6-402(1) Meter loops shall be installed so that: (a) Conductors between the service box and the meter are inaccessible to unauthorized persons;
and
6-404 (1) Instrument transformers used in conjunction with meters shall be installed in metal enclosures, except where access is only to authorized persons.
Just trying to keep people from modifying the wiring I guess.
As well 36-208 may come into play for installations above 750V. This rule requires that fuses (usually mounted with the PT) can't be accessed unless the power is off, and can't be re-energized while accessable. This typically requires an enclosure interlocked to a disconnect, or a drawer style that physically is disconnected while open.