We are going to install a service entrance ATS with a utility disconnect switch MCCB, input power coming from utilities 13.8KV secondary trafo of 220Vac,3Ph and from 200KVA,3Ph, 525 Ampere generator
Downstream is a 2 x 30KVA AC UPS (parallel Redundant) to supply power to the plants SCADA Control Room.(max.load in the room)
The engineer incharge for this project (client side) ask for a 400A service entrance ATS(with a utility disconnect switch MCCB), however i find it too big for its total load requirement (assumming UPS full load of 60kVA/72KW [p.f=0.8])
generally i would size the ATS rating using [Total System Load] divided by [square root of 3]*[220V] = 72,000 / 381 = 189 Ampere, hence i would say 200 Ampere Service Entrance ATS is more suitable.
Am i correct with my sizing calculation? if im wrong, please advice me correct sizing calculations.
what will be the effects if we use the 400A SE ATS??? technical issues???