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A load reaches its maximum limit due to one of the following:

1. Power Transfer limit -- typically, caused by long distance from generation to load
2. Voltage collapse -- typically, caused by reactive power deficiency

One way to do it is to gradually increase your load (MW and/or MVAr) and run the case every time you increase until you hit a point where the case diverges. Do it in small steps (for examples increase 10% as you go up).

Another way to do it is to check the PV or QV curves:

• PV curve indicates the point of power transfer limit
• QV curve indicates the point of lack of reactive power support (voltage collapse)

Usually, in a power grid, MWs are the main concern form a load point of view. So I would more recommend doing a PV analysis to determine your load limit. Whereas a QV analysis is usually applied to see if you need reactive power compensation.

However, in industry what really dictates for us the limit of how much power you can have on a bus is the rating of the transformers installed between the bus and the load. In power system literature cases, these kind of details are usually ignored/neglected for the sakes of simplicity.

So say for example, you have 3 transformer, each of rating 30 MVA. That means you want to make sure that you can safely deliver 90 MVA without hitting your power transfer limit or voltage collapse limit. Keep in mind that loads tend to have high power factors (0.8-0.9) so most of that is going to be MW. As such, I would say your main concern is to determine the power transfer limit, rather the the reactive power limit.