Regarding the question of power and an extra power cable:
There have been detailed discusssions here in this forum about the fact that HP's PCIe supplemental power cables are designed by HP engineers to have higher wattage capacity than the "ATX Standard" of just 75 watts each. Use the search bar at top left to do some searching. It is 18A each. There are special HP quality power adapters that let you split out one of those to two 6-pin ends or one of those to one 8-pin end.
And, my guess is that the original poster does not know that he has two of those PCIe supplemental power cables plugged into the side of his front bottom black plastic fan holder. The two plastic receptacles there do nothing but hold those neatly in place waiting to be used. So neatly that many owners think the two extra cables are doing something there. They are not..... just go pull them free and use them.
Finally, there are some real junk adapters being sold on the internet.... buy the HP ones for the best quality. They have all the proper wiring; many you'll see on the internet are missing wires. At least buy from a very good source if you don't buy HP. That also has been posted about in here......