i used to run a 25A esc until a few weeks ago, after i hooked up my wattmeter to my 4100kV w/8T and got 9A at full collective (+12 degrees pitch); this was at the battery, with a long can geared brushless tail spinning ~half speed, so the main motor was probably drawing closer to 6A. ever since i've been running a 10A esc to save weight.