http://en.wikipedia.org/wiki/American_wire_gaugeTable suggests that AWG14 is suitable for constant currents of 20A with wire temps up to 65C. Thats a pretty thick wire...
As an example, I use standard 220 celsius 18 and 20 AWG silicone insulated multistrand wires up to 35-40CM lenghts on FCP-HL with ESC current limits of 13A and 15A.
The constant current is usually way under 50% of the motors current limit, so I scale wiring for the expected constant current under normal operating conditions with a slight overhead to handle spikes. So for a 20A limit with constant current in the 5-8A range, 18AWG is probably still just fine.