I have some USB 3.0 ports on my motherboard which can deliver up to 900 mA of current I believe. If I plug in a USB 2.0 cable, I can still get up to 900 mA right? I have some devices that uses USB charging and outputs from 1A to 2A but I'm not sure if the cable provided is a factor or not since I'm just using a generic USB cable for this.

  • I doubt it. The USB 3 cable is different to USB 2, so I would expect the maximum to be dependent on the USB 2 cable. – user3463 Jul 22 '12 at 1:18
  • 2
    USB 2 supports up to 500mA and USB 3 supports up to 900mA (are you sure your devices pull the full 900mA?) The specs are more for supporting the higher data transfer speed than the higher power draw. A USB 2 cable may get hotter than a USB 3 cable when drawing 900mA, but I highly doubt that a USB 2 cable is going to “burn” from having 900mA drawn through it (at least not if used briefly as a short-term stop-gap). – Synetech Jul 22 '12 at 1:28
  • @Synetech Well the power supply/charging adapter that came with the devices output 5V 1A+ so I'm pretty sure it supports that much at minimum. Since that is the case, I know that at least the USB cables that came with the charger can support 2A but I'm not sure if that's the general case for all other USB cables. – Jack Jul 22 '12 at 2:18
  • 2
    You have it wrong. The power adapter says it supports up to 1A, meaning that a device (whatever it happens to be) that is connected to it can draw no more than 1A from it. That does not mean the device itself actually draws that much (especially when merely charging). Don’t look at the power-adapter; look at the device itself (near the power connector or on the bottom) to see what the actual input current rating is; it may easily charge from a USB 2 cable if it is low-power enough to not use the extra USB 3 conductors. Is the device even a USB 3 device? – Synetech Jul 22 '12 at 2:26
  • 1
    @Synetech No, I know that. You don't get my point. The fact that the charger can output 1A or 2A with the supplied USB cable must mean that the cable is capable of supporting 1A or 2A of current. You're not reading my question correctly. I'm not asking about the device. I'm asking about the USB cable. And most devices only supply a 700 mA charger because it's cheaper. There is absolutely no point for them to supply a more expensive charger 1A+ if the device can't draw at least that much. I'm looking at what is being supplied to me by the manufacturer. – Jack Jul 22 '12 at 2:59

If the output is only 900 mA, your device will still charge, albeit, a little more slowly.

USB 3.0 is designed to be backwards compatible with USB 2.0, and for this, I believe it is. See everythingusb.com and the Power Requirement Comparison table (from Buzzle.com for details.

  • Oh I know it will still charge, I just wondered if a USB 2.0 cable can support 900 mA. – Jack Jul 22 '12 at 2:15
  • "maximum current drawn by the device may be as high as 1.8 A" Wikipedia.org. – wizlog Jul 22 '12 at 2:20
  • there are 2 modes of charging. most laptops usually max out at 250 or 500ma i cant remember off hand. many wall worts are designed for high speed charging of up to 2A. Some systems may support such high speed charging from bios but that depends on the system. The wire size inside the chargers and usb cables are the same size as far as I know, though cheepos may not be. wire size is what limits charging capibility in the cables. I havent read the usb spec to see if its required or not but suspect considering wire size they wouldent want to have to mess with more sizes of wire. – Kendrick Jul 22 '12 at 3:59
  • Powered usb hubs should be able to handle high speed charging as well. My usb2 powered hub is rated for 1A. – Kendrick Jul 22 '12 at 4:02

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.

No, not all cables are capable of carrying the full 900mA. It will depend on what size the wires are in both diameter (gauge) and length. The length is seldom considered when using cables. 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.

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 do go ahead, then make sure you test to see if the cable or plug gets hot after using it for a while. 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. 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.


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. USB2 is 500ma, so all USB2 connections supply 500ma. USB3 is 900ma, so all USB3 connections supply 900ma (some may supply more to comply with the charging spec). HARDWARE wise, the power pins on a USB3A or B connector are the SAME pins as on a USB2A or B connector. The USB3 connector simply adds 5 pins which are +Rx, -Rx, +Tx, -TX, and GRD/DRAIN. Notice, not more power pins. SO, if you plug a USB2 device into a USB3A or B connector you can draw up to the limit of that connection.

What this means in real life. A USB2 device will charge faster in a USB3 connector. A USB2 portable HDD will probably work without the need to plug in both A connectors or to add an external power source. If the USB3 port supports charging currents, you could probably use a USB2 4 port expander without an external power source. 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 it is labeled as backward compatible, then is must draw 500ma or less.

PLEASE note: Current or AMPS or ma is the amount of current AVAILABLE. How much current the device DRAWS is a function of the device. 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. VOLTS on the other hand is the pushing force of electricity. Applying 120V to a 12V device will push the electricity clear thru the device and out the other side taking no hostages. This is where you research Wall Wart VOLTAGES. 12V on a 5V device is not good.

I hope that helps


No, it can’t because a USB 2.0 cable is different than a USB 3.0 cable, and both the cables have different outputs. For example, USB 2.0 delivers 500 mA and USB 3.0 delivers 900 mA.

  • 3
    The difference is more data wires. Power transmission isn’t changed. – Daniel B Apr 28 '17 at 9:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.