What is the maximum power supplied by a USB 3.0 port?
USB 3.0 specifies 900mA (0.9A) per port.
What happens when you plug a non-powered (no AC adapter) hub into an USB 3.0 port? Do all of the items plugged into the port need to use 900mA maximum total combined, or can they safely exceed that amount?
For example, on a 4-port non-powered USB 3.0 hub, could you have:
- Port 1: 400mA device
- Port 2: 400mA device
- Port 3: 400mA device
- Port 4: 200mA device
Or would it stop working when trying to plug in the device into Port 3?
Solution 1:
For example, on a 4-port non-powered USB 3.0 hub,
This example is ambiguous. There are two kinds of "a non-powered" hubs, aka "bus-powered" hubs.
(1) The certified ones would have a special logical switch if plugged without external AC-DC adapter, and report to host as being "bus-powered", with an information on how much the hub silicon consumes itself.
(2) Hubs that fake their descriptors and falsely report themselves as "self-powered" hub, no matter with AC adapter or without.
In case (1) the host will make note of hub being bus-powered, and of its own power needs. Then, when sequentially enumerating devices from Port1 and so on, the host will read the device nameplate requirements, and check it against available port power budget of 900 mA. If the total doesn't exceed 900 mA, first device will be allowed to connect and operate. The same will happen with a device connected to Port2, etc, until the upstream (host) port budget is exhausted. The next attached device won't be allowed to operate. (Note: this USB 3.0 power budget schema is more elaborate than in USB 2.0, where a bus-powered hub would allow only under 100 mA devices per each port, no matter if all other ports may consume nothing).
In case (2) all devices will be allowed to connect, but the delivered power will likely sag (and cause hub malfunction and/or unstable operation), or host port would trigger its overcurrent protection, or cable might smoke out. You choose.
Solution 2:
I tested it: Put an USB3.1 Hub in USB3.0 port, then connected 2x external HDDS to it:
- first needing 500 mA;
- second needing 800 mA.
Both drives started spinning. Then started a copy from the second to the first. ASAP the operation started, the second drive turned off abruptly. So in conclusion: An USB3.0 port will not provide more than 0.9A total current to the underlying bus-powered hub no matter how much ports it has.
Edit: USB3.1 and 3.0 mean one and the same in my case, USB IF renamed USB3.0 to USB3.1 Gen1.
Solution 3:
For a USB-A port on a computer to comply with the USB 3.0 spec it must be able to supply at least 900 mA. For USB-C the minimum to meet the USB 3.0 spec is 1.5 amps. The maximum for USB-C is 3 amps, unless using USB-PD at 20 volts or more then it's 5 amps.
Power out of a hub cannot exceed what goes in. The USB spec says a port must be able to provide 900 mA minimum, but there are exceptions for hubs. On a hub it may only be required to supply 150 mA. A compliant hub cannot have more than 4 ports, likely so it's not dividing up the power too much. A USB 3.0 device may draw up to 150 mA before being enumerated and allowed to ask for more power. With a hub powered only by the host then any request may simply be denied.
If the hub is getting 3 amps then perhaps one device can ask for 1.5 amps and have that request granted. The others would then have to fight over what's left in the power budget.
If the host and hub are using USB-PD to blast out power to connected devices then there's all kinds of things that could happen. Is the hub capable of stepping voltages down for the different ports?
The USB-C and USB-PD spec was extended to allow 240 watts. Assuming the spec didn't change the 4 port max then that's potentially 60 watts per port. That happens to be the maximum power allowed at 3 amps.
Looking at the behavior of my own USB-C dock in the system information tool it says the USB-A ports are allowed 500 mA, the dock internal SD-card reader is allowed 900 mA. So, my guess is no device is allowed more than 900 mA from a hub. Lower limits can be imposed but no lower than 150 mA.