# How many devices can be plugged into a USB hub, and how is power distributed amongst them? [closed]

1. Is it possible to connect a USB hub to a hub that is connected to a hub and a hub and.. and.. and to a laptop?

2. I was thinking there are bound to be some "overhead" like if without the hub we have 100%, with the hub we lose 1% to overhead and have 99% left to split, and if i add it to another hub I will lose another 1% overhead, but is that assumption right?

3. Are the power divided evenly among the devices? so if I have 5 connected to the hub, each will receive 20%? And what if one has "finished using" (like say I'm charging a iPhone and it's already at 100% / or say i connect to an external mouse but the mouse is switched off), does the power of the other 4 stay capped at 20% as long as the 5th one is attached?

• You mean a USB hub? You may want to break this down into a few different, more precise questions, about specific hardware. – Ƭᴇcʜιᴇ007 Sep 1 '11 at 1:09
• @techie007 ok let me edit it – Pacerier Sep 1 '11 at 1:09
• USB hub. Why do you need the maximum number? The power depends on the devices. – Tamara Wijsman Sep 1 '11 at 1:09
• ok i've edited the question – Pacerier Sep 1 '11 at 1:27
• @techie take a look at the edit – Pacerier Sep 1 '11 at 1:27

The Wikipedia article will tell you everything you need to know, but the basics are that you can have a total of up to 127 devices, this includes the hubs themselves when daisy chained. Each device has available a specified amount from the port it is plugged into, not a division based on how much is plugged in. Each port will provide no less than 4.75 volts and no more than 5.25 volts and up to 500 mA to the connected device. A hub can supply no more than 500 mA in total load. If a device needs more than 500 mA, it needs to draw external power. If the hub is supplying power to multiple devices, the draw is divided amongst them as they require it (all devices start out as low-power (100 mA), but may change mode to high-power after and pull current as needed if available). If the hub is powered from another USB port and does not have external power, it will supply no more than 100 mA to any port at any given time.

USB as a battery charging specification works differently. If you plug your phone into your computer it will charge differently than when plugged into a wall or your car. When connected to a computer, you are bound by those limits above as it is treated as a connected device, but to a wall or your car it can pull up to 1.8 A (though 1.5 is pretty standard for dedicated charging ports).

EDIT: Updated post after reading specifications a bit more in depth.

• i don't quite understand the part when you say each device has a specified amount from the pot it is plugged into. do you mean to say that if i plug Only 1 device onto a hub which is plugged into the laptop, the device will not receive 100% of what the hub can provide and will only give a "fixed" amount? – Pacerier Sep 1 '11 at 2:50
• @Pacerier, yes, each device is allocated 100 mA from the hub, if it requires more than that it needs external power. It also has a set range between 4.75 and 4.25 volts. – MaQleod Sep 1 '11 at 5:20
• cool, btw what do you mean by "A hub can supply no more than 5 total 100 mA loads". Do you mean that if I have 8 holes in the hub and i plug in them all, only 5 will actually receive power? – Pacerier Sep 1 '11 at 6:26
• This is interesting and explains my observation of why my phone can charge completely from nearly dead in about 2 hours when plugged into the wall using the AC->USB charger, but will take over 4 or 5 hours from the same charge level when plugged into a computer. – Ben Richards Sep 1 '11 at 7:07
• @Pacerier, I updated the post to clarify the difference between how a hub acts when it has external power vs when it is powered from another hub. So if the hub has external power, it will share the 500 mA to all connected devices as they request it, if there is available current. If the hub is powered from another hub alone, it will never allocate more than 100 mA per device to the maximum it can pull from the previous device (500 mA). – MaQleod Sep 1 '11 at 14:33

It is technically possible to daisy chain 127 devices on a single USB connection in a PC. However, as you suggest, power draw will become an issue. Unless these devices can get power elsewhere, the USB port will not provide sufficient power to the devices. As far as it being evenly distributed, I don't believe it is. I think each device gets a proportion of the power available according to its needs. But if you have too many devices or if they require more power than is available, then they will not work properly. The OS usually can recognize this and will not attempt to use the device if it is underpowered.

• are the OS and hubs and the other devices "safe" no matter how many amount i plug into them simultaneously? – Pacerier Sep 1 '11 at 2:15
• If by "safe" you mean they will not be damaged, then yes, they should be fine. However, the more devices you plug into a USB hub, the more power will be drawn. The common practice is to avoid using hubs and daisy chaining as much as possible, to avoid running into power issues. – Ben Richards Sep 1 '11 at 2:24
• yea by "safe" i meant will not reduce their lifespan – Pacerier Sep 1 '11 at 2:51