OSPi was originally only designed to deliver 500mA output current. In practice it can probably go up to 800mA, but the problem with RPi 3 is that it can draw a lot more depending on the processes running on it. So we recommend powering it additional using a USB adapter and microUSB cable.
Actually, more recently we found that the 3.3 ohm (3R3) resistor that’s inline on the 24VAC line is limiting the amount of output current. By bypassing that resistor, or replacing it with (or stacking) a 0 ohm resistor it will work much more reliable with RPi 3 without additional USB power. Because most of our RPis are already packaged it’s going to be a pain to replace these ourselves. So you can do it on your own if you have a soldering iron.
That resistor used to be a 500mA PTC fuse, but as soon as RPi 2 was released, we found that RPi 2 draws more current than RPi 1 and can sometimes trigger that fuse. So we replaced it with a resistor. After that, RPi 3 was released and that resistor also became a problem, so next time we will have to just remove it. It’s hard to keep up with the RPi releasing game, and the ever increasing power consumption 🙁