See in power supply there is a rail which support and provide current to all the components present in the power supply. Now in rail current is measured in amperes instead of wattage. So in any component specifications there should be wattage consumption written somewhere. Now you don't know if your power supply rail will support it or not so what you do is divide that wattage with 12 to get the amperes, for example hd 7850 consumes 130W so thats make 13 amperes: 130/ 12 = 12.8{13amperes overall}. Now in some power supplies there is 2 rails instead of one to divide the burden so in in case of dual rails there are different amount of amperes like one rail give 20 amps and other give varying amount like 20, 17, 18 amperes. In these cases the rail with the most amperes only give current to processor and the graphic card while the other give current to rest of the components. Same rule apply here you calculate the amount of amperes your graphic card and processor consume and see if it support both your graphic card and processor otherwise your power supply will burn. Remember in this case you can leave other rail and only worry about the one with most ampere cause it the one going to give current to processor and graphic card. Now there is a disadvantage in dual rail too. But that i will explain later.
In you case you have a single rail with 30 amperes. I calculated the ampere consumption of your processor and graphic card and added them which made the total 20 amperes while the 10 amperes is for other components. As for why don't we measure amperes of other components present in the pc that is because they all consume very little amount of ampere like they all will consume 5 to 7 amps max. But in case you cutting on short on amperes you should consider a new power supply. Hope this explained things to you a bit.