The DC and Ground wires supply 5 volts DC to power devices connected to the port. My laptop has the latest version of USB port, known as USB 3.1. The specifications for this version say at the connector on the PC, the DC line must be within +/- 5% of 5 volts, and supply current from 0 to 3 amps. The PC contains the electronic equivalent of a fuse, or circuit breaker. If you try to draw more than 3 amps, it will simply drop the voltage to zero, and keep it there until you power cycle the PC. I want to model the 5-volt DC power source in the PC as an ideal voltage source, in series with an internal resistance.

What is the maximum possible internal resistance the PC can have in this model, and still have the port meet the USB 3.1 standard?

Part II

I connect the USB 3.1 port on my PC to my phone using a 3-meter-long USB cable. The DC and ground wires in the cable coper and #22 AWG.

What is the maximum amount of power, in watts, that I can deliver to my phone?