Molex Clarification


Can anyone clarify what the requirements are for the Molex connections to the backplane?

Specifically, how many separate cables are required for the 4x Molex connections? Can you daisy-chain 1 cable (i.e. 1x4) or do you need 2x2 or do you need 4x1?

I previously purchased a “Quad Molex” (i.e. 1x4) cable from CableMod for my Dark Power 13 750w PSU, but now I am questioning that decision and want to make sure I have ordered the right cable(s). Configurator – CableMod Global Store

Here are the 12V specs listed for my PSU

edit: Is this the cable (1x4) the standard RM750e uses? (taken from RMe Series RM750e Fully Modular Low-Noise ATX Power Supply)

You should be fine to use that cable. In the end, all the power is still coming off the same 5V and 12V rails in the PSU normally.

The only issue you might see is when the drives are under load they might not have enough power at which point you would just want to swap that cable out for 2 separate cables instead

1 Like

Thanks for the clarity. One interesting thing to note is that “Dark Power 13 offers 750W of continuous power with 4 independent [12V rails](12 Volt rails - what is that and what makes it so special?).”

The 12V rail that the Molex cable plugs into rides the “12V1” rail (the first 12V rail), which is listed at 25A. Just checking if that is an issue? Also, it sounds like I can combine the 12V rails into 1 single rail if need be?

“The overclocking key gives you full overclocking control by manually switching between the default four 12V-rail mode and the alternative massive
single-rail operation

edit: Also worth noting that it looks like 12V1 shares the rail with the 24-pin and CPU

No, 12V 25A is 300W which should be more than the combined inrush of 15x HDD spinning up.


Perfect, thank you. And I assume the single daisy-chained wire can support the power of 15 HDDs without melting?

This isn’t really a helpful answer, but a lot of stuff on the internet said a normal molex wire/the connector was only good to 5, 8 or 11 amps of continuous use depending on what source I read, so I just bit the bullet and had cablemod make me some. (1 to 1)

Main reason? 45 Drives has their supply thats provided with power supplies with 4 separate cables.

I wish 45 Drives would put their cables in the store, but you’d get someone who plugs in the cosair wired molex power cable into some other brand and they would fry their drives I guess (to anyone reading this that doesn’t already know, not all modular power supplies have the same wiring on the PSU side of the cable, you have been warned)

edit: also also in regards to your first post, no the PSU->molex in the non-chassis build use custom cables 1 to 1


I might end up just doing the same to be safe (4x1). What length cables did you acquire? I was looking at all 150mm.


There’s a decent write up here:

I interpret this to say a 4-outlet inline using 14AWG can drive 11A to each part of the backplane which is more than sufficient for this requirement, but I’m not sure, and ampacity tables for 120V/240V usually don’t consider 18AWG and “small cables”.

If it isn’t 14AWG and is say 18AWG then it’s going to be a bit less — in the link above, stated as 9A per pin or 36A collectively, but I expect 36A is over the 12V, 5V and their respective grounds?

Number of amps required for each subsection of the backplane has gotta be <= 5A continuous, possibly a bit more during initial power on unless the drives are staggered spin up.

If 45Drives is selling theirs with 12V individually wired then I’d assume the PSU they’re using splits it’s 12V over multiple discrete rails and to deliver 20A to the backplane they had to do it that way, not that the cable itself could not carry the amperage as a 4-outlet setup?

Would be great to hear from @Hutch-45Drives about how we should think about this?


For my drives:

Seems like 9.4W each so 141W across all fifteen? That’s around 11.75A so if 18AWG should be carrying 12V and 9A maximum (across all four) and 14AWG is 12V and 11A maximum then I would be slightly over if I use a single cable into the Molex + SATA ports and plug in all 4 outlets…

But looks like the answer could be two outputs from the PSU using 2x Molex on each?

1 Like

Apparently CableMod uses 18AWG for PCI-E cables, so it should be 9A per pin (36A collectively as you stated).

I agree, 2x cables at a minimum seems like a safer option. I am using essentially the same Exos drives.

Now I am just wondering if my PSU’s 12V1 being 25A (shared w/ 24-pin and 8-pin CPU) is an issue. From what I read, HDDs can pull up to ~2.5A during startup each (15 * 2.5 = 37.5A) which is well over the 25A rail, but I can combine them to single-rail which would be well over my need.

edit: and if I should cancel my 4x1 CableMod order and update it to 2x2 to save some $?

edit2: Also I am using Unraid which does not support staggered spin up I believe

Edit: corrected some misworded parts

If 45Drives is selling theirs with 12A individually wired then I’d assume the PSU they’re using splits it’s 12V over multiple discrete rails and to deliver 20A to the backplane they had to do it that way

I don’t think it does, its an RM750e and from what I understand splitting the 12v rail is rare and only for very high wattage power supplies, the tech document says nothing about split rails just a total amperage.

From a homelab users perspective even if you had 15, 20TB Exos drives they pull less than 1A per in use it seems when being all hammered with reads, thats less than 12.5 amps total with the datasheet and napkin math, spin up is transient and shouldn’t matter really as long as the power supply doesn’t trip. With that amperage I’m not sure if 1x4 is a problem, 2x2 is definitely safer, 4x1 is probably overkill and this is coming from someone who did 4x1

I got 200mm which was a touch too long on a RM850 but whatever

I think this is down to the HBA and not the choice of Unraid, TrueNAS, etc?

I was debating getting 150mm and then 50mm extensions. Two cables with 150mm to the first molex, and then 50mm to the second molex. Sounds like this might work great, I’m using an MSI PSU (MEG Ai1300P PCIE5 | Power Supply | Overflow With Power) instead of a RM850 and its a bit deeper.

I am currently running a 9207-8i, but will switch to a 9400-16i. I don’t know the staggered spin capabilities of either.

edit:Staggered spin up - Storage Devices and Controllers - Unraid

I really wouldn’t worry about spin up with a 750w power supply, and the cable(s) should be able to take that high amperage spike for a few seconds. Cable amperage rating is a lot about heat dissipation, heat won’t build up during the short spin up (especially in a case with active fans).

I have 9300-16i that started having issues with firmware I think with truenas so I’m on a leftover 9300-8i, if I can’t fix the 9300-16i firmware based on the truenas post I just found I’m headed to the 9305-16i which is a 9400 but without NVME capability I think

Take a look at 9400-16i. Very similar price to 9305-16i.


Hi All,

I am going to get @mhooper to jump in on this as he has done lots of testing with both the HL15 and our enterprise servers in terms of Power delivery and requirements.


Are the molex connectors glued on the backplane or just extremely tight?

They are just extremely tight, I know in our lab when we are doing testing I sometimes need to use a pair of pliers to grab the connector to pull it out

1 Like