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https://www.raspberrypi.org/help/faqs/#powerHub indicates that certain USB hubs can "backpower" the the Raspberry Pi by its USB ports. Apart from the fact that it seems to bypass the overvoltage protection circuit which is afforded to the "normal" micro USB power connection, there doesn't seem to be a reason not to do this.

However, this could be specific to previous models (RPi <3) as I read elsewhere that it's not possible to power the RPi 3 via its USB ports. I haven't seen a schematic for the RPi 3, so I cannot confirm. Can anyone help, with either an schematic or a citation as to whether it's possible?

Thanks.

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The official answer is NO. My experience backs this up.

The USB ports are (said to be) powered by a AP2553 Current-Limited Power Switch (U13). http://hackaday.com/2015/04/06/more-power-for-raspberry-pi-usb-ports/

As well as controlling current to USB these chips prevent back-powering.

NOTE The published schematics for the B+ and Pi3 are incomplete, and do not include detail of this part of the circuitry and the designation on the chips on my boards are BK4.F BK5.0 so this must be regarded as informed speculation.

The Pi3 visibly lacks the MOSFET used on the B+ & Pi2 to switch the current limit from 600mA to 1200mA.

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Depends on your definition of power.

As far as I can remember you can backpower via the USB ports AFTER the PiB+/2/3 has been booted by powering via the microUSB power socket or the expansion header.

EDITED TO ADD

I can confirm that a Pi3 can be backpowered via the USB sockets AFTER it has been booted by powering via the microUSB power socket or the expansion header.

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