I am very concerned about incorrect and misleading information I am seeing posted on forums and the Internet in general about electrical specification for GPU mining. There are a lot of new miners building rigs and the AMD shortage is pushing people to use 1080 ti's and other Nvidia cards (up to 250 watt cards stock). You need to accept that there are individuals on these forums that make a profit off of your inexperience, and want to sell you stuff. That is not a bad thing by itself, but if facts are distorted and people make incorrect decisions, bad things can and will happen.
I am an electrical engineer with 30 years experience and I will simply present the facts in this post. I sell no mining equipment and have no motives other than to keep people safe in their mining endeavors. You need to make up your own mind what you think is safe for you, your rigs, and your home or facility.
1) New risers have NO power connection to the mobo, only data via the USB cable. They make their own 3.3V on the riser from the 12V rail. You NEED to test your risers to confirm this. Ultimately you are responsible for building your rig in a safe and reliable way. Not me, or any other person on this forum. Having said that, each GPU should be powered from one PSU (both riser and 8 or 6 pin connector). IF you power the riser from the mobo PSU and the 6 or 8 pin connector from a different PSU you can potentially tie together the 12V rail from two PSU's. They will fight to regulate the 12V rail if they are just a little bit off. No one, especially you, wins that battle. This can cause melted cables and fires. Others on this forum insist you must power the riser from the same PSU as the mobo. This is not correct and ties back to very early Bitcoin mining when the risers had power connections from the mobo. Just because you wire it wrong and it works does not mean you are OK. Most GPU vendors have circuitry to isolate the PCIe 12V rail from the 6 and 8 pin 12V rail.... if you have it wrong, and you find one that does not, you will not be happy.
2) SATA connectors and therefore SATA risers are rated for 4.5 amps on the 12V rail. The PCIe 3 specification allows a GPU to draw 5.5 amps from the PCIe 12V bus. A 1080 ti GPU draws 4.4 amps at 80% power (link below), and more than 4.5 amps at 100% power. Therefore a 1080 ti at full power draws more than 4.5 amps and you are drawing too much current through a SATA connector. Anyone who says that its fine, and perfectly safe to use SATA cables on risers with more than 4.5amps draw, has no idea what they are talking about, and absolutely no electrical engineering experience.
The older 4 pin molex connectors are rated at 11 amps for the 12V rail. Therefore, do NOT use the molex to SATA adapters. Run a 4 pin molex cable from your PSU directly to your riser get rid of the SATA adapter. Even if you power two 1080 ti's, and they draw 5.5 amps each (the maximum), you are still within the rating of the 4 pin molex cable and connector. Its simply not worth the risk to overpower a connector when it is so easy to use the right connector.
3) In the US an 8 GPU 1080 ti rig will draw about 90% load on a 20 amp circuit. In the US 80% maximum continuous load is code, and can deal with inrush current. Circuit breakers have one purpose: to keep the wires in the wall from catching fire from too much current. They could care less what is plugged into the line, if its on fire, or not, as long as it does not draw more current that the wires can handle. So circuit breakers do NOT protect your rig or your house from a fire, they are not designed to do so. Keep total current draw on a rig to 80% of the breaker. If you want to build big rigs with 250 watt cards run 20 or 30 amp dedicated lines. Don't try to plug different PSU's into separate residential 15 amp lines. Other countries with 220V mains line voltage is different.
A 7 GPU 1080 ti rig at full power draws 79% continuous load, perfect for a 20 amp line.
4) plugging multiple PSU's from the same rig into different circuits / breakers is a bad idea. If one breaker blows and its not the master PSU, you have half a rig powered. While not normally an issue, it could damage your cards, depending on how you have your rigs wired.
5) Molded SATA connectors (like the ones sold with risers) are known to have the internal crimp connections that can move in the molding process and can be close enough to each other to eventually short (even at normal SATA loads, not over rated). Heat will help to facilitate this. While this can take months or years, once you get a short, the SATA connector WILL burst into flames. Now, you have a fire. Don't use them, especially on 1080 ti's, just use the 4 pin molex connectors and cables from your PSU, or buy non molded SATA connectors for use with GPU's that draw 4.5amps or less.
6) Just because something works on one GPU from one vendor does not mean in will work on another. I do use SATA cables to power my Nitro Sapphire RX480 rigs (they draw less than 4.5 amps on the 12V PCIe bus, one SATA cable / GPU, non molded adapters). However, I do NOT use SATA on the XFX RX480 cards. The XFX cards draw more power from the PCIe 12V bus and will get warm SATA connectors. So each vendor has different power draws and you need to be very careful. If you do not know how to measure, just use the 11amp molex cable and don't worry. I have had a SATA connector get very hot on an XFX card (you could smell it).
Ok so that is all I have for now. I expect some backlash from this post, but if I can help one person avoid a dangerous situation I will be more than happy.