There is nothing in the NEC that bears the title, "Grandfather Clause." But there is a statement that has an effect something along those lines. I am not yet at work, and I don't keep an NEC at home, so someone else will have to look it up. I believe it is in article 80 somewhere. That has been moved into the Annex at the back of the book, so it is not an enforceable article.
The words say something to the effect that an existing condition that does not meet current NEC requirements does not have to be brought up to modern standards, unless the authority having jurisdiction believes it to represent a threat to safety. The words do not, as written, say anything about the installation having been in compliance at the time it was built. But words to that effect have been added to several local codes, including the one that applies in my area.
However, no code, federal, state, local, or other, is going to agree with the proposition that, "Yea, I know it's a violation, but it has been that way for a long time, so therefore I am grandfathered, and I don't have to fix it."