This is where you research Wall Wart VOLTAGES. Applying 120V to a 12V device will push the electricity clear thru the device and out the other side taking no hostages. VOLTS on the other hand is the pushing force of electricity. You could have 10,000 amps available, and no harm will come to your device because it only needs 500ma and that is all it will draw - unless of course your DEVICE is broken, then it might be a pretty fireworks show - or not so pretty. How much current the device DRAWS is a function of the device. PLEASE note: Current or AMPS or ma is the amount of current AVAILABLE. If it is labeled as backward compatible, then is must draw 500ma or less. If you plug a USB3 device into a USB2 socket, it may or may not work depending on how much current the USB3 device needs. If the USB3 port supports charging currents, you could probably use a USB2 4 port expander without an external power source. A USB2 portable HDD will probably work without the need to plug in both A connectors or to add an external power source. A USB2 device will charge faster in a USB3 connector. SO, if you plug a USB2 device into a USB3A or B connector you can draw up to the limit of that connection. The USB3 connector simply adds 5 pins which are +Rx, -Rx, +Tx, -TX, and GRD/DRAIN. HARDWARE wise, the power pins on a USB3A or B connector are the SAME pins as on a USB2A or B connector. USB3 is 900ma, so all USB3 connections supply 900ma (some may supply more to comply with the charging spec). USB2 is 500ma, so all USB2 connections supply 500ma. WHY is this so difficult? Because people keep talking about the SPEC and not the hardware! The spec refers to how much power a USB device CAN use and therefore how much power the USB connector must supply. I personally keep one of those laser thermometers next to me to check the temperature of the wire and plugs before starting the test, and the temperature as time passes. 10 to 15 minutes at least, and do not just plug it in and then come back after the time is up - make sure you keep an eye on it for the full duration. If you do go ahead, then make sure you test to see if the cable or plug gets hot after using it for a while. Some of the makers of USB cables try saving money by using wire so small that it cannot cope with the extra 400mA difference between the 500mA of a USB2 and the 900mA of USB3. If you try to draw 900mA down a short cable then you may be able to, but if you try to draw the same 900mA down a 3 meter cable then most likely you will fail and it could even cause a fire by either the connector or the cable getting hot. The length is seldom considered when using cables.
It will depend on what size the wires are in both diameter (gauge) and length. No, not all cables are capable of carrying the full 900mA.
I think the answer you are looking for is all about the cheap cables some companies supply, and if they can cope with carrying the full 900mA.
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