GRA wrote:SCE provides power at Tuolumne Meadows, and PG&E covers the Valley, Wawona and presumably Badger Pass.
That doesn't necessarily mean that the POCO (power company) owns the transformers, in some case the "service point" is upstream of the transformers and the customer owns them. So you need to find out where the service point is and who owns the transformers.
If the customer installs loads that are larger than can be carried by the existing POCO-owned transformer, then sometimes the POCO just upgrades the transformer without charging the customer. Or sometimes the POCO wants to be paid by the customer for the upgrade. That's a question for the POCO in each case.
Of course, if the customer (the Park Service?) owns the transformers, then any cost of upgrading them is on the customer.
GRA wrote:I was thinking this might count as a feeder, and thus would need the de-rating.
Ratings tend to be continuous on the POCO side of the serivce point, while the NEC applies on the customer side of the service point. So if you have a 25kva 3 phase transformer at 208Y/120, that means the transformer can provide up to 25000/120/3 = 69.4 amps on each phase continuously. If the customer has a continuous load that draws 69.4 amps on each phase, then the NEC requires sizing the conductors after the service point with a factor of 125%, or a minimum of 86.8 amps. In practice you'd install a "100 amp" service (a standard size), using conductors sized for at least 100 amps under the NEC, all fed by that 25 kva POCO transformer.
In fact, the NEC load calculations are generally very conservative. So the NEC might tell you that you need a 200 amp 208Y/120 service, and you'd install 200 amp conductors from the service point to your main panel. But the POCO knows that calculation is conservative, so depending on your load type, it might choose to feed your 200 amp service with only a 25kva transformer. It knows that in practice your demand per phase will seldom exceed 69.4 amps, and that the transformer will be OK with occasional short term overloads.
If the load consists principally of EVSEs, that that is one case where the NEC calculation isn't overly conservative, other than the 125% multiplier for continuous loads. So three 32 amps 208V EVSEs really could draw 64 amps on each phase for a long period of time.
BTW, if you want to balance the EVSE loads across all 3 phases, another option is to install (2) 32 amp 208V EVSEs, one on, say, phase A and B, one on phase B and C, and then (4) 16 amps 120V EVSEs or (single) receptacles, two on phase A and two on phase C. When all 6 are then running flat out, the current on each leg will be 32 amps.
GRA wrote:They're all pad-mounted
Did you take photos of the name plates?