Molex Power and Current

Discussion in 'DIY and Makers Spot' started by lumdol, Apr 3, 2019.

  1. lumdol

    lumdol Member

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    Hi there.

    I have one big ol JBOD with the following inside:

    4x M28SAB which take 2 Molex connections each.

    2x Intel RESCV360 which take one Molex each.

    1x M14TQC

    So, 11 Molex connections.

    I also have a modular power supply which has 4x peripheral plugs for Molex/ SATA.

    The obvious idea would be 3 of 3x molex and one 2x Molex. However this would make the chassis layout pretty messy.

    I was thinking of using molex splitters on the four hot swaps, so each molex line with 4 connections, then a third line with 3x molex powering the expanders and M14.

    Inside the unit I’ll have about 36 drives totaling about 30A of total current max (spinup). But, I’ll also be using staggered spinup.

    Each M28SAB states max current around 10A per 8x hot swap, so around 50A total capable amperage.

    I’d rather configure for a possible upgrade to SAS disks and keep current handling on the max spec of the unit rather than my current drive configuration.

    Any thoughts on this?

    Where can I find the best Molex cables? I’m thinking 16Awg would be sufficient.

    Debating making my own, using cablemod for a clean layout and least possible daisy chains, or just going cheap off the shelf and dealing with a less than perfect arrangement.

    Ideas and experiences on cabling a diy JBOD?

    I’ve heard a few horror stories about poor cabling and it’s pitfalls and would like to avoid it. Seems to be frequently the area where least attention to detail is kept. On the other side of the coin, many consider most of the concerns unwarranted and the disaster stories merely deep outliers.
     
    #1
  2. funkywizard

    funkywizard mmm.... bandwidth.

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    A molex "string" is good for no more than 200 - 250w max, for AWG 16 wire. No more than 125w per molex connector on AWG 16 or 100w on AWG 18. A lot of newer PSUs, especially desktop ones, don't expect you to draw a lot from the molex ports.

    Also depends upon the quality of the power supply or power distribution board. A supermicro 825 PBD will have 18 AWG molex wiring and you're lucky to get half the numbers mentioned above without voltage dipping and the server rebooting. 826 PDB will have 16 AWG going to the first plug in the string and 18 AWG to the other two plugs.

    Splitters and adapters are dicey on a good day. I've had a few burned up plugs before. Make sure the connections are solid and the molex wire doesn't pop out of the plug when plugging it in.

    For splitters, get ones with 18 AWG wires (avoid 20 AWG), and plug them in to a molex port connected via AWG 16 wire. Do that amd you should be good for 125w or thereabouts for what's downstream of the splitter. Drawing less is better if you can.

    A 7200rpm drive will pull down, typically, 7w each. I wouldn't use 15k rpm drives in your situation.
     
    #2
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