Power Supply Requirements

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Bill1950

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Aug 12, 2016
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Trying to determine the size of the power supply needed to handle this workstation.

Starting to accumulate components for a workstation build. The following parts will be used:

Supermicro X9DR3-F mb
2 x E5-2670 SR0KX
2 x Supermicro SNK-P0050AP4 fans
Half a dozen case fans
32gb mem
GTX 750Ti video card
2 x 250gb SSD (SATA)
2 x SATA electro-mechanical hard drives
2 x DVD drive

I think that's pretty much everything that would draw current. I have no idea how to compute how big the power supply needs to be. Any help or suggestions would be appreciated.

Thanks for your consideration.
 

Bill1950

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Thank you. Especially thanks for the link. I recalculated using worst case operational considerations plus a "buffer" range. That puts my shopping in the right range.
 

fractal

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I don't think you will be able to purchase a power supply with the proper plugs that is not WAAAAY overkill for your system. Power supplies with two x 8 pin atx12v power plugs are rarely smaller than the 500 watts you were recommended.

The least expensive unit I found on newegg with a pair of 8 pin plugs was an EVGA 650GQ.
 
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Bill1950

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Thanks for comment. You're probably right. I wonder whether gett8ing a good power supply with a single 8 pin plug could be used with a splitter / adapter to create 2 x 8 pin power plugs. Does such a thing exist without building one DIY?
 

shhyguuy

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Jul 10, 2015
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Thanks for comment. You're probably right. I wonder whether gett8ing a good power supply with a single 8 pin plug could be used with a splitter / adapter to create 2 x 8 pin power plugs. Does such a thing exist without building one DIY?
I actually had the same problem about a year and a half ago and found a guy that custom makes them on ebay.

Look at this on eBay NEW EPS 8 PIN Y SPLITTER POWER CABLE---10" MADE IN USA

Sent from my SM-G935T using Tapatalk
 

fractal

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Oh man, these conversations quickly degrade into "I believe", "I think" and "The guy down the street / on some random forum says...". It is hard to argue with belief when silly facts get in the way.

As with many things, whether or not you can "get away with" using an 8 pin splitter .. "It depends".

I used an 8 pin y splitter before I bought the proper supply to run a pair of L5640's. Those are 60 watt TDP processors. And, we all know how reliable it is to use TDP for your power budgets. I think I bought mine on Amazon. They work fine within their limits.

One thing to consider is the power supply. Is it single rail or multi rail. If it is multi-rail, what is the limit on the rail feeding the ATX12v/EPS12v plug? Is it the UL 240VA limit? Many multi-rail supplies are limited to 18amps at 12v per rail to comply with the 240 VA UL limit. If so, you probably won't melt the plug going into your splitter with a pair of 135 watt TDP processors before the over current limit kicks in. (The UL limit is specified in VA, and VA == watts for direct current so we can use them interchangeably in this discussion. This is not necessarily true for AC. But, I digress). But, you won't be able to run a pair of 135 watt processors flat out before the over current limit in the power supply shuts you down.

If you have a single rail supply, or the rail feeding the ATX12v/EPS12v plug is NOT limited to the UL 240VA limit, you have to ask yourself, how lucky do you feel? Do you think the pins in that 8 pin connector are good for 5 amps? 6? 8? A conservative number is 5 amps, or 60 watts per pin, or 240 watts per 8 pin connector. How about 6 amps per? Four pins at 6 amps will give you 24 amps or 288 watts which will run your pair of 135 watt processors. Allowing for some variation in plug resistance, you may have some pins pulling 8 amps and others only pulling 4 amps but even 8 amps is unlikely to melt the 8 pin connector feeding the splitter.

So, if you have a single rail supply rated, or if you have a multi-rail supply, and that multi-rail supply can provide 25 amps on the rail feeding the ATX12V/EPS12V plug, then you can probably get away with splitting the single ATX12V plug into two ATX12V/EPS12V plugs to run your 2670's.

A safer choice is to use one of the dual 6 pin PCIe to single ATX12V adapters if you have spare PCIe power plugs. They are likely on a separate rail for a multi-rail supply. They will be using separate wires than the 8 pin ATX12V plug so you are less likely to overload the wires or the pins. They are harder to find than the 8 pin Y splitters.

And, of course, the best and safest way is to use a supply with dual 8 pin ATX12V/EPS12V plugs.

You might get away with using the splitter. And, you might burn down your house by using one. The former is far more likely than the latter. Please don't construe this as legal advise and don't sue me if you do burn down your house using an 8 pin Y splitter.
 

ttabbal

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Each setup will be a little different, but from what I found with a couple google searches, those pins are a fair bit better than some people think.

http://www.formfactors.org/developer\specs\atx2_1.pdf

Mentions using these connectors or equivalent. http://www.molex.com/webdocs/datasheets/pdf/en-us/0039012040_CRIMP_HOUSINGS.pdf

Max current per contact is rated at 13A with 16Ga wire. There are 4 circuits, so each EPS12V connector is good for 52A.

According to this datasheet for the crimp pins, http://www.molex.com/pdm_docs/ps/PS-45750-001.pdf, the ratings are based on a max temp rise of 30C over abient. The housings are Nylon, unless your system is in a pretty hot room, you won't even hit the glass transition temp for Nylon, let alone melt it at a 30C rise. I 3D print the stuff at about 270C.

Now, if you are using a Y-cable, you must not draw >52A from that single connector, it's the weak spot. That's 624W@12V, to be within spec for the connectors. I'm not aware of any system that can pull that level of current for the CPU regulators, so you're pretty safe. Obviously, your 12V rail needs to be able to support whatever power level you want to draw. That's another issue entirely. Are there better options? Probably. Is a well made Y-Cable "good enough"? Probably.

If you use one, keep an eye on it. I run burn-in tests that max the CPU out for days at a time. Part of my testing is to measure the temp rise of the power cables and connector pins. It's never been more than about 10C, but particularly with extenders and Y-Cables, it's good to be safe. At least unplug them and feel the pins with your fingers. That should tell you if there is a possible problem.
 

fractal

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If you use one, keep an eye on it. I run burn-in tests that max the CPU out for days at a time. Part of my testing is to measure the temp rise of the power cables and connector pins. It's never been more than about 10C, but particularly with extenders and Y-Cables, it's good to be safe. At least unplug them and feel the pins with your fingers. That should tell you if there is a possible problem.
Excellent advice for any application and particularly important for anyone running a splitter or extender. I tend to run full load for a while with all covers on, then open it and grab each of the connectors if I absolutely must run a splitter or extender. I figure if it is so warm I don't wanna hold it, I don't wanna run it like that.

You probably want to check http://www.molex.com/pdm_docs/ps/PS-45750-001.pdf to get the proper de-rating for in a multi-row housing. 13 amps is the single conductor rating. It drops to 11 amps maximum for 16 awg wire in a two row, 8 pin package. 8.5 amps for 18 awg wire like the power supply I grabbed out of the pile and the splitter listed above uses. You need to find a qualified packaging engineer to tell you how much to further de-rate it for your application. Heck, we cut your 13 amp rating almost in half just by looking at the proper line in the proper table. I tend to cut it in half which is how I came up with my 5 amp number. But then again, I prefer being over cautions in what I do at home.

There are all too many threads from the "bitcoin" farmers who started off saying "splitter / schmitter, it works so who cares!" with their multi-GPU rigs. They came on a few months later with pictures of the melted mess crying about the loss. I just smirked. I learned my lesson when I melted the ATX power connector by overclocking my BP6 like so many of us back then ...
 
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ttabbal

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Ahhh, yes. I missed the derating factor for the housings. I thought it seemed a bit high. I do know my PSUs are 16awg, the ones above 600W anyway. The smaller units use smaller wire, as one would expect. I even measured the copper on one, but that was because it died and I was recycling the wire for an application that needed that size. Needed to know which ones to use. :)

It is a good point to double check things. This stuff is never quite as simple as it looks.

Also watch for suspiciously thin wires. I've seen some get pretty toasty while under the nameplate rating on the PSU.
 

Bill1950

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The research I can dig up indicates short runs of 16 gauge are rated for 20A@12V. WE'RE TALKING WIRE ONLY. That's 240watt rating per circuit coming out of a power supply. 4 circuits are 960watts. Even using a splitter, that's 480watts per processor.

When I used the power supply calculator on OuterVision.com, it indicated a different of 110watts when adding a second E5-2670 processor. That makes me believe that the processor draw is 110 watts even though the TDP is 115watts. (still ballpark)

So, even assuming the actual processor draw peaks at 50% higher, that's ~ 175 watts (conservatively) per processor. 2 processors @ 175w@ would draw 350watts total (considering 150% indicated draw). That 350 watts is well below the rating for the single EPS run from the power supply before the split.

If we use the TDP value of 115watts (above the value used to computer power supply requirements - does that defy physics?), then the total load on the single EPS run from the ps is only 230watts, below the rating for a single run of 16gauge copper.

That's for the wires, ONLY.

If we look at the limitations for the connectors, we must max out below 600watts combined across the 4 connector from the single EPS run from the power supply. That means less than 300watts per processor. The worst case (full TDP) combined total for both processors plus 50% is 350watts or 175watts per processor. Still in the safety margin for the connector.

So, please tell me where I'm off base. (seriously)

Certainly, I would test "feel" the connector temps. Only makes sense. IF, I go with a single EPS run ps.
 

ttabbal

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I think in 99% of cases, the EPS12V run is a non issue with a single splitter. I can certainly see the issue previously mentioned in the thread with GPUs though. They draw more power than most CPUs and in the high end setups, there could be 4+ cards. Then if someone cheaps out on the PSU, things can get hot fast.

Now, if you're doing a 4 socket board and trying to use 3 splitters from one EPS line, I would worry a bit. Add in someone getting cheap and using thin wire splitters, and they could have a bad day coming soon.

All that said, if you are buying parts, make sure you get at least 2 EPS lines on the PSU. That's not very hard to come by these days. And if you use extension cables, don't get the cheapest ones from eBay. :)
 
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Bill1950

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I ended up getting a good deal on an EVGA Supernova 850B2. It's overkill, but is quiet. Plenty of "basic connectivity." Plenty of headroom. No splitters needed.