Transformer

[the juicer]

Changed out a sub panel 400 amp -277 /480 volt , being fed from a 2000 amp mdp, turned on the 400 amp feed in mdp,all breakers turned on in sub panels feeding lights came on with no issues. We turned on the 2- breakers In Sub panel feeding 2 separate 75 kva dry type transformers,well we tried. When either transformer got power turned on it would trip the 400 amp in the mdp instead of transformer breaker. Did the same thing with either transformer. Scratching head now. These things have been working fine since mid 80s. Changed out sub panel because of ac units frying some breakers in sub panel.also installed a main in sub panel,which didn't trip.

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[augie47]

Possibly the trip curve on the main is set too low and the inrush current is causing a trip.

 

[jim dungar]

Did you adjust the magnetic/instantaneous setting of the new 400A breaker?

 

[Jraef]

Was the 400A breaker in the MPD new, or is it the same one that has always fed the old sub-panel?

If it’s the same one, it’s still possible that the mag trip coils in the old feeders were dampening the transformer inrush just enough to keep the 400A breaker mag trip from tripping, and your new feeders for the transformers are different enough that more gets through. It might just require a tweak to the mag trip setting of old 400A breaker. So either way, that’s where I would start.