If the transformer is an autotransformer with wye connections on both the primary and secondary (I don't know if there is any other kind), it does not create an SDS. The neutral is common to both sides.
There are not two "sides" with an autotransformer, just a tapped winding (if single phase) or windings if, as here, 3 phase.
Yes, I agree about autotransformers. I was asking whether one can choose to install an isolation transformer as a non-SDS, or if the NEC requires that they be installed as SDSs.
So 250.20 (and 250.21) tell us which AC systems are (and are not) required to be grounded. When an isolation transformer is fed from a grounded AC system on the primary side, and the secondary side provides an AC system that is required to be grounded by 250.20, is there some section that says the second side grounding must occur as an SDS, with a separate GEC to the GES? Or may one instead elect to ground the secondary side AC system by directly connecting the primary and secondary grounded conductors, creating a non SDS?
There's no direct prohibition. I remember once discovering an indirect prohibition but I can't remember what it was. Right now I am thinking about sizing conductors for ground fault current on the secondary side, especially for a step-down transformer. If the primary has a grounded neutral, sizing it for rated primary current or "not smaller than the required EGC" could make it too small for a ground fault on the secondary side of the transformer.
I know how an autotransformer is constructed. What I meant by the two "sides" are the 480V (one side) and 208V (the other side) terminals. The neutral is common to both "sides" so the transformer does not create an SDS.
