Table 18 exist in BC again. the 2006 code made som very big changes to Grounding and bonding conductors. For some very sound technical reasons the BC code committee did not adopt the changes to tables 17 and 18.
T17 is the minimum size of grounding conductor for AC systems or common grounding conductor. and would apply for services and secondaries of isolated distribution transformers or other seperately derived systems.
T18 is for grounding conductors for service raceway and service equipment. Bonding jumpers and ground conductor sizing for ungrounded systems.
It should also be mentioned that t16 is now based on conductor size not the overcurrent device. It can have a big impact if you say upsize conductors for voltage drop on a string of parking lot lights and forget to upsize the bonding conductor too. Alternately you will be good with the number 12 bonding conductor for a motor with a 24 amp fla fed with #10 on a 40 or 50 amp breaker. Thus making teck cable a good wiring method for a motor.