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I'd like to upgrade my hashcat rig, a 4 GPU box. The PSU can deliver more than enough power (pro redundant rack hardware), cards are on real (physical, not lane-wise) x16 slots but the number of physical connectors on the PSU power backplane is limited. So now I've got a bunch of options. What will work / will be the best option according to you pros?

1) 1080Ti connected with a single 8 pin power connector only. Will it work to start with? If so: e.g. power limit on 225W?
2) 1080Ti connected with a 8 + 6 using a splitter (reading advice telling not to use them).
3) Use 1080s.
4) Something else?

And in general:
- How many Watts can you safely pull through a single 8 pin GPU power cable?
- How many Watts will a 1080/1080Ti burn in -w4 mode, worst case seen and what cracking mode will that be?

You need to connect both power connectors

Get a PSU that's build for this situation. eg: https://pcpartpicker.com/product/v7PzK8/...20t21600x1

Whatever you're thinking about doing is NOT safe.
Nvidia drivers will limit a 1080 Ti to 300W to keep it within spec, and a 1080 will probably stay ~<200W without any overclock and depending on the model.  

I would never use splitters or short the cards of power. Get 2 (or 3) HQ supplies. Perhaps some of the hardware gurus on here could give more info on this.
What kind of system is this? You say it is rack mount hardware, but what form factor? I assume since you say the cards are in physical PCI-E x16 slots, it is a 4u system. Does it take a standard atx power supply? If so, just pull the old PSU and upgrade to a higher capacity power supply that can deliver the 8+6 that you need.

If you insist on keeping your old power supply or are not able to source a new one, what/how many connectors do you have to work with? I have seen some adapters that will take two sata power and convert to one 6 pin. I have no idea if two sata power cables can combine to supply 75W though, since a single 6 pin from the power supply is rated to supply that much. Also check your power supply. 4x 1080Ti is a decent amount of power, and you don't want to be maxing out your power supply.
Yeah, this very much depends on your system. For example, the HP Z series have some serious headroom over their stated power delivery. There are many threads on the web where folks have successfully installed and used GPUs with much higher power ratings than HP officially support.

The key is to look at the power supply sticker, and do the math yourself to see if it can deliver. Part 2 of that is to use high quality 6 to 8 pin adapters. From memory, I believe HPs will reliably deliver 212W or 216W from a quality 6 to 8 pin adapter. It's comfortably above 200W.

I was almost exclusively a Mac user when I started dabbling in Hashcat. They stopped offering competitive hardware, so I incorporated a bunch of Z820s into the workflow, all loaded up with Pascal cards which are unsupported (1080 Tis to 1030s). They work beautifully, even in Hackintosh mode.

Found a relevant thread:

Thanks for the replies so far!

We're talking about 4U pro HP hardware. So no ATX power supplies I can replace, but 2 or 4 1200 Watt modular PSUs. So max power for sure is not a problem (total = ~2400 / 4800 Watt).

There are 2 x 8-pin GPU connectors on the PSU board (plus some proprietary plugs, info more than welcome). HPE provides certified cables that split to 2 x 6 + 2 x 8 pin, see HPE part http://partsurfer.hpe.com/ShowPhoto.aspx...667266-001. So this will mean 150 + 75 or even 150 + 150 per connector right? Since some years I'm running 4 single connector GPUs using the 2 GPU plugs. Rock solid. Officially they do not support > 3 GPUs but 4 works, that's for sure.

Opinions about using / quality splitters? And any figures about real-life hashcat power usage of the 1080 and 1080Ti? Or other options I didn't think of? Thanks!
Stick with genuine HPE or other quality cables and you should be good.