It’s not really a service to the building. Does this make any difference?
Physics-wise, one might think it shouldn't, but code-wise it does. Physics doesn't care about transformer ownership and circuit classification, but the NEC does, and has different standards that apply in each case.
Customer-installed conductors from the secondary of a utility-owned transformer, are service conductors, and comply with article 230. 240.21(C) doesn't apply.
Conductors from a customer-owned transformer, are transformer secondary conductors governed by 240.21(C). These rules are very similar to tap conductors, from 240.21(B). The underlying idea is that such conductors are only protected in excess of their ampacity, and "not yet protected" at their ampacity, and thus have a higher risk. Lengths are limited to reduce this risk, but still given to allow a practical length to work with, in order to connect to a grouped piece of equipment that hosts the OCPD & shutoff. Or with no limits on length, for outdoor circuits.
240.21(C) doesn't really offer guidance on how the unlimited length outdoor rule, interacts with the length limits inside a building, for a circuit in both environments. My reasoned judgement on this, would be to pick one of the length limits for the scope of the wire inside the building and comply with all context rules for both applicable subsections of 240.21(C). As an example, 20 feet inside a building, and 100 ft outside a building, would mean complying with context rules of both the 25 ft rule and the outside unlimited rule. You'd probably have to deliberately go out of your way of what most people would do anyhow, to not comply with context rules of both sections.
