I agree with "Winny" (Jon) on this one.
The Distribution Equipment being fed from this Tapped feeder will require a Main Disconnect / OCPD with GFPE - since the Main Disconnect / OCPD for this Gear is 1,000 Amps and larger, and the System is a Solidly Grounded Wye - exceeding 150 Volts to Ground, but not exceeding 600 Volts Line to Line.
Article 100 Definition of "Feeder" covers this type of Tapped Feeder; and the GFPE Requirements will include Tapped Feeders on Separately Derived Systems (AKA Customer Owned Transformers).
The Hazard of an uncontrollable Line to Ground Arc Fault is not limited to Systems derived solely, or directly, from Utility Transformers.
The Hazard (more correctly, the Potential for hazard) exists on any type System - be it SDS, Emergency Generation, Go-Generation, On-Site Generation, or Utility Derived.
If the Equipment from which the Tapped Feeders are derived from had GFPE, then this new Distribution Equipment would not require a Main Disconnect with GFPE - as the protection would already be in place.
In other words, if the Utility provided the Customer with a 4160Y/2400 3Ø 4 Wire System, and the Customer has a 4160V Delta x 480Y/277V 3Ø 4 Wire 2,500 KVA Transformer feeding a 4,000 Amp Section; and this Section had a GFPE Main Disconnect + the new Distribution Equipment you are installing was fed from this Equipment On The Load Side Of The GFPE Main Disconnect
, then the new gear would not require a GFPE Main.
If the Secondary Feeders between the 2,500 KVA Transformer and the 4,000 Amp Gear were tapped, and this Feeder Tap is used to feed the newly installed Distribution Equipment, then the new gear requires a GFPE Main, because there is no Ground Fault Protection on the Line Side of the Main Disconnect for that Distribution Equipment.
Of course, there are the exceptions for Continuous Production Industrial setting and Fire Pumps, which removes the requirements of GFPE, but I think this installation does not fall into these two areas...???
Anyhow, just wanted to post this information for redundant reasons