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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?

  • Your question asks about power but the limits you mention are for current. USB can supply various voltages in practice. – Turkeyphant Jul 16 '19 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 '19 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. – Turkeyphant Jul 16 '19 at 16:17
  • USB-C most certainly has a spec, especially because it is much more than just a different physical USB connector. No compliant USB port would ever output 9 V, not even with USB-BC or USB-PD. // Because the OP started out with 900 mA, it’s safe to assume this question is about “plain old USB”. – Daniel B Jul 17 '19 at 12:33
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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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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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