Voltage drop in the EGC is never _explicitly_ dealt with, but 250.122(B) is a crude and imprecise requirement that does deal with voltage drop. Essentially what 250.122(B) says is that if you do anything to deal with voltage drop in your circuit conductors, then do the same thing to you EGC. Of course, low voltage drop is _not_ a requirement of the code, and thus you could do something silly like run a 2000 foot long 120V 20A circuit using #12 conductors along with a #12 EGC; a bolted short between hot and ground would _never_ trip the breaker.
-Jon
