Let's look at the power executioner of a quad, the electronic speed control, commonly known as ESC.
The ESC has three major sets of wires coming from it: One pair receives power from the battery. Another set of wires connect to the motor, in which order is not of essence, as in the case of a BLDC (Brushless DC) motor. The third set of wires, power the receiver and also receive inputs from the receiver.
ESC in a UAV performs two major functions; controlling the power to the motor and powering the receiver, by the use of an inbuilt battery eliminator circuit. This reduces the use of an extra battery. It is of great importance that, while building a UAV, we choose an optimum ESC, with a certain amount of tolerance. The transmission of current and voltage have fluctuations while functioning. Hence, when selecting an ESC, always keep in mind to choose the ESC with a higher rating, by a minimum of 5 amps, more than the maximum current drawn by the motor and the receiver combined.
Here are some commonly used ESCs:
ESC 20 Amps
ESC 40 amps