I have 15kVA dry type 480V single phase to 240/120V single phase transformer.
Following:
1. Does the primary breaker protect the secondary conductors or does it need to have secondary main breaker?
2. Is the transformer separately derived need grounding electrode or not?
3. Would rules of 450 apply sizing primary breaker and secondary breaker?
1. There is a very limited selection of qualifying topologies that enable you to protect the secondary conductors indirectly by the primary OCPD. Anything with a center-tap connected to your loads, or a WYE system, does not qualify, because there is a possibility that an overload on the secondary will be in the "blindspot" of the primary OCPD.
The qualifying topologies are a 2-wire to 2-wire transformer, or a delta-delta 3-wire on both sides, the overcurrents are going to line up winding-to-winding, and not get re-distributed, which is why the primary OCPD can indirectly protect the secondary conductors. Given a 480V primary, and a 240V secondary for a qualifying topology, a 100A breaker on the primary effectively acts as if it were a 200A breaker on the secondary. Another example: 277V to 120V 2-wire to 2-wire 1-phase transformer. In this case, a 100A breaker on the 277V primary, would effectively protect the secondary conductors at 231A. It's the voltage ratio that governs this calculation
2. Most transformers produce a separately-derived system, because transformers can only transmit a change in voltage across the isolation through the magnetic coupling. They cannot transmit absolute voltage, if the two sides are electrically isolated from one another. You need to bond to ground and to use a grounding electrode connection, if you want to re-establish the secondary as a grounded system of voltages.
The kind of transformers that do not produce a separately-derived system, are called autotransformers. The secondary of an autotransformer shares a reference to the same grounded conductor as the primary.
3. Article 450 applies for protecting the transformer itself. Article 240.21(C) is for protecting the transformer secondary conductors.