Under the Alberta OHS Act when does a work site require a prime contractor?

At every construction site that is more than $20$ minutes from a hospital

A construction worksite where there are $2$ or more employers at the worksite.

All type of work activities at a worksite where the Owner has identified a Prime Contract to be responsible for the worksite

All types of worksites if there are $3$ or more employers involved in the work at the worksite