Welcome to ECN!
First off, I would like to clarify a few things, per the EE's Design.
Does the Single Line show the 300 KVA Transformer having Over Current Protection for the Feeders only on the Primary Side Feeders, or Both Primary and Secondary Side Feeders?
Since the Separately Derived System ("SDS") is "Dual Voltage", the Primary Over Current Protection cannot be used to protect the Secondary side feeder Conductors.
Since the Secondary rating is 208Y/120V, there will need to be Over Current Protection for the Feeder Conductors on the Secondary side - as well as the Primary side.
Example setup + values for OCPD (Over Current Protection Device):
Primary rating = 360 Amps @ 480V 3 Phase 3 Wire.
Secondary rating: 830 Amps @ 208V (208Y/120V 3 Phase 4 Wire).
Maximum Primary side OCPD: 900 Amps (250% of 360 Amps) - refer to Table 450.3(B).
Primary Feeder Size: 500 MCM THHN Cu. or equivalent parallel conductors.
Maximum Secondary side OCPD: 1200 Amps (125% of 830 Amps, brought up to the next highest common OCPD rating).
Secondary Feeder Size: (3) sets of 500 MCM THHN Cu, or equal.
Basically, if the Single Line shows OCPD on the Primary Side only, the Design is not compliant, since the Secondary is Dual Voltage (208/120V).
If the Secondary was a Single Voltage - such as 240V 3 Phase 3 Wire, then the Primary OCPD could protect the Secondary Feeder Conductors, and the design would be compliant.
Since it appears that the Secondary is Dual Voltage, the Secondary Feeder Conductors require OCPD.
This may be done in Two different ways:
- 1200 Amp Feeders brought directly from Transformer, to a 1200 Amp OCPD (Circuit Breaker or Fusible Safety Switch).
This option would relate to feeding a gear section from the Transformer, and the gear contains a 1200 Amp "Main Breaker" or "Main Fusible Switch".
*** FYI, The OCPD does not have to be an intregral part of the gear - it may be separate equipment. The OCPD must be "In Front Of" (on Line Side of) the gear section.
- Separate feeders - say of 400 Amp capacity, brought out of Transformer, and run to individual Panelboards - each Panelboard contains a 400 Amp Main Breaker (or Fusible Safety Switch with 400 Amp Fuses).
Review the Single Line for this information, and let us know what's indicated.
As it stands, this issue requires an RFI submitted to the EE, requesting clarification for Primary & Secondary Conductors' OCPD.
Reference NEC Article 450 & Table 450.3(B) in the RFI document.