National Science Foundation (NSF) in US enforces a limit on salary compensation over all grants, often referred to as the two-ninths-rule:
Summer salary for faculty members on academic-year appointments is limited to no more than two-ninths of their regular academic-year salary. This limit includes summer salary received from all NSF-funded grants.
So, for a 9-mo faculty who can fill up to 3 months of their own summer salary, NSF as the sole source of salary compensation doesn't work no matter how many grants one has.
Compared to the NIH model where there is no such limit, in fact 100% of salary can be recovered in theory via NIH grants, this is a rather strong limit.
What is the motivation of the NSF two-ninths-rule? Why not three-ninths to cover the whole summer salary?