Here are some of the airspaces where FAR 91.215 requires aircraft to have transponders--
(2) All aircraft. In all airspace within 30 nautical miles of an airport listed in appendix D, section 1 of this part from the surface upward to 10,000 feet MSL;
(3) Notwithstanding paragraph (b)(2) of this section, any aircraft which was not originally certificated with an engine-driven electrical system or which has not subsequently been certified with such a system installed, balloon or glider may conduct operations in the airspace within 30 nautical miles of an airport listed in appendix D, section 1 of this part provided such operations are conducted -
(i) Outside any Class A, Class B, or Class C airspace area; and
(ii) Below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport or 10,000 feet MSL, whichever is lower; and
Consider the airspace near Honolulu International Airport (now Daniel K Inouye International Airport), HNL/ PHNL. (Link to sectional chart.)
May an aircraft with no engine-driven electrical system operate at 9500' MSL directly over Wheeler AAF (HHI/PHHI)?
This airspace is within one of the 30-nm Mode C veils described in FAR 91.215 (2).
The intent of the question is to ask whether the "Below the altitude of the ceiling of a Class B or Class C airspace area" clause of 91.215(3)(ii) applies only within the actual lateral boundaries of (i.e. within, above, or below) Class B or C airspace, or throughout the 30-nm circle. Wheeler AAF is within the mode C veil for HNL, but beyond the actual lateral boundaries of any of the Class B airspace. The ceiling of the Class B airspace at HNL is 9000' MSL.