its not 'just' about the cable able to withstand x-ampere..u need to understand oven & aircon are switched on for a long period most of the time..oven especially draws high constant current for a long period of time (few hours)..the more current it draws, the hotter the cables becomes..with bigger cables, the copper able to deal with the heat generated by the current, less so with smaller cables

If you wan real time experience, buy one of those extension cord, use it with your vacumm cleaner. Turn and leave ur vacumm on for 30mins or so..then feel the extension cable with ur hand and u will understand

I'd recommend u use 4mm for your oven & your aircond
Remember, this is one off thingy u have to do..u probably wouldnt wanna rehack your wall to conceal the cables again if anything happen to the smaller cables.