The FACTS about GPU mining electrical specifications


#1

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.

http://www.jonnyguru.com/forums/showthread.php?p=130311

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.


How to connect 1080ti (with 2x 8-pin PCIe as power connectors) with cable which has (6+2pin & 6pin)
NVIDIA GTX 1080 ti GPUs
#2

Thank you. Thats a great read.


#3

I don't think you need to expect a backlash. Miners are pretty open to info here and I think we all want to be safe. So basically you're saying to skip the connector and run direct PSU to molex to two risers (per molex cord), correct?

Also, you're saying that direct to sata connctors are not safe for 1080 ti's >= 80%. Is there any power level that you would consider these safe - say 70%?

What about the 6-pin risers - is a molex to 6-pin adapter safe for those?

Thanks


#4

Yes, for cards that draw 5.5 amps, two risers are safe on 11 amp rated 4 pin molex cables. However, don't forget that when the AMD RX470 and 480 reference cards came out they initially failed the PCIe 3 specifications and were drawing north of 6 amps on the riser (two would be over 12 amps, over molex rating too). AMD had to fix this as they were denied PCIe compatibility rating, but quite a few SATA cables burned in the time before they fixed it.

1080 ti at 80% TPD is at 4.4 amps and fine for a SATA cable... but who wants to limit their GPU to 80% TPD? Why not just go 4 pin molex (or as you say 6 pin PCIe power) and run full power if you want, then no worries.

6 pin cables are rated at rated at 75 watts (6.25 amps) and 8 pin cables are rated at 150 watts (12.5 amps). So 6 pin can handle 1 GPU riser / cable and 8 pin can handle 2 GPU's risers / cable (the best option with most power rating). While properly made PCIe power cables, 6 and 8 pin with 18 gauge wire are known to be under rated, and can deliver significantly more power than specified. Many argue that an 8 pin has only 2 more ground wires than a 6 pin, so theoretically a 6 pin can carry the same current at an 8 pin. While this is true, this does not prevent a cheap manufacturer from making cables that just meet specifications and cant handle the higher power. So you can not count on this unless you know your cables have the right gauge wire and can handle more than 2 risers / cable.

I buy 6 pin risers now but will use 4 pin molex too. The issue with 4 pin molex is that most PSU's don't ship with enough cables to power a full 7 or 8 GPU rig, but typically have ample 6 and 8 pin cables.


#5

Great post, though there are some spots I couldn't follow.

Rumor says you can only put two risers on one molex cable (they usually have 3 connectors).
Can you plug extra molex cables into the PSU sata slots? I think they have the same pin connection. If so, where can you buy extra molex cables for your GPU?


#6

Thanks, Great question.
You can go up to two risers on a 4 pin molex cable and still stay at 11 amps (unless you are dealing with an old RX480 reference card and old AMD drivers, not likely).

They should be the same ports. The PSU's I use, the ports are labeled sata/perif so either cable can be plugged in. However, I have other PSU's from the same vendor that have separate sata and perif ports (but the vendor did say they can be plugged into either). So I would check with your PSU vendor on that.

There is no simple answer on how to get enough cables to power all the GPU's for your rigs. The problem is that its hard to get enough cables from any off the shelf PSU to not use SATA cables somewhere, that is why SATA cables are used in the first place. I buy all my PSU's from the same vendor and have boxes of cables. When I have a RX480 rig that is below 4.5 amps draw on the riser, I will sometimes use SATA cables (but NEVER the molded connector adapters that came with the risers). This then gives me extra 4 and 6 pin Molex connectors to use on a 1080ti rig since all my PSU's are from the same vendor and I can interchange cables.

So if you want to use the cables that came with the PSU, you will have both 4 pin molex risers and 6 pin PCIe risers (not using any of the SATA adapters). You need to plan out the rig and the PSU's you buy based on the ports they have available. The 4 pin molex cable uses up one sata/perif port on the PSU, and the 6pin PCIe cable uses one 8 pin Video port on the PSU. You need one 8 pin port for each GPU already so you cant power all your risers from 8 pin Video port as you will not have enough, some must be 4 pin sata/perif ports.

You can also find ATX 6 pin to 4 pin adapters on the Internet but they will always have the wrong female connector for the 4 pin since they are meant to go from 4 pin to 6 pin (not the other way around). These will work fine, so you can either change the 4 pin connector to a male or just make your own 6 pin to 4 pin cable very cheaply.

All you need is:
Crimping tool
18ga wire of the appropriate colors
molex connectors.

So I just talked myself into that. I have everything I need except the molex connectors. Ill get those ordered and make up a bunch of 6 pin to 4 pin, and 8 pin th 4 pin. then I can just use the 4 pin risers exclusively without the any SATA adapters (molded or not).

Thanks for forcing me through that mental exercise.


#7

You said 80% was safe but I want to make sure that a single molex cable rated to 11 amp can handle two risers with 1080 tis at 100% power.

Thanks


#8

a 1080 ti at 80% is safe for a single SATA connector. 2X 1080 ti's at 100% should not ever draw more than 5.5amps for a total of 11 amps. Its right at the limit of a 4 pin but there is room to spare.

The problem with SATA connectors they have small pins meant for HD's, and were never intended for high current loads. As opposed to the ATX 4, 6, and later 8 pin standards that were designed for high current loads. Nvidia and ATI pushed for higher currents when the PCIe specifications were written for the 6 and 8 pin connectors. While the SATA connectors will work most of the time the cheap molded type from China used in a mining rig is a recipe for disaster with their known failure issues. I have seen them get really hot, its scary stuff.


#9

Thanks for the clarification from the other thread. I see what you mean about the rails and volts. Like you said, getting enough molex cables is the issue PSU. What can handle the more load, molex or the 6 pin? I stopped mining yesterday because I was concerned about what you said :sweat_smile:

I was pushing my rig to the max and pulling around 320W per card right when you said SATAs were dangerous and I said I like having a place to live and pulled the plug.


#10

Strictly by the specification the 6 pin are rated at 6.25 amps and the 8 pins are rated at 12.5 amps. The 4 pin is rated at 11 amps. So the 4 and 8 pin can handle two risers (but I have never seen an 8 pin riser). The 6 pin can handle one. But again these connectors ARE designed for high current loads and should never get hot, or even warm, unlike the SATA connectors. Just use 4 pin molex, you should have some with your PSU.

I would not stop mining. just cut back to 80% TPD, over clock to get some performance back and you should be good to go until you decide what you want to do. And if you ARE not using the SATA to molex adapters and just using the PSU SATA cable directly (has to be non molded type), you should have some extra buffer built in. I make it a regular activity to feel my power cables on my rigs when I am inspecting them, and use a IR thermometer to make certain nothing is getting warm that shouldn't be.

If you are really worried, as I was when I started finding hot connectors. Get a smart things hub, a smart outlet, and a smart smoke alarm. Set up ITTT to kill power to the master PSU if the smoke alarm goes off. Just place the smoke alarm right were your rig is. I do this for the rigs I have in my house. This gives me some peace of mind when I am out of town and my rigs are on their own. You can also set up a webcam and can always kill your rig on your cell phone if you see something you don't like.


#11

That I think was one of the most helpful posts I have seen on this forum. Thank you very much. I just need to find molex cables now and get the smart devices as I am somewhat paranoid when I am not at home.


#12

Good post. For the benefit of those here who have the molex risers, are you saying to run those from the PSU molex connections and power each GPU from the 6/8 pin PCI-e cable?

I've popped 4 cheapo PSU's in the past 6 weeks, and I can say that every miner should get good PSU's and spend the cash. None of my GPU's went out, fortunately, but the cheap ones are not worth it and can be overloaded easily...stay especially far away from Apevia!


#13

Can you give me some links for that proposed ITTT / smart outlet/alarm setup?
I am trying to find parts on ebay, but can't even find a smart smoke alarm.
Also i ordered 6 pin to 6 pin cables from china to feed my risers directly (v 006c) Do you think thats ok?
Right now i am using the sata to molex cable. My rig is a 8x 1080 TI running now at 70%. so i must be safe the upcoming weeks till the cables arrive.


#14

There may be others but Nest Protect will work e.g. see this https://ifttt.com/applets/184132p-turn-your-lights-on-if-your-nest-protect-detects-a-smoke-alarm-emergency. You'd obviously need a smart outlet something like the Wemo at the other end to power the machine off http://www.belkin.com/us/Products/home-automation/c/wemo-home-automation.


#15

4-pin Molex is rated for 40W, SATA something like 22.5 - PCI-E spec allows 75W from slot. Basically, while out of spec, you can skate with using 4-pin Molex for a riser. Using a SATA connector is only recommended if you're feeling suicidal.


#16

So I have a 1600W EVGA G2 and one of the cables is 2x molex and 2x SATA. The rail should be rated for 11 amps since it has 2x molex, right?


#17

In reality this is all speculation based on plausible assumptions. Nobody knows for sure how much a riser pulls from the PSU because nobody even knows how much a card pulls from the PCIe slot when connected directly to it. Besides that, I prefer to use 6 pin or 4pin to connect risers although some people use sata risers without any problems.


#18

How is the power draw from the riser related to the power supplied by the 6/8 pin connectors for the card? From memory an 8 pin cable can supply 150W, and a 6 pin can supply 75W.

If a card is drawing 5.5 amps from the riser, that is ~66W.

I'm guessing the card will favour taking power from the 6/8 pin over taking it from the PCIE, but will take from the PCIE if it needs too. For a card drawing 250W with a 6 pin and 8 pin connector, that is a max of ~225W supplied by the 6+8 pin cables, requiring the card to draw ~25W from the PCIE. That would only be around 2 amps which should be okay, but that assumes the card takes the maximum amount from the 6+8 pin cable before taking from the riser. If this is correct, that means that cards with 2x 8 pin connectors should have less of a problem.


#19

Actually, we do - first of all, there are these things called amp clamps. Put an unpowered riser on a GPU and connect. Second, the PCI-E specification allows a maximum of 75W from the slot for x16, simple fact. If the GPU exceeded this, it would violate the standard and AMD would be in all kinds of mess.

Some people play Russian Roulette and end up just fine, too - wanna have a go?


#20

Can some give me a tldr summary? this looks serious..