If your utility provider has a "star to star" xfmr (grounded conductor required) and you are feeding a 480/277v 3 phase main panel for a/c, heat and refrigeration only, are you required to pull a grounded conductor with the feeders? This particular situation includes an additional secondary set of feeders from main panel to 208/120v xfmr with taps for delta/wye (no grounded conductor needed) for lighting panel.

250-24(a) 1999 NEC
You are required to bring it to the service, and required to land it on the proper terminal. It is for fault situations. If you don't, you have an ungrounded system, and that ain't gonna fly without a REAL good reason.

Why is this not required in a delta/wye situation then?

For all purposes the equipment will run reguardless (w/o neutral) and the ground fault can be achieved through bonding. Additionally how would you size a grounded conductor with no knowledge of load. Possibly by main breaker size or combination of breaker sizes for less than (6) means of disconnect?

Above- I meant delta/delta

The service equipment requires the grounded conductor. The feeder panel does not require a grounded conductor unless there are line to neutral loads. The feeder panel always requires a grounding conductor.
Also are you sure about the utility transformer being wye/wye? That is somewhat rare. Most are delta/wye.
Don(resqcapt19)

I'm positive about the wye/wye xfmr. It was brought to my attention by Central Power and Light. The original wiring was just a three wire 480/277v application (aparently improper from the start) and it was to be upgraded from a 400amp 500 kcmil service to 600amp, 500 kcmil parallel (x2). That is what brought up all the questions. However, I was trying to learn if what is needed is a ground or neutral since no load will be present. How is this to be sized? Under NEC art 250-122?

If the utility has grounded XO at their thransformer, then 250-24(b) requires that you bring a grounded conductor to the service equipment. This conductor is sized per Table 250-66 or the 12 1/2% rule for services with service conductors exceeding 1100 kcmil. This is the only path for fault current to return to the transformer in the event of a ground fault. Once you leave the service equipment and run feeders to the sub-panels, the grounded conductor is only required if the sub-panels are serving line to neutral loads. In either case an EGC sized per Table 250-122 must be installed between the service equipment and any sub-panels.
Don(resqcapt19)

Thanx for your response. I thought I needed a grounding conductor, not a grounded conductor, but now I see what you mean. I will need to bond the main panel (first means of disconnect) to the neutral. Without a grounded conductor, I got bupkis.