When is a resource considered over loaded?

Summary:

We have setup work definition with a primary resource and alternate resources. We would like to see how the alternate resource is consumed when primary resource is over loaded. At what point is the primary resource considered over loaded? We have a constrained supply plan and resource is a soft constraint.

With Planned orders with quantity as high as 30000 units, we still see that the primary resource is being used with resource utilization over 300%-400%

How do we ensure that the alternate resource is getting consumed instead of over loading primary resource to a point where resource utilization is shooting up to such high levels