I use the official premium 8 pin x 2 to 12v cable that with the Corsair hxi 1500w PSU. No idea if that's "safer", but no issues with temps so far.
It's technically in spec but also sketchy and still at risk of all the usual 12-pin problems. Full ramble below on why:
To explain, a standard single PCIe 8-pin connector itself is rated for 150w (6 pin is rated for 75w). Where this gets confusing is that I assume you're talking about a different 8-pin connector. The modular power supply EPS or PCIe connector from the PSU side is not the 8-pin PCIe standard. These have different pin mappings depending on manufacturer and generally have a slightly different moulding on the plastic to prevent incorrect insertion.
Manufacturers for years have provided modular cables that go 1 x PSU input to 2 x PCIe 8-Pin outputs for convenience. The PCIe standard hasn't changed and cards can draw up to 150w over the PCIe 8-pin, meaning these cables when fully loaded will result in 300w being pulled over the 8-pin modular PSU port. The safety of this is mildly questionable as the PSU 8-pin ports usually use the same number of hot wires (3 x 12v pins) and 5 neutral / ground. They also seem to use the same pin connection standards in a lot of cases. Ultimately however they are a manufacturer's standard and not an industry one. Quality can vary widely with wire gauge usually sitting in the 18-16 range (For example corsair's gen 3 cables are 18 gauge and the type 4 cables are 16 gauge, lower is thicker / better).
There's a few reasons why manufacturers probably went ahead with this in the past and used lower wire gauges. Cards never came close to using their full power spec in the past. The 2080 Ti was rated for about 250w with 2 x PCIe 8 pin. The PCIe port on your motherboard will provide a standard 75w base, so the expected pull from the 2 x PCIe ports was the remaining 175w, no where near a full 300w load. On the other hand the 5090 today is pulling 615w from some testing I've seen and going insane on power draw, easily maxing out those 300w loads. There's also some arguement that the 8-Pin 150w limit was overly safe but that's only assuming everything is intact.
For some math on why this could be unsafe, here's a table I pulled regarding 12v wiring in cars. It goes over the current limits at each distance for different wire gauges (Lower wire gauge = thicker wire):
Current Limit @ 12v --- 5 A ---- 10 A -- 15 A -- 20 A --- 25 A --- 30 A
18 Gauge Wire (AWG) - 7.3 ft -- 3.7 ft -- 2.4 ft -- 1.8 ft
16 Gauge Wire (AWG) - 11.5 ft - 5.8 ft -- 3.8 ft -- 2.9 ft -- 2.3 ft -- 1.9 ft
There's probably a mountain of futher research to do on current, wire gauge, resistance, etc but this is a quick example resouce. Basically trying to upull more than 20 amps on a short run 18 wire gauge cable could cause some problems. Under normal circumstances you should only get about 8.33 amps per 12v line when the connector is under full load:
12v * X amps = 100 W (100w per 12v pin for 300w total)
100 W / 12v = X amps
8.33 amps = X amps
So ok under a 300w load we're only really putting ~8.33 amps on each pin and that's well under the spec in the chart above. However if say 2 of the 3 pins for 12v are not well connected or the cabling is cut and the 300w load makes it way onto a single pin:
12v * X amps = 300 W (300w on single 12v pin, assume broken cable)
300 W / 12v = X amps
25 amps = X amps
That's in to melty territory depending on the length of the modular cable, wire gauge, and any resistance that might be incurred at any joints (Like the 2 x 8-Pin PCIe split off, or actually pin connections). It's missing from the chart above but assuming linear scaling 18 wire gauge @ 25amp should cut off at around ~1.4 ft and most PSU cables are around that length. This is very unlikely to happen in reality though and the reality is most melting seen on PCIe cables in real life is likely due to high resistance points at pin connection, and very little to do with the manufacturing standards or 300w power spec out. The PCIe 8 pin standard is pretty well tested at this point. Even in the nightmare scenario of 300w ending up on 1 pin it's still within spec for 16 wire gauge, you might just see incorrect pin connectivity causing resistance and melting there.
So with all that rambling basically said:
- PCIe 6 and 8 pin are safe at 75 / 150w.
- 300w over a modular PSU 8-pin port is generally fine / safe even if it's out of spec with PCIe 8-pin power limit of 150w. It only really causes problems in super improper connectivity scenarios (ie 25 amps ends up on 1 pin).
Where we run into super unsafe bullshit is the 12 pin high power connector... It's specced for 100w per pin over 6 pins @ 12v, so about 8.33 amps per pin, similar to the math above. The spec is usually 16 american wire gauge, so great 8.33 amps is within current spec. Should be no problem right? Unfortunately the cables are really poorly designed and do nothing to balance power across those six lines (In most cases they seem to just be joined together on one bus). So what we've seen people experience is wild amperage differences between each pin, with some pins going as high as 22-23 amps, even on a cable that has all 6 lines connected. It's speculated that because the 12 pin cable uses smaller pins / connections that there is just more room for mis-alignments and poor connectivity causing resistance on some pins. Electricity will usually seek the path of least resistance if you connect multiple wires in parallel. Even worse if some voltage lines of the 12 pin connector are cut, the current just loads itself onto the other wires. If all the load wound up on one wire you'd end up with 50amp on a 16 gauge wire. That's a fire scenario.
So all this is to say in theory your 2 x 8-Pin to 12-pin cable should be fine if balanaced and well connected, but it carries the exact same risk as all the other 12-pin cables. It's specced for 8-9 amps per each of the 6 pins but frequently we see scenarios where amperage varies and over loads some pins. The 12-pin cable is especially dangerous for this because it's carrying double the power and when the cable becomes unbalanced we can go well over the 22-25 current limit of the individual 16 gauge wires. A similar failure scenario under the old PCIe 8-pin standard only put half as much current on wire, and those cards rarely even drew 300w which is why incidents were rare.
Edit: Two quick disclaimers as well.
1) I am not an electrician but am in the IT sector dealing with data centers, high power server loads, etc. (ie I have some knowledge but am not an electrical expert and usually defer to them.)
2) The math above only goes over wire gauge. The pin connectors likely have much lower limits for current as that's where we see most melting so the real scenarios are likely worse and I would apply wide safety margins if I was using any of this math professionally. This was just a quick example of how some bad wiring can result in fire.
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