I have an incoming service 600v 100amp going to a 3 storey commercial bldg, it is being fed overhead, my question is this, can i run the conduit though the parking garage ceiling 50'to the electrical room into the main then into the meter center.Something tells me i can,t but if i do underground from the weatherhead to the electrical room that is ok?

That "something" that is "telling you", might be 6-208. Underground is OK but 50 ft through the parkade is not allowed.

Where judgement required about "as close as practicable", the following may be used for
guidance on the maximum length of run:

(i) 1.5 m into rooms or areas of combustible construction; or

(ii) 7.5 m into other areas provided:
„h the wiring method is rigid conduit, electrical metallic tubing (EMT) or a cable
with a continuous metal sheath or interlocking armour, (No PVC greater than
625mm2 as per Building Code), and
„h the conduit or cable is mechanically protected where it is within 2 m of the floor.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority