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It seems that still a lot of people may overlook the importance of the power supply when building a multi-GPU mining rig and the more the number of video cards, the more the issues may arise. Normally people think that if you have six video cards with a TDP of 150W, then any decent 1000W power supply should be just fine for the job in providing enough power for the mining rig. Doing the simple math it will, however when we get to actually connecting everything things may actually start to get messy. You might discover that you have not properly planned the number of power connectors you need for the video cards that you need, or the number of 4-pin Molex connectors needed for the powered risers. The simplest solution here would be to just add some splitter adapters and got a few extra connectors available to be able to just connect everything. Unfortunately the simplest solution here is definitely not the wisest thing to do as you might end up overloading some of the wires going to the PSU…
The first thing you need to ensure is that the power supply you go for has enough PCI-E power connectors available to power all of your video cards in the mining rig. So if you have 6 video cards getting six separate PCI-E power connectors from the power supply is the best solution. Unfortunately not many power supplies are designed with single cables for 6 or even more PCI-E power cables as normal computers rarely have more than 2 or up to 4 video cards in rare cases. Even if a power supply has let us say 8 power connectors these are usually double connectors going from a single wire to the PSU and this is originally designed for up to 4 video cards (if they have double PCI-E power connectors). Adding additional adapters splitting single PCI-E power connector to dual PCI-E or using Molex to PCI-E power adapter can cause problems, so if possible it is wise to try and avoid it. If not, then you at least need to make sure that you are not overloading any of the lines going to the power supply, the easiest thing to check for that is to touch the cables and if they are getting hot, then it is not Ok.
If you are using a motherboard with multiple PCI-E slots like the AsRock H81 Pro BTC motherboard that is actually designed to be sued for GPU mining rigs you will need to use extenders. Although the motherboard has extra 4-pin Molex power connectors in order to properly power video cards that need more power from the PCI-E slot. Even in this case it is still better to go for powered PCI-E risers and do not try to draw more power through the motherboard, especially if you go for 6 GPUs. The x16 PCI-E slot that is normally being used for video cards on the motherboard is designed to provide up to 75W of power to the video card by specifications, though most of the consumption comes from the 12V line and there the max power is rated at up to 66W. If you however have 6 video cards that are maxing out the power draw from the motherboard it will come up to a pretty high number, even though in most cases video cards do not go with the maximum power that the PCI-E slot can provide them with.
There are a number of different PCI-E risers, but one that is very popular choice is the x1 PCI-E to USB 3.0 riser as it is really well designed and works very good, unlike the case with not so well made ribbon cable extenders. These risers use a USB 3.0 cable, but they are not actually using USB interface, the cable is just a good choice for the data communication without issues. PCI-E risers like the one on the photo rely on 4-pin Molex power connector to supply any power that the video card may need, completely cutting any power draw from the motherboard’s PCI-E slot. We already know that the video card can draw up to 66W over the 12V line and this should normally not be a problem for a 4-pin Molex connector as these connectors are rated at 10-11A per wire and since there is just one 12V wire you should in theory be able to go for up to 120-132W over it, but that is only if you have a single Molex power connector.
The actual problem with 4-pin Molex power connectors are not the connectors themselves, but the cables that are being used to route them to the PSU, because they also have a maximum power rating depending on their thickness. Power supply manufacturers and cable adapter manufacturers do not go for the best option as it would make things more expensive and in most cases it will not hurt to go for a thinner cable. To be able to get up to 10A current over a 4-pin Molex connector you would require a 14 AWG or 16 AWG wire, 18 AWG ones are rated at up to 8 Amps and 22 AWG wires are rated for up to 6 Amps. Unfortunately most manufacturers do not go for 16 AWG let alone for 14 AWG wires on Molex connectors (usually 16 AWG wires used on PCI-E power connectors on high-end power supplies). In most cases the power supply wires for the 4-pin Molex connectors are 18 AWG, though there are cases, especially for adapters or extenders where even cheaper 22 AWG wires are being used.
If the maximum power draw for a video card over the 12V line of a PCI-E slot is 5.5A (66W) then even a 22 AWG wire rated at 6 Amps should be enough. It should, but only if you connect only one such 4-pin Molex power connector to a line going to the power supply. That is rarely the case however as power supplies normally have a couple such 4-pin Molex connectors on single line going to the power supply and that goes over a 18 AWG wires. In order to be safe a maximum two PCI-E extenders should be powered by such cables to stay within specs or at least close to them, but people often use three or even four of these connected and that can lead to cables overheating and melting or even burning and damaging your expensive mining hardware as a result.
Normally with video cards that have external PCI-E power connector the maximum of 66W over the PCI-E slot is never reached, it will be more like up to 30-40W for example (or less), though there are exceptions as well – the reference design Radeon RX 480 GPUs. Even at 40W if you connect three or four of the 4-pin Molex power connectors that use a single cable line to the power supply, then you will still be over the maximum rating of the cables. What happens is the cables starting to heat up and as a result they may even melt and short out and that can do things like start a fire, damage your video cards, motherboard or power supply etc.
The easiest way to discover such a problem is to just touch the power cables a few minutes after you have the mining rig up and mining, so that it is under heavy load and using a lot of power. Cables that are hotter to the touch than the others are probably near or over their limit and you should think of a way to reduce the power draw over the specific power line to the PSU. If you have a thermal camera you can just take an image like the one above and see the problem clearly and diagnose which cable might be overloaded and do something about it. But even if you don’t have extra tools to assist you, even just touching and noticing a hotter cable can be enough to find an issue with the power distribution of your mining rig. Do not overlook possible issues like that as later on they may cause you serious headaches and even cost you a significant amount of money, taking you on the red, instead of you actually making profit from mining.