This is close as I could get for you. 12 watt USB A Apple charger going through a USB A to USB C cable. Its the shortest I had, 8" (least resistance).
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Even that short 8" USB A to USB C has too much resistance to get 12 watts. Voltage has dropped below USB spec acceptable limits at 4.71vdc. If I increase the load by even a faction of an amp higher the power supply trips off on its safeguards.
I think with a USB C using a USB C to A adapter on the charger the results would have been a tad bit better.
You might find this interesting. We know that 12 watt power supply is doing as it advertises from the above pic.
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BRAND NEW APPLE Lightning cable can deliver around 10 watts max, trying to increase the load again to get as close as I could to 12 watts only gets me to 11.3 watt and WELL below 5 volt we need.
Moral of the story....volts x amps = watts. When you restrict the voltage to 5 vdc the only way to increase watts is to increase amps. And higher amps at a low voltage makes the requirements for the cable (to minimize all the resistance you can) just too unrealistic.