Reddit mentions of Cable Matters 2-Pack 8-Pin PCIe to Molex (2X) Power Cable 4 Inches

This.

Or this.

It doesn't look like your power supply is modular because those Corsair PSUs use ribbon type cables.

These types of cable adapters are usually frowned upon. They tend to be very cheap in quality and have been known to start fires. Many people buy a power supply designed with at least two 6+2 pin PCIE power cables instead of using cable adapters.

Consider this.

I've seen a couple of people say molex to molex, not sure how that would work? There are only 6/8 pin slots on the PSU itself

Cables would look something like this, except I can't find the female version https://www.amazon.com/gp/product/B01DV1Z428/

Could this work for your setup?
https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428#

It would help to know the model of MSI RX 580 you're referring to, the model of the power supply, and the exact nature of the power adapter you're using (e.g. Molex to PCIe 8-pin, etc.)

So like this?

And then connect one molex to the PSU, and then plug these two into that, and those into the GPU?

mine came with one of these actually
https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Molex/dp/B01DV1Z428/ref=sr_1_3?ie=UTF8&qid=1504144795&sr=8-3&keywords=molex+to+8+pin

https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Molex/dp/B01DV1Z428/ref=sr_1_3?s=electronics&ie=UTF8&qid=1519919611&sr=1-3&keywords=molex+gpu

You need something like this molex adapter to get your graphics card to work with your power supply.

As for the amount of wattage needed for overclocking. In this case, it's not necessarily the amount you should worry about, but rather the quality of the components. Unfortunately, your brand of power supply has very low component quality. A high quality 550W PSU would be more than enough for an overclocked RX 5700 XT.

As long as the power supply has enough juice, this would solve the other potential issue.

https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428/

You are looking for something like this:
https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Molex/dp/B01DV1Z428/

These are PCI Express power cables (PCIe) There are several types 6-pin or 8-pin, there are both ones to use Molex or SATA power cables. You probably are better off with a properly equipped power supply though. If yours didn't come with these connectors, it will probably get pretty stressed from the video card it wasn't designed to handle. Some just include one connector, to not have any, the power supply must be pretty old or cheap. It would be a better investment in my opinion to sink the money into a decent bronze series or better PSU than some cables.

The one linked in the OP looks like a bad idea. There should be 2 molex to a single 8-pin PCIe connector and 4 wires coming off each molex connector to ensure it's enough to carry that amount of current. Something like these will be safer: https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Molex/dp/B01DV1Z428/ref=sr_1_2?ie=UTF8&qid=1486114393&sr=8-2

Here's one I bought a while ago. Works well

Are there any open 8-pin connectors for GPU's in there?

https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428

https://www.amazon.com/dp/B07F2ZWGB3/ref=sspa_dk_detail_3?psc=1&pd_rd_i=B07F2ZWGB3&pd_rd_w=Lr1ZS&pf_rd_p=8a8f3917-7900-4ce8-ad90-adf0d53c0985&pd_rd_wg=LH1GN&pf_rd_r=EPNVPP6RGMT5JBVP5AZZ&pd_rd_r=4d661eb1-78c1-11e9-a045-19daa5a404f5

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They make PCIe- 2x 2.5" drive bay cards too, they are plastic but work great (basically just space to secure the 2.5" drives)

I'm sorry, I don't think I understand. But I'm using a 2 separate power cables. One of them is straight from the PSU and the other is via a Molex to 8-pin cable. This one right here: https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428

Yikes, that is rather close for 6x1070s. I would have bought a 1200W PSU just to be safe, as this one will be running at almost 100% capacity 24/7. You can always use a Molex to 6-pin adapter like the one shown here on Amazon. I would avoid adapting SATA to 6-pin if possible though, those can be a fire hazard if you are drawing too much current. If you are out of spots on the actual PSU, you can use a splitter, but again, make sure to be careful you are not pulling too much current or it could pose a fire hazard.

Next time, remember to take your total system TDP and multiply by at least 1.3 to properly size your power supply. In your case, 6x150w for the 6 1070s, then add an extra 50 watts for the cpu. So your sytem TDP is about 950. 950 * 1.3 = 1235. So, you should have gone with a 1200W power supply. If you still have the box, maybe return it and size up a bit.

If you found this information helpful, please donate to me at 35X6priTi9P8aseAjnyQGUNg5guKtU3hH7.

Of course. Molex uses 12v, pcie uses 12v. Molex is rated at 75w, pcie 6 pin is rated at 150w. 2 molex = 6 pin pcie. If you're in the US, here's an amazon link.
https://www.amazon.com/dp/B01DV1Z428/ref=cm_sw_r_cp_apa_AEf4ybZAQJ749

Something like this if you have spare 4 pins.

https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428/ref=sr_1_3?keywords=MOLEX+TO+8pin&qid=1568493892&sr=8-3

If you are not going to replace your PSU, the RTX 1650s that come without a power connector are the fastest cards.

But if you are certain your PSU is good enough, you can use a molex adapter with a faster card:

https://www.amazon.com/Cable-Matters-2-Pack-8-Pin-Inches/dp/B01DV1Z428

Buy a dual molex to 8-pin PCI-e connector.