First, I'm confused: is that circuit a source or a load type?
In case it is a load, if you apply 120 VAC to it the circuit will "pull" 20A from the source, giving you the 2400 W. And it will never go over that threshold unless it was badly projected or it is defective.
In case it is a source, and it characterized as a *voltage* source, if you connect a 20 A load it should (in a perfect world) work at its maximum ratings, but still as expected. Now, if you gradatively require more current from it, you should expect the output voltage to drop (because output power would exceed 2400 W).
Example: Your source circuit is 120 VAC and 20 A max.
First case: you connect 10 incandescent light bulbs in parallel (let's say, 120 VAC, 0.5 A each)
This way you would require 5A from your circuit. 120 VAC x 5A = 600 W, which is below 2400 maximum rating, so your circuit will be ok and output will be steady 120 VAC.
Second case: You connect 40 light bulbs in parallel, giving you the 20 A.
120 VAC x 20 A = 2400 W, which is exactly the maximum rating. Your circuit should still behave just like the first case.
Third case: You connect 50 light bulbs to it. (25A)
120 VAC x 25 A = 3000 W, exceeding the max output power for your circuit.
In this case, the output voltage *will* drop, and the output current will also be lower than 25A. But, you can assume they will be values that will still give 2400 W when multiplied (E x I less or equal to 2400 at all times).
So, in this case, you could expect 107,3 V and 22,36 A output, giving you 2400 W.
