Power Supply 101: A Reference Of Specifications

Power Supply Form Factors

The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When designing a PC, the engineers can choose to use one of the popular standard PSU form factors, or they can elect to build their own custom design. Choosing the former means that a virtually inexhaustible supply of inexpensive replacement parts will be available in a variety of quality and power output levels. Going the custom route means additional time and expense for development. In addition, the power supply is unique to the system and generally available for replacement only from the original manufacturer. This precludes any upgrades as well, such as installing higher-output replacement models.

If you can’t tell already, I am a fan of the industry-standard form factors! Having standards and then following them allows us to upgrade and repair our systems by easily replacing physically (and electrically) interchangeable components. Having interchangeable parts means that we have a better range of choices for replacement items, and the competition makes for better pricing, too.

In the PC market, IBM originally defined the form factor standards, and everybody else copied them; this included power supplies. All the popular PC power supply form factors up through 1995 were based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting thing is that these three original IBM power supply form factors had the same motherboard connectors and pinouts; where they differed was mainly in shape, maximum power output, the number of peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three designs were popular up through 1996 and beyond; in fact, even the current industry standard ATX12V models are based on the PS/2 Model 30 physical form factor, but with different connectors.

Intel defined a new power supply form factor in 1995 with the introduction of the ATX form factor. ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX and the standards that have followed since use different connectors with additional voltages and signals that allow systems with greater power consumption and additional features that would otherwise not be possible with the AT-style supplies.

Note: Although two power supplies can share the same basic design and form factor, they can differ greatly in quality and efficiency. Later in this chapter, you’ll learn about some of the features and specifications to look for when evaluating PC power supplies.

More than 10 different power supply form factors have existed that can be called industry standards. Many of these are or were based on designs IBM created in the 1980s, whereas the rest are based on Intel designs from the 1990s to the present. The industry-standard form factors can be broken down into two main categories: those that are currently in use in modern systems and those that are largely obsolete.

Note that although the names of some of the power supply form factors seem to be the same as those of motherboard form factors, the power supply form factor relates more to the system chassis (case) than to the motherboard. That is because all the form factors use one of only two main types of connector designs: either AT or ATX, with subtle variations on each. So, although a particular power supply form factor might be typically associated with a particular motherboard form factor, many other power supplies would plug in as well.

For example, all modern ATX form factor motherboards with PCI Express slots have two main power connectors, including a 24-pin ATX main connector along with a four-pin +12 V connector. All the modern power supply form factors include these same connectors and therefore are capable of plugging into the same motherboards. In other words, no matter what the form factor of the motherboard (ATX, BTX, or any of the smaller variants of either), virtually any of the modern industry-standard power supplies will plug in.

Plugging the power supply connectors into the motherboard is one thing, but for the power supply to work in the system, it must physically fit inside the chassis or case—and that is what the different modern power supply form factors are all about. The bottom line is that it is up to you to ensure that the power supply you purchase not only plugs in to your motherboard, but also fits into the chassis or case you plan to use.

The following two tables show the industry-standard power supply form factors, their connector types, and the motherboard form factors with which they are usually associated.

Modern Industry-Standard Power Supply Form Factors
Modern Power Supply
Form Factors
Normally Associated Motherboard
Connector Types
20/24-pin Main, 4-pin +12VATX, microATX, BTX, microBTX
20/24-pin Main, 4-pin +12VmicroATX, FlexATX, microBTX, picoBTX, Mini-ITX, DTX
24-pin Main, 8-pin +12VATX, extended ATX
20/24-pin Main, 4-pin +12VmicroATX, FlexATX, microBTX,

picoBTX, Mini-ITX, DTX

20/24-pin Main, 4-pin +12VmicroBTX, picoBTX, DTX
24-pin Main, 4-pin +12VpicoBTX, nanoBTX, DTX
Flex ATX2007
24-pin Main, 4-pin +12V‑microATX, FlexATX, microBTX,
picoBTX, nanoBTX, Mini-ITX, DTX
*SFX12V also includes the PS3 form factor, which is a shortened version of ATX12V.

12 V versions include a four-pin or eight-pin +12 V connector.

You may encounter power supplies using obsolete form factors if you work on PCs built in the 1980s through the mid-1990s. I cover them in more detail in the 18th and earlier editions of this book.

Obsolete Industry Standard Power Supply Form Factors
Obsolete Power Supply
Form Factors
Normally Associated Motherboard
Connector Types
PC/XT, Baby-AT
ATFull-size AT, Baby-AT
ATFull-size AT, Baby-AT
ATFull-size AT, Baby-AT
**LPX (PS/2)1987
ATBaby-AT, Mini-AT, LPX
*PC/XT connectors were the same as AT connectors, except one +5 V pin (P8 pin 2) was not used.
**LPX is also sometimes called PS/2 or Slimline.

Each of these power supply form factors is, or has been, available in numerous configurations and power output levels. The obsolete LPX form factor supply originated in the IBM PS/2 Model 30 in April 1987 and was the standard used on most systems from the late 1980s to mid-1996, when the ATX form factor started to gain in popularity. Since then, ATX and the many variants based on ATX have become by far the dominant form factors for power supplies. It is interesting to note that IBM’s legacy lives on even now because ATX, PS3, and EPS are all based on the LPX (PS/2) physical form factor. Any power supply that does not conform to one of these standards is considered proprietary. In general, avoid systems that use proprietary power supply designs because replacements are difficult to obtain and upgrades are generally not available. When you consider that the power supply is one of the most failure-prone components, purchasing systems that use proprietary designs can be a significant liability in the future. If you need a replacement for a proprietary form factor supply, one of the best sources is ATXPowerSupplies.com. They maintain replacement models that cover a huge number of both proprietary and industry standard designs.