic calibration is 100x10=1000(+-20% calibration range). As you can see it may be a bit marginal. If you look at art 450-3 of the NEC with a 1ph3w or a 3ph4w secondary you not only have to provide a pri. OCPD but you must provide a sec.OCPD. If so you are allowed to use a OCPD rated up to 250% the FLA of the transformer.
My advice has always been to at least know the risk and then understand the results of those risks.
You shouldn?t have any issues unless you carelessly undersize a pri. MCCB without taking the risk into consideration or apply a MCCP of a small KVA transformer as a large one, or respect the fact the inrushes for 115, 80deg, K4-K13 transformers are going to be higher than the common 150degC transformer use. Use as much of the 250% as you can unless you are confident that 125% will work in your application. But going under that may be a bit risky and 125% may not be sufficient with transformers under 75kva.
