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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?

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  • Your question asks about power but the limits you mention are for current. USB can supply various voltages in practice. Jul 16, 2019 at 10:10
  • @Turkeyphant I don’t think so. USB-C excluded, there’s 5 V and that’s it. This question is about USB as a data connection.
    – Daniel B
    Jul 16, 2019 at 11:46
  • Firstly, USB type-C is a connector not a spec and nothing in the question says it's about data only. I know some USB ports put out 9V but I don't know whether they comply to USB 3.0. Jul 16, 2019 at 16:17
  • @DanielB USB-PD does allow 9 volts on USB-C. 9 volts is allowed up to 3 amps. I have 3 USB-C power adapters that provide 9 volts at up to 3 amps. I guess they could all be non-compliant but it would be easy enough to check the spec to be sure.
    – MacGuffin
    Jun 11, 2021 at 14:57
  • @MacGuffin Yeah dunno what I was looking at back then. Keep in mind that this question is 2 years old.
    – Daniel B
    Jun 11, 2021 at 15:28

4 Answers 4

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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.

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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.

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  • Thanks for the test. What happened to the file system on the second drive? Was there corruption? May 21, 2020 at 15:15
  • @RockPaperLizard that's a funny question. It is ext4 FS. It managed to auto-recover. This drive was reading the file. Similar power-outage on the destination drive led me to a "nice" SMART warning that the HDD has pending sectors for relocation. So now I'm performing a full surface scan of the 4TB drive to see if it's still reliable.
    – A. Genchev
    Jun 3, 2020 at 20:28
  • Thanks for the update. More than the surface scan, I am curious about the integrity of the data on the destination. Jun 4, 2020 at 4:31
  • The file was truncated, the filesystem replied the journal and that was it. I guess the filesystem reserved some space ahead and this was discarded. The rest of the files are readable. Note this was not the system disk with many open files.
    – A. Genchev
    Jun 4, 2020 at 13:42
  • Thanks for the update. What do you mean by the "the filesystem replied the journal"? Was that a typo? Jun 5, 2020 at 9:05
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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.

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  • Thanks for your answer. Upvoted. When your wrote *So, my guess is no device is allowed more than 900 mA from a hub. *, to which type of hub are your referring? Jun 12, 2021 at 2:46
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    " to which type of hub are your referring? " Those that take power only from the USB host. I'm pretty sure the USB 3.0 protocol is limited to 900 mA even if there is a separate power supply on the hub. A USB port can obviously supply more than that but by using USB-BC, USB-PD, some proprietary protocol, Thunderbolt, or the newer USB 3.2 or USB4 protocol. If this is strictly about USB 3.0 then the max current draw allowed is 900 mA, which will have to be shared by the hub and all devices. In reality few hubs and hosts are USB 3.0 only and may allow more.
    – MacGuffin
    Jun 12, 2021 at 6:55
  • Thank you for your reply. That helps a bunch. Jun 13, 2021 at 7:24
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Disclaimer: I'm mostly making this up as I go.

What is the maximum power supplied by a USB 3.0 port?

You kind of answered that yourself: "USB 3.0 specifies 900mA (0.9A) per port." That's the standard maximum limit for normal ports.

That said, if the port implements the USB Battery Charging spec (and usually is marked with the corresponding logo), it can provide quite a bit more.

Do all of the items plugged into the port need to use 900mA maximum total combined, or can they safely exceed that amount?

The power used by the hub itself is the sum of the power needed by all its downstream devices (plus some for the hub's circuitry itself) – e.g. if you have three devices using 5V×400mA each, together they'll be using 5V×1200mA, which means the hub itself will need 5V×1200mA from its upstream port as well.

The "per port" limit of course also applies to the upstream port that the hub itself is connected to. So if you plug an unpowered hub into a upstream port that supplies 5V up to 900mA, it is physically impossible for the sum of hub's downstream ports to supply 5V at anything more than 900mA.

(I don't know how hubs react to that, I expect that some smarter ones will reject the last device, some will provide as much as they can, and some will stop working entirely.)

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