"61.129 Aeronautical experience.
(a)For an airplane single-engine rating. Except as provided in paragraph (i) of this section, a person who applies for a commercial pilot certificate with an airplane category and single-engine class rating must log at least 250 hours of flight time as a pilot that consists of at least:
(1) 100 hours in powered aircraft, of which 50 hours must be in airplanes......."
Interestingly, if you have 50 hours of motor glider time that should count towards the 100 hours of powered aircraft time.