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Step 7: Select A Power Supply

How To Build A PC: From Component Selection To Installation
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Although it doesn't get its fair share of recognition, the power supply is the single most critical component for system stability and longevity. We've seen cheap models literally go up in flames, taking out several key pieces of hardware in the process. Picking an underpowered model might get you crashes or even boot failures. Since low-quality parts often fall short of their specifications, we'll start off with a link to our power supply reviews and a list of reputable units that have surpassed the expectations of our forum experts. You’ll notice that power supplies don’t get updated as often as other parts, because that technology doesn’t progress as quickly. Quality units have “staying power”.

How much capacity your system needs depends on its hardware configuration. Graphics cards are the most power-hungry components in gaming systems, while CPUs take priority if you're using integrated graphics. Several power supply calculators are available on the Web, though some are more up-to-date than others. The good news is that oversized power units can easily sustain undersized systems without damage, though efficiency sometimes drops when the unit is loaded by less than 20% of its rating.

Power supplies are divided into multiple primary (12 V, 5 V, 3.3 V) and secondary (-12 V, -5 V, 5 V standby) voltage outputs. Better-quality power supplies provide separate over-current protection on each of these output levels, called "rails". Additionally, Intel specified that each rail could provide no more than 18 amps, to reduce the risk of connector meltdown/cable fire.

As the need for more than 18 A of 12 V power became obvious, most manufacturers started dividing their 12 V output into multiple 18 A rails. That created load-balancing trouble as, for example, a two-rail unit could have two highly-loaded cables on one rail and two relatively unloaded cables on the other. This would trip the amperage protection circuitry, even though the internal transformer had power to spare. So-called single-rail power supplies were then devised that violated Intel's mandate, but allowed these systems to at least function. And "smart" power protection circuits have since been employed to reduce the risk of a fire from a single connector (which was the reason for the mandate in the first place).

Simple calculators might do the job for basic configurations, but the highest-end graphics cards place higher load bias on +12 V rails (so much so, in fact, that AMD's Radeon R9 295X2 even has a very specific +12 V rail requirement). Most of today's highest-performance power supplies are correspondingly designed to serve up lots of current on the +12 V rail, though cheaper parts occasionally skimp in that specification. Be on the lookout for this as you shop. AMD and Nvidia originally guided customers to the PSUs with enough 12 V amperage through their lists of CrossFire- and SLI-certified supplies. However, 80 PLUS and its efficiency ratings are also popular sources for determining higher-quality products.

Power supplies are rated in output, and one benefit from 80 PLUS reports in that they contain efficiency data from 20% to 100% load. This enables Tom’s Hardware readers to find a similar configuration in one of our builds, read the input power that we report, and calculate the required output power using 80 PLUS efficiency ratings. For example, a complete machine that draws 647 W through our meter at 85% efficiency needs a 550 W-rated unit (647 x 0.85). Even if you add a little over-capacity for USB-powered peripherals and future drive upgrades, that same machine can run comfortably on a high-quality 600 W unit.

Power supply form factors are not named after motherboard standards, in spite of the way they’re often sold. The ATX motherboard form factor does specify how they’re wired however, and an ATX-compliant power unit could follow one of several sizing standards. These include PS/2, PS3, SFX, or TFX, plus propriety parts.

Power Supply Form Factors
TypePS/2PS3SFX*TFX
Height5.875"5.875"2.50"70 mm
Width3.375"3.375"5.00"85 mm
Depth5.625"4.00"4.00"175 mm

Often called “ATX”, the PS/2 power supply form factor is a carry-over from the 1980s, long before ATX even existed. Its mounting pattern continues to be used in most mid- and full-tower ATX systems, but large-capacity units are often far longer (deeper into the case) than required by the original specifications. The odd-appearing metric dimensions are artifacts from an original design based on fractional inches. But the inch-based screw threads aren’t as friendly to metric conversion.

Using the same mounting holes as standard PS/2 units, PS3 allowed Hewlett Packard to shorten the overall depth of its 1990s full ATX mini-tower cases. Confusion over PS3’s age can be attributed to the extensive time it took for Intel to add the existing standard to its power supply guidelines. Conflation with SFX can also be blamed on Intel’s placement of its physical dimensions within SFX design guidelines.

One might say that SFX is two form factors, one that’s 5” by 4” and the other 4” by 5”. As a potential third candidate for SFX naming, Intel also specifies a 50 mm-tall version as “SFX, 40 mm Profile” in reference to its fan size. The three (sub-standards) can be differentiated by visual inspection as being wider, deeper, or thinner than the other two. The wider one is more common in consumer-level cases, and the one that’s coincidentally (and mistakenly) most often referred to as microATX. This form factor also allows up to 17 mm of fan housing to extend from one side of the lid, into the computer case.

The narrow TFX form factor allows some companies to make their slim cases even slimmer, though it also intrudes farther into the case. Because PS3, SFX, and TFX are often sold side-by-side under the microATX banner, buyers must often look at the pictures to determine what the seller is actually selling.

EPS supersedes ATX as the electrical standard for high-amperage power supplies, with a 24-pin “EPS” main connector powering most on-board devices and an 8-pin EPS 12 V connector delivering power to the CPU. Most manufacturers make these connectors divisible, with 4-pin sections breaking away to allow fitment in 20-pin ATX and 4-pin CPU power headers.

Also shown is an 8-pin PCIe supplemental power cable for high-end graphics cards, from which two pins can be split away to make it work with 6-pin headers. The plastic insulator surrounding these pins is shaped differently from the 8-pin CPU power connector, preventing accidental misuse.

There’s also some cross-compatibility between wider and narrower cables. Many systems with 8-pin CPU power connectors will operate sufficiently from a 4-pin cable, lacking the extra current needed to support a high overclock. And it’s often possible to hang the end of a non-divisible cable over the end of a narrower connector.

Drive power cables include the old-fashioned 4-pin “ATA” style, a smaller “floppy” style, and the more modern “SATA”. Increasingly, power supplies lack the floppy power cable, but, because some accessories use it to power other things, you often get an adapter for one of the ATA-style connectors. In this day of SATA-based storage, the four-pin ATA leads rarely hook up to drives, but rather power cheap fans, fan controllers, and multi-drive backplanes.

In total, builders must find a power supply that’s quality-made, fits their case, has enough capacity, and has all the required cable ends. If that last measure isn’t met, adapters are usually available.

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  • 6 Hide
    gofasterstripes , June 23, 2014 2:11 AM
    Can I once-again say a big "Thanks" to the Tom's team for writing a great article which will save me lots of time and hassle in explaining PC building and component selection to n00bs who ask me for help.

    Cheers!
  • -1 Hide
    JOSHSKORN , June 23, 2014 2:57 AM
    What about NUCs and cases alike? Yes, I realize those are too small for discrete GPU cards, but depending on your usage and space, it just might be what one needs.
  • 4 Hide
    Crashman , June 23, 2014 3:06 AM
    Quote:
    What about NUCs and cases alike? Yes, I realize those are too small for discrete GPU cards, but depending on your usage and space, it just might be what one needs.
    It's addressed in the last two sentences of page 1:
    Quote:
    Intel even jumped on the tiny bandwagon with its similar-appearing NUC (Next Unit [of] Computing) form factor. Most of these machines are available either as a barebones system (no drives or memory) or a complete PC, and all of them use external, notebook-style power adapters.
    There's really nothing more to say. You don't build these from separate components, you just finish them with a drive and memory as you would any of the other systems covered in this article.
    Quote:
    Can I once-again say a big "Thanks" to the Tom's team for writing a great article which will save me lots of time and hassle in explaining PC building and component selection to n00bs who ask me for help.

    Cheers!
    You're welcome!
  • -1 Hide
    revanchrist , June 23, 2014 3:25 AM
    Should put in a Cooler Master Elite 110 to showcase a mini-ITX instead of that huge Lian-Li.
  • 1 Hide
    illuminatuz , June 23, 2014 3:47 AM
    Guess most of the issues in the forums will be ironed out by this one simple post..
    Wonderful as usual toms.. Appreciate it..
    :D 
  • 1 Hide
    rolli59 , June 23, 2014 5:46 AM
    Most if not all issues brought up on the forums have answers in some or the other article on Tomshardware. This article compiles answers to build questions.
  • 1 Hide
    AMD Radeon , June 23, 2014 6:37 AM
    This article is very good reference newbies asking how to build PC in toms forum.
  • 0 Hide
    DookieDraws , June 23, 2014 6:54 AM
    Long time reader of Tom's Hardware, even before I finally became a member. This has been a very helpful site to me and my friends over the years. In fact, this IS our go-to site for when we need to figure something out. I am always learning something new.

    Great article! No doubt this is going to help a lot of folks.

    Thanks, guys!
  • 1 Hide
    Yuka , June 23, 2014 7:25 AM
    Very good write up guys.

    I think you missed a section for "SLI - XFire", but it's great overall. Since its a guide for folks with little to no knowledge, I think it would help them to dispel myths and get some facts over XFire and SLI.

    Cheers!
  • 2 Hide
    Xeres Forteen , June 23, 2014 7:53 AM
    Good article with excellent info. I'm going to just add my 2 cents on how I like to do the actual assembly of the PC after receiving the parts.

    First I put the motherboard into the PC (not fastened) to see where the standoffs are going to be placed onto the case. Also I note what routes I'm going use for my cabling. Then I take the motherboard out and insert the standoffs and port plate into the case. Also I take my case cables (power sw, reset sw, USB, front audio and mic cables and put a twist tie around them all and place them near where they are to be plugged into the motherboard. These cables are easy to lose track of.

    Next I place the power supply, and "bay devices" (optical drives, non-removable storage, etc) into the case and have those cables attached and either hanging over the outside of the case or routed behind the motherboard tray. This obviously depends on how you determined the cables will be routed earlier.

    Then I take my motherboard, put the CPU, RAM, and cooling system on as much as I can. Then I place the whole thing into the case - usually at an angle at first, leading with the side with the RAM (which is normally going behind the case bays in smaller cases) in first.

    At this point it's just a matter of aligning the motherboard with the standoffs and port plate. Plug it all in (including the case plugs which are conveniently out of the way and together).

    Power it all on and volia!
  • 1 Hide
    Onus , June 23, 2014 8:03 AM
    I would like to have seen the article begin well before case selection, with questions as to purpose for the build. Define the purpose, then you know what sort of parts you'll need for it. The case is often the last component I choose, and only once was the first, when the purpose was to build the smallest system possible that would meet my needs.
    Otherwise, it was a good article. People who are uncertain of building their own PCs can learn a lot from it.
  • 0 Hide
    zelgar , June 23, 2014 9:10 AM
    I'm a little confused with the section dealing with Choosing your Power Supply. If the power requirements of the components is 647 W as shown in the example, how could you use any power supply that isn't designed to higher wattage? Is it expected that only 85% of the design load will ever be used (i.e., that you wouln't be using the max energy requirements of all of your components at the same time)? Wouldn't it be better to have your power supply based upon the max usage of the components since this may occur (e.g., running a modern game at max settings that your computer can run)? Aren't Power supplies rated Bronze/Silver/Gold/Platnium at 80% of the Power Supplies capacity, so you'd want a PS that is would be running at 80%+ at max load (e.g., 647/.80 = 809 W or 647/.85 = 761W)?
  • 1 Hide
    Damn_Rookie , June 23, 2014 11:35 AM
    Quote:
    I'm a little confused with the section dealing with Choosing your Power Supply. If the power requirements of the components is 647 W as shown in the example, how could you use any power supply that isn't designed to higher wattage? Is it expected that only 85% of the design load will ever be used (i.e., that you wouln't be using the max energy requirements of all of your components at the same time)? Wouldn't it be better to have your power supply based upon the max usage of the components since this may occur (e.g., running a modern game at max settings that your computer can run)? Aren't Power supplies rated Bronze/Silver/Gold/Platnium at 80% of the Power Supplies capacity, so you'd want a PS that is would be running at 80%+ at max load (e.g., 647/.80 = 809 W or 647/.85 = 761W)?

    The 647W is measured at the wall socket, as the article mentions input power. After taking into account the 85% efficiency of their power supply in this example, the PSU is only outputting 549.95W to the PC components at max load. Adding some headroom they come to the 600W PSU recommendation.

    Personally I'd like a little more headroom, but the calculations in the article are correct.
  • -1 Hide
    zelgar , June 23, 2014 12:00 PM
    I think the article will confuse noobs of talking about the power measurement at the wall socket versus the power rating of the power supply. If the sum of the power requirements of the computer components is X, they need a power supply the provides more than X. Making them aware that the power supply will be using more energy than X at the wall is useful to know so that people don't buy a PSU that is too wasteful. Since undersizing a PSU is more catestropic than oversizing one (e.g., crashes/boot faiolures vs electricity costs) getting one that meets the needs of the components is the starting point of evaluating PSU.
  • -1 Hide
    n3cw4rr10r , June 23, 2014 12:37 PM
    I usually buy my pc components from Amazon. #1 customer service. They dont give you any bs. Hell they even send you a pre-paid shipping label to ship the item back. I have never had they charge me a "re-stocking" fee on any item, even when I returned item because I did not like it.
  • 0 Hide
    PapaCrazy , June 23, 2014 1:16 PM
    After all this time, why do I still get the most excited about the cases? Maybe cuz the components are logical forgone conclusions, but the case is the artistic, subjective bit? I don't know, years at this stuff and a sexy case still turns my head. The mobos are starting to look really sexy nowadays too.
  • -1 Hide
    Jim Maher , June 23, 2014 3:03 PM
    Great article! This is a wonderful overview, and should prompt most readers to research their more detailed questions on the forums.

    Building your own is great fun, and most serious users should probably give it a try at least once in their lives. Given that, I'd recommend an annual "refresh" of this article, with updated info and re-validated links to corresponding reference articles and resource forums.

    A great service to your readers!
  • -1 Hide
    chaz_music , June 23, 2014 5:24 PM
    Excellent article with lots of interesting information.

    I wanted to comment on the power supply part of the article. One is the efficiency and the total cost to use versus the front end purchase cost. A less efficient system will obviously create more total heat as wasted energy. But aside from possibly making someones room rather uncomfortable, it also increases your airconditioning energy use. A good rule of thumb is that an AC system will use 50% of the heat energy. To add the total annual cost, multiply that times the percentage of the year that the AC is on. So your example of a 647W system with 85% PSU would give (550W used):

    647W - 550W = 93W at plug

    93W * 50% = 47W AC energy

    Total Energy (summertime) = 93W + 47W = 140W

    If the AC were on the while year and the PC were on continuously, this is about $140 annually, or almost $12 per month added electricity in the summer. If you did the same thing with a cheap 70% efficient system, you get $248 annual cost which is $20.63 per month summertime cost. At a difference of $8, it does not take many months (of continuous on!) to make the more efficient PSU make much more sense.

    The other topic I wanted to comment on is ESD. I am an engineer and work with ESD issues everyday. It is a very real an poorly understood issue by many because of the often hidden or delayed failures that it causes. ESD many time causes walking wounded damage without an immediate failure, which finally fails several months later. And if you look at websites sell PC parts, many people complain of DOAs. Many, many DOAs are caused by ESD. Memory, CPUs, motherboards, HDDs, and other sensitive systems are often returned as DOA, driving up the cost of the PC enthusiast market and adding frustration. In research texts, they estimate the global electronic failures due to ESD to be 40-60% of the total failures over product life.

    So that little $5 ESD wrist strap is money well spent. Buy one and reduce your heartburn.

    Charles
  • 1 Hide
    Crashman , June 23, 2014 5:49 PM
    Quote:
    The other topic I wanted to comment on is ESD. I am an engineer and work with ESD issues everyday. It is a very real an poorly understood issue by many because of the often hidden or delayed failures that it causes. ESD many time causes walking wounded damage without an immediate failure, which finally fails several months later. And if you look at websites sell PC parts, many people complain of DOAs. Many, many DOAs are caused by ESD. Memory, CPUs, motherboards, HDDs, and other sensitive systems are often returned as DOA, driving up the cost of the PC enthusiast market and adding frustration. In research texts, they estimate the global electronic failures due to ESD to be 40-60% of the total failures over product life.

    So that little $5 ESD wrist strap is money well spent. Buy one and reduce your heartburn.

    Charles
    The vast majority of "DOA" returns are for undamaged products (simple things like having a standoff or cable in the wrong place that caused power fault/no boot/no damage are number 1, followed by fraudulent returns at Number 2). And the majority of damaged products have physical damage from the amateur technician cramming things together. I used to be in that business.

    The only problem with wrist straps is that most people don't want to be "tied" to anything. They're a great idea that's really rarely needed. Feel free to say otherwise if you live in the desert.

  • -1 Hide
    cutiemech , June 24, 2014 12:31 AM
    Skimmed through it but didn't saw anything about tools required for assembly like what screwdriver etc
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