all about the various pc power supply cables and connectors

7/31/2019 All About the Various PC Power Supply Cables and Connectors

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Power supply information

Compatibility issues for ATX power supplies and motherboards

A short history of PC power supply voltage railsSo what's all this rubbish about multiple 12 volt rails?

All about the various PC power supply cables and connectors

Rail complications #1 - current limit problems: too much current

Rail complications #2 - cross loading problems: unbalanced current

Rail complications #3 - minimum loading problems: too little current

Using PC power supplies in things other than PCs

Last updated: July 15, 2008

All about the various PC power supply cables and connectors

The various power supply cables

Original PC main power cables 4 pin peripheral power cable Floppy d

bout the various PC power supply cables and connectors http://www.playtool.com/pages/psuconnectors/connectors.htm

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6 pin auxiliary power cable SATA power cable

20 pin ATX main power cable 24 pin ATX main power cable 20+4 pin A

4 pin ATX +12 volt power cable 8 pin EPS +12 volt power cable 4+4 pin +12 vol

6 pin PCI Express power cable 8 pin PCI Express power cable 6+2 pin PCI Expre

General info

If you'd like more technical data about ATX power connectors then you can find the current specifications and

more at formfactors.org

Wires and connectors are not perfect conductors. They have resistance. When current passes through wires and


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connectors there is a voltage drop and that energy is lost as heat. As long as you don't overload them the voltage

drop and extra heat don't matter. But the losses get worse as the current rises. That's why you see some power

cables with more that one wire for the same voltage. Having multiple wires reduces the losses. If you seriously

overload a line, the wire can get fairly warm. The resistance of connectors tends to increase as they are plugged

and unplugged so after enough uses they can overheat and even melt when passing a large current. So a lot of

changes in connectors over time has to do with adding more wires and connectors to make sure that none of

these problems occur.

Some of the tables below provide the maximum wattage supported by a power cable and its associated power

connector. Some specifications clearly spell out the maximum allowed wattage. Other specifications just providethe suggested connector and wire gauge and never specify a maximum wattage. And there's never really an

absolute value to the maximum wattage anyway. If you draw a little more wattage then the maximum, the

hardware doesn't immediately burst into flames. The voltage drop and power dissipation increase as you increase

the current so there isn't a clear maximum wattage at which it stops working. Most of the specifications which

spell out the maximum wattage provide a wide safety margin by defining a value which is far below the

maximums supported by the connector and wire. If the table below provides an "Official cable/connector

maximum wattage" then that specification has spelled out the maximum wattage. In most cases that wattage will

be significantly below what can actually be handled by the suggested connector and wire. If the table provides

an "Unofficial cable/connector maximum wattage" then that specification doesn't provide the maximum and the

value in the table is a practical maximum wattage defined by the maximums for the connector and the suggested

wire. An unofficial wattage doesn't have a wide safety margin built in because people differ on how much

margin to provide. Some people happily use connector and wire maximums and other people like to have a wide

safety margin.

Power connectors included with various kinds of PCs

Version Introduction date Included connectors

PC 1981original PC main power cables

4 pin peripheral cable

ATX 1995

20 pin main power cable


floppy cable

ATX12V 1.0 2000

20 pin main power cable4 pin 12 volt cable

6 pin auxiliary cable


floppy cable

ATX12V 1.3 2003


4 pin 12 volt cable

6 pin auxiliary cable

SATA cable


floppy cable

ATX12V 2.0 2003

24 pin main power cable4 pin 12 volt cable

6 Pin PCI Express power cable

SATA cable


floppy cable

EPS12V 2003


8 pin 12 volt cable


SATA cable


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floppy cable

PCI Express 2.0 2007


4 pin 12 volt cable

8 pin 12 volt cable



SATA cable

4 pin peripheral cablefloppy cable

Original PC main power cables

The original PC debuted in 1981 and used two cables to connect the PSU (power supply) to the motherboard.

The two cables plug side by side into the motherboard connectors. Sometimes they are keyed so they only plug

in one way and sometimes they aren't. Even if they're keyed you can insert them the wrong way if you put a

little effort into it. You always have to remember to plug them in so the black wires are next to each other. It's

either "black to black" or smoke and a shower of sparks.

Pinout

Pin number Wire color Description

1 orange power good

2 red +5 volts or connector key

3 yellow +12 volts

4 blue -12 volts

5 black ground

6 black ground

1 black ground

2 black ground

3 white -5 volts


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4 red +5 volts

5 red +5 volts

6 red +5 volts

Connector part numbers

Motherboard connector Cable connector Terminals Maximum current per circuit

Molex 15-48-0106 Molex 90331 Molex 08-50-0276 5 amps

Unofficial cable/connector maximum wattage delivery for main rails

Voltage rail Number of lines Maximum current Maximum wattage

+5 volts 3 or 4 15 or 20 amps 75 or 100 watts

+12 volts 1 5 amps 60 watts

In old PCs, almost all of the chips ran directly off of the 5 volt rail. As a result the PSU delivers most of its

wattage at 5 volts. There are three or four lines dedicated to the 5 volt rail. The other main rail is 12 volts. That

was used primarily to run disk drives, motors, and fans. The two negative rails are "bias" supplies which only

have to provide small amounts of current. Just for interest's sake, here's the specification for the PSU which

came with the original IBM PC. It provides a maximum of 63.5 watts. Boy, those were the days.

Original IBM PC power supply

Voltage Maximum current Maximum wattage

+5 volts 7.0 amps 35 watts

-5 volts 0.3 amps 1.5 watts

+12 volts 2.0 amps 24 watts

-12 volts 0.25 amps 3 watts

4 pin peripheral power cable


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The four pin peripheral power cable dates back to the original PC. It was used for floppy drives and hard disks.

It's still around and is now also used for all kinds of things including add-on fans, extra video card power,

supplemental motherboard power, and case lighting. It's as old as the hills but is still very widely used. The

connector is shaped so that it only fits in one way. You don't have to worry about inserting it the wrong way.

People often use the term "4 pin Molex power cable" or "4 pin Molex" to refer to a four pin peripheral power

cable. It's not a technically useful term because the 4 pin 12 volt cable is also a 4 pin Molex cable (Molex makes

lots of connectors) but "4 pin Molex" is commonly used to refer to peripheral cables anyway.

PinoutPin number Wire color Description

1 yellow +12 volts

2 black ground

3 black ground

4 red +5 volts


Socket housing Socket Pin housing Pin Maximum current per circuit

AMP 1-480424-0 AMP 60619-1 AMP 1-480426-0 AMP 60620-1 13 amps

I don't know of any official definition of the maximum current allowed in a peripheral cable. The connector can

handle 13 amps according to the manufacturer. But you normally find 18 awg wire in the peripheral cables. If

you have an 18 inch cable (about a half a meter) and are running 13 amps through 18 gauge wire then you get a

voltage drop of about 0.25 volts counting both the power wire and the ground (it's got to go both ways) and the

dissipation is about 3.3 watts. That's not good. I've just played it safe and listed the maximum current as 5 amps.

Unofficial cable/connector maximum wattage delivery




Current power supplies usually have at least two separate peripheral power cables, each of which has two or

more peripheral connectors. When you're plugging in multiple high powered devices it's a good idea to spread

the load between all of your cables. Don't just plug all your devices into one cable unless they're relatively low

load devices. Spreading the current between the cables reduces the voltage drops and power loss. If they're

relatively low current devices like fans or it's just a disk drive or two then it doesn't really matter. But if you're

putting lots of hard disks into a computer (some can draw almost 3 amps at 12 volts when doing some

operations) or connecting a video card's auxiliary power, then spread the loads between the peripheral power

cables. It's also helpful if you use a connector as close to the PSU as possible rather than sticking things at the

end of the cable. Extra wire just means more voltage drop. And if you're using a peripheral connector to PCI

Express adapter then be sure to plug each of the adapter's peripheral connectors into a separate PSU cable. They

gave you two peripheral connectors for a reason. Plugging them both into the same PSU cable forces your videocard to draw its 12 volt power through one 18 gauge wire. That increases your voltage drop and power

dissipation in the cable. Some current high-end video cards can suck up more than 10 amps at 12 volts with most

of it coming through the PCI Express connector so it pays to be careful. It will probably work if you don't spread

the load but there's no excuse for not doing it properly. They gave you multiple cables. You might as well use

them. Plus there's just something creepy about having warm wires even if they're not melting.

You will occasionally run into peripheral connectors which don't have all four wires. They are usually 12 volt

only cables intended for fans. Never plug one of those into a disk drive. Drives expect both 5 and 12 volts to be

provided. Some of the two-wire peripheral connectors are for speed-controlled fans. That means that the voltage

changes depending on the desired fan speed. The connector will only provide 12 volts when the fan is going full


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speed and the voltage decreases to slow the fan down. Definitely don't plug that one into anything but a fan!

Normally this kind of peripheral connector has "fan" printed on it to warn you. As long as a peripheral connector

has four wires: one yellow, two black, and one red and it doesn't have some kind of printed warning attached

then it's a standard peripheral cable and you can plug it into anything.

Floppy drive power cable

The four pin floppy drive cable showed up when PCs started including 3.5 inch floppy drives. This kind of cable

is also sometimes used as an auxiliary power cable for AGP video cards which use more power than can be

drawn from the motherboard slot. The connector is shaped so that it only fits in one way so you don't have to

worry about inserting it the wrong way. Floppy cables are built with small connectors and 20 awg wire so they

are limited to relatively low current uses.

Pinout


1 red +5 volts

2 black ground

3 black ground

4 yellow +12 volts


Socket housing Socket Maximum current per circuit

AMP 171822-4 AMP 170262-1 3 amps





6 pin auxiliary power cable


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The aux power cable was added to provide extra wattage to motherboards for 3.3 and 5 volts. This connector is

rarely used anymore. It's most commonly found on older dual CPU AMD motherboards. You're more likely to

sight Bigfoot than a motherboard which uses this connector. It plugs into the 6 pin version of the motherboard

connector used by the original PC main power cables.

Pinout


1 black ground

2 black ground

3 black ground

4 orange +3.3 volts

5 orange +3.3 volts

6 red +5 volts



Molex 15-48-0106 Molex 90331-0010 Molex 08-50-0276 5 amps



+3.3 volts 2 10 amps 33 watts


SATA power cable


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SATA was introduced to upgrade the ATA interface (also called IDE) to a more advanced design. SATA includes

both a data cable and a power cable. The power cable replaces the old 4 pin peripheral cable and adds supportfor 3.3 volts (if fully implemented). The connector is shaped so it can only be plugged in the correct way.

Pinout

Pin number Wire number Wire color Description

1 5 orange +3.3 volts



4 4 black ground

5 4 black ground6 4 black ground

7 3 red +5 volts

8 3 red +5 volts

9 3 red +5 volts

10 2 black ground

11 2 black ground

12 2 black ground

13 1 yellow +12 volts




Cable connector Terminals Maximum current per circuit

Molex 67582-0000 Molex 67581-0000 1.5 amps

Official cable/connector maximum wattage delivery



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+3.3 volts 3 4.5 amps 14.85 watts

+5 volts 3 4.5 amps 22.5 watts

+12 volts 3 4.5 amps 54 watts

You have to be careful about SATA power cables. Some of them are missing the 3.3 volt wire. People with older

power supplies often use adapters which convert from 4 pin peripheral cables to SATA power cables. But since 4

pin peripheral connectors only supply 5 and 12 volts, the SATA connector is missing 3.3 volts (there's no orange

wire). There are also a few older power supplies which inexplicably have SATA power cables which are missing

the 3.3 volt wire. Currently, SATA drives rarely use 3.3 volts. That may be because there are too many people

using adapters so the drive makers don't want the headaches which come with using 3.3 volts. But in the future,

3.3 volt drives may become common so you need to be careful when using SATA power cables which don't

implement 3.3 volts.

ATX 20 pin main power cable


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In 1996 PC makers started switching to the ATX standard which defined a new 20 pin motherboard power

connector. It includes a 3.3 volt rail which is used to power newer chips which require a lower voltage than 5

volts. It also has a standby 5 volt rail which is always on even when the power supply is turned off to provide

standby power to the motherboard when the machine is sleeping. The new connector also allows the

motherboard to turn the power supply on and off rather than depend on the user to flip a power switch. This

connector is polarized so it can only be plugged in pointing in the correct direction.

Pinout

Pins 1 through 10 Pins 11 through 20

Description Wire color Pin number Pin number Wire color Description+3.3 volts orange 1 11 orange +3.3 volts

+3.3 volts orange 2 12 blue -12 volts

ground black 3 13 black ground

+5 volts red 4 14 green PS_ON#


+5 volts red 6 16 black ground


PWR_OK gray 8 18 white -5 volts (optional)

VSB +5 volts purple 9 19 red +5 volts

+12 volts yellow 10 20 red +5 volts

Some of the voltage lines on the connector may have smaller sense wires which allow the power supply to sense

what voltage is actually seen by the motherboard. These are pretty common on the 3.3 volt line in pin 11 but are

sometimes used for other voltages too. The -5 volt line on pin 18 was made optional in ATX12V 1.3 (introduced

in 2003) because -5 had been rarely used for years. Newer motherboards virtually never require -5 volts but

many older motherboards do. Most newer power supplies don't provide -5 volts in which case the white wire is

missing.


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Molex 39-28-1203 Molex 39-01-2200Molex 39-00-0168,

Molex 44476-11116 amps



+3.3 volts 3 18 amps 59 watts



Dell used to ship many computers which used motherboards and power supplies which looked like plain old

ATX but were not really ATX at all. If you plugged a real ATX power supply into the motherboard then things

went kaboom (or frzzzzzap or ...). Sadly, some large computer vendors like to lock customers into buying

upgrades from them rather from a third party. That's nasty enough but Dell built their non-standard computers

and motherboards with what looked like standard ATX components. You only found out otherwise once sparks

started flying. The Dell PCs which were carrying this ticking time-bomb were made between 1996 and 2000. If

you're replacing a Dell power supply from this era then you should call Dell and make sure it's not a proprietary

design. And if they say yes then be sure to voice your displeasure. This is an evil business practice which shouldstop.

You can plug a 20 pin ATX power cable into a motherboard with a 24 pin ATX connector. The image above

shows the 20 pin power cable plugged into a 24 pin motherboard. A 20 pin power cable only fits into one end of

a 24 pin motherboard connector so you can't insert it incorrectly. The 24 pin motherboard connector is actually

just the 20 pin connector with 4 extra pins added on the end. The original 20 pins were unchanged. The extra 4

pins are not separate rails. They're just extra lines to provide more current to the same rails. On every 24 pin

motherboard I've seen, the extra lines for ground, 3.3, 5, and 12 are just connected to the other lines on the same

rail from the 20 pin connector. I've never heard of exceptions and it doesn't make sense for motherboard makers

to create any. As a result, you can plug a 20 pin cable into a 24 pin motherboard and it will work just fine. At

least, it will for a while. They added those extra 4 pins for a reason. When you plug a 20 pin cable into a 24 pin


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connector you're not providing the extra current carrying capacity which may be needed by the motherboard. If

your motherboard's current requirements are low enough then it will work properly with only a 20 pin power

cabled plugged in. But if the motherboard draws enough current, then you can overheat the 20 pins you're using

on the 24 pin connector. I've seen enough pictures of burned ATX main connectors to assure you that this

happens. In adding the extra four pins, 3.3 volts went from 3 to 4 lines. 5 volts when from 4 to 5 lines. 12 volts

went from 1 to 2 lines so its capacity doubled. Connectors really do get hot if you overload them so the safest

thing to do is use a real 24 pin power supply on a motherboard with a 24 pin connector. Note that your 24 pin

machine may work fine with a 20 pin power supply until you add a PCI Express card later on down the road.

PCI Express cards can draw up to 75 watts through the motherboard connector so adding an expansion card can

substantially increase power draw through the main power cable. Many PCI Express video cards which have the

6 pin or 8 pin PCI Express power cable still draw a substantial portion of their 12 volt load through the PCI

Express slot. The extra 4 pins doubled the current capacity of the 12 volt rail so that one is easy to overload

when only using a 20 pin main power cable.

The picture above shows an adapter which allows you to plug a 20 pin power supply into a 24 pin motherboard.

These do not solve the problems mentioned above. In fact, they usually make things worse. You still have only

the same old 20 wires and pin&socket connectors that you would have if you plugged it directly into the

motherboard. But with the adapter you also have longer wires and another set of pins&sockets so you have a

higher voltage drop and more things which can go wrong. All the adapter does is remove the problem from the

24 pin motherboard connector and move it to the 20 pin connector on the adapter. About the only good thing it

does is to make sure the 20 pin connector on the adapter melts rather than the 24 pin connector on the

motherboard. You shouldn't underestimate the potential problems which come from adding another connector

between the power supply and the motherboard. The resistance of connectors can increase due to being plugged

and unplugged. Increasing the resistance makes the connector drop more voltage and dissipate more heat. Any

number of physical changes can also cause higher connector resistance which causes more problems. You

shouldn't put an extra connector in a high current path unless you've got a good reason. That's a good rule for

power cables in general but it goes doubly for the high current draws you sometimes find through motherboard

main power cables. The best practice is to avoid these 20 to 24 pin adapters and get a real 24 pin power supply.

ATX 24 pin main power cable


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The 24 pin main power connector was added in ATX12V 2.0 to provide extra power needed by PCI Express

slots. The older 20 pin main power cable only has one 12 volt line. The new 24 pin connector added one line

apiece for ground, 3.3, 5, and 12 volts. The extra pins made the auxiliary power cable unnecessary so most

ATX12V 2.x power supplies don't have them. The 24 pin connector is polarized so it can only be plugged in

pointing in the correct direction.

Pinout


Description Wire color Pin number Pin number Wire color Description

+3.3 volts orange 1 13 orange +3.3 volts

+3.3 volts orange 2 14 blue -12 volts


+5 volts red 4 16 green PS_ON#


+5 volts red 6 18 black ground


PWR_OK gray 8 20 white -5 volts (optional)

VSB +5 volts purple 9 21 red +5 volts



+3.3 volts orange 12 24 black ground

Some of the voltage lines on the connector may have smaller sense wires which allow the power supply to sense

what voltage is actually seen by the motherboard. These are pretty common on the 3.3 volt line in pin 13 but are

sometimes used for other voltages too. The -5 volt line on pin 20 was made optional in ATX12V 1.3 (introduced

in 2003) because -5 had been rarely used for years. Newer motherboards virtually never require -5 volts but

many older motherboards do. Most newer power supplies don't provide -5 volts in which case the white wire is


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missing.







+3.3 volts 4 24 amps 79.2 watts



If you have an ATX power supply with a 24 pin main cable, it's okay to plug it into a motherboard with a 20 pin

connector. It was designed to work that way. You can see an example in the picture above. The extra 4 pins on

the cable just hang over the end of the motherboard connector. The 24 pin cable only fits into a 20 pin socket at

one end so you can't plug it in incorrectly. The extra 4 pins were added to the 24 pin version of the cable to

provide one extra wire for ground, 3.3, 5, and 12 volts. But it's okay to leave those 4 pins disconnected because

a motherboard with a 20 pin connector doesn't need them. The only problem you can bump into (literally) is if

there is something blocking the spot where the 24 pin cable hangs over the end. Or sometimes the end of the 20

pin motherboard connector is too thick to fit between the pins of the 24 pin cable. You can solve that problem

by carefully shaving down one end of the 20 pin motherboard connector. It's just plastic. You won't miss it. Ifyou can't get them to fit together then you can get an adapter cable which will make it work. The 24 pin cable

plugs into one end of the adapter and then the adapter plugs into the 20 pin motherboard. But you should avoid

using that kind of adapter if you can because the extra wire and connector are just more things which can go

wrong. Adapters also slightly increase the voltage drop which is something worth avoiding. It's better to first see

if you can get a 24 pin cable to fit into a 20 pin motherboard before resorting to an adapter.

ATX 20+4 pin main power cable


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Motherboards can come with either a 20 pin main power connector or a 24 pin main power connector. Many

power supplies come with a 20+4 cable which is compatible with both 20 and 24 pin motherboards. A 20+4

power cable has two pieces: a 20 pin piece, and a 4 pin piece. If you leave the two pieces separate then you can

plug the 20 pin piece into a 20 pin motherboard and leave the 4 pin piece unplugged. Be sure to leave the 4 pin

piece unplugged even if it fits into another connector. The 4 pin piece is not compatible with any other

connectors. If you plug the two pieces of a 20+4 power cable together then you have a 24 pin power cable

which can be plugged into a 24 pin motherboard.

4 pin ATX +12 volt power cable


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Older computers put most of their load on 3.3 and 5 volts. As time passed, computers drew more and more of

their load from 12 volts (see here). Before this power cable was introduced there was just one 12 volt line

provided to the motherboard. This cable added two more 12 volt lines so more of the load could be shifted to 12

volts. The power coming from this connector is usually used to power the CPU but some motherboards use it for

other things as well. The presense of this connector on a motherboard means it's an ATX12V motherboard. For

dual 12 volt rail power supplies, this connector provides the voltage refered to as 12V2. The power cable which

plugs into the 4 pin connector has two black wires and two yellow wires. This cable is sometimes called an

"ATX12V" cable or "P4" cable although neither of those are technically accurate descriptions.

If you have one of these connectors on a motherboard then you must plug a power cable into it or your CPU

won't get any power. The one exception is that when this connector was new, some motherboards shipped with a

socket into which you could plug a 4 pin peripheral power cable as an alternative. That helped people who had

older power supplies which didn't have the 4 pin 12 volt cable.

The 4 pin 12 volt cable is polarized so it can only be plugged into the 4 pin motherboard connector correctly. If

you look carefully at the picture above you can see that two of the pins are square and the other two have

rounded corners. The motherboard connectors also have the same square and rounded arrangement so the power

cable only fits in one way. At least that's true unless you try really hard to force it into the connector. With

enough force you can sometimes get a cable with a small number of pins into a connector which doesn't match.

If you look carefully you can also see that the square and rounded pattern matches various positions on other

motherboard connectors like the 20 pin main power connector and 24 pin main power connector. Do yourself a

favor and only plug the 4 pin 12 volt cable into the motherboard connector where it belongs (unless you enjoy

smoke and fried components).


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If your power supply doesn't have a 4 pin 12 V cable then you can provide one with the adapter shown above. Itconverts a 4 pin peripheral cable into a 4 pin 12 V cable.

Pinout

Pins 1, 2 Pins 3, 4


ground black 1 3 yellow +12 volts

ground black 2 4 yellow +12 volts









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You can plug a 4 pin 12 volt power cable into an 8 pin EPS motherboard connector but there's no guarantee that

it will work. If the motherboard expects only one 12 volt rail then a 4 pin 12 volt cable may work. If the

motherboard expects two 12 volt rails (many dual CPU motherboards require one 12 volt rail per CPU) then it

won't work. Even if the motherboard works with a 4 pin 12 volt cable, you are still only providing half of the

current carrying capacity which would be provided by an 8 pin EPS cable. That can overheat both the

motherboard connector and 4 pin cable. Scorched or melted connectors can be a result. A motherboard which

has the 8 pin EPS connector expects a lot of current and you are taking a serious risk by plugging in a 4 pin

cable. The 4 pin cable only fits at one end of the 8 pin EPS motherboard connector so you can't plug it in

improperly. That is, it only fits into one end of the motherboard connector unless you force it. If the cable

doesn't go into the socket easily then you're probably trying to plug it into the wrong end. But then again, it's nota good idea to try to run with a 4 pin cable in an 8 pin motherboard anyway.

8 pin EPS +12 volt power cable


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This cable was originally created for workstations to provide 12 volts to power multiple CPUs. But as time has

passed many CPUs require more 12 volt power and the 8 pin 12 volt cable is often used instead of a 4 pin 12

volt cable. Depending on the power supply, the connector may contain one 12 volt rail in all 8 pins or two 12

volt rails taking up 4 pins apiece. It is often refered to as an "EPS12V" cable.

The 8 pin 12 volt cable is polarized so it can only be plugged into the 8 pin motherboard connector correctly. If

you look carefully at the picture above you can see that four of the pins are square and the other four have

rounded corners. The motherboard connectors also have the same square and rounded arrangement so the power

cable only fits in one way. At least that's true unless you try really hard to force it into the connector. With

enough force you can sometimes get a cable with a small number of pins into a connector which doesn't match.

The 8 pin cable has enough pins that it's pretty hard to insert it in the wrong direction but determined people

might be able to do it. If you look carefully you can also see that the square and rounded pattern matches

various positions on other motherboard connectors like the 20 pin main power connector and 24 pin main power

connector. You should only plug the 8 pin 12 volt cable into the motherboard connector where it belongs unless

you enjoy the smell of fried electronics.

You can also plug an 8 pin 12 volt cable into a 4 pin 12 volt motherboard connector. I don't have a picture of this

one but it looks similar to this. Four of the pins on the 8 pin cable fit into the motherboard connector and the

other four pins hang off the end. The 8 pin cable only fits into one end of the 4 pin motherboard connector

unless you try hard to force it into the wrong position. The 8 pin cable is electrically compatible but it may not fit

into a 4 pin motherboard. There is often a component which blocks the area where the 4 pins would hang off the

end. And sometimes the plastic end of the 4 pin connector is too thick to fit between the pins of the 8 pin cable.

Make sure that you don't try to plug an 8 pin 12 volt cable into the 8 Pin PCI Express power connector on a

video card. The two cables look very similar so it's easy to get the two confused. 8 Pin PCI Express power

cables are usually labeled to distinguish them from 8 pin 12 volt cables. The PCI Express cable usually has

"PCI-E" printed on the connector. If there are no labels then you can usually use wire colors to tell the two kinds

of cables apart. An 8 pin 12 volt cable has yellow wires on the same side as the connector clip. An 8 Pin PCI

Express cable has black wires on the clip side. The two power cables are also keyed differently so you can't plug

one kind of power cable into the other kind of connector. But as with this kind of connector, you can sometimes

force the wrong kind of cable into a connector if you push hard enough. Make sure you have the right kind of

cable before plugging it in. The two are definitely not compatible with each other.


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Motherboards can come with either a 4 pin 12 volt connector or an 8 pin 12 volt connector. Many power

supplies come with a 4+4 pin 12 volt cable which is compatible with both 4 and 8 pin motherboards. A 4+4power cable has two separate 4 pin pieces. If you plug the two pieces of a 4+4 power cable together then you

have a 8 pin power cable which can be plugged into an 8 pin 12 volt connector. If you leave the two pieces

separate then you can plug one of the 4 pin pieces into a 4 pin 12 volt connector and leave the other 4 pin piece

unplugged.

If you look carefully at the image above then you can see the polarization of the pins which prevents you from

plugging the cable in improperly. Some of the pins are square and some of them have rounded off corners. The

motherboard connectors have matching square and rounded off corners to prevent the cable from being plugged

in the wrong way. But if you lookreally carefully at the right half of this particular cable and then look at the 8

pin 12 volt cable pictured above you'll notice that they don't match. A regular 8 pin cable has four square pins

and four rounded ones but the 4+4 cable shown above has two square pins and 6 rounded ones. The left half of

the 4+4 matches the left half of an 8 pin cable but the right half is different. Hmmmm... And this isn't somebizarre cable either. I've seen plenty of 4+4s which look like this one. And then there are other 4+4 cables which

look just like an 8 pin cable split in two (which makes sense). Since rounded pins fit into square holes in

motherboard connectors, this particular cable will fit just fine into an 8 pin 12 volt motherboard connector. But

both halves of this 4+4 will fit into a 4 pin 12 volt motherboard connector. You're supposed to use the left half of

the cable shown above when plugging it into a 4 pin motherboard connector but the right half will also fit. As it

happens, either half will work fine in a 4 pin motherboard because both halves of the 4+4 just provide 12 volts.

The pinouts are the same for both halves so either one will work. I'm not sure why they make cables like this one

because you'd figure a 4+4 cable would just be an 8 pin cable which splits in two. And you only need one half of

a 4+4 cable to plug into a 4 pin motherboard. The other half is unused. But the kind of 4+4 cable shown above is

pretty common so don't let it throw you.

6 pin PCI Express power cable


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This cable is used to provide extra 12 volt power to PCI Express expansion cards. PCI Express motherboard

slots can provide a maximum of 75 watts. Many video cards draw significantly more than 75 watts so the 6 pin

PCI Express power cable was created. These high-power cards draw most of their power from the 12 volt rail so

this cable provides only 12 volts. These are sometimes called "PCI Express cables". They are also occasionally

called "PEG cables" where "PEG" stands for PCI Express Graphics. If your power supply doesn't have a 6 pin

PCI Express cable then you can use the adapter shown above on the right to convert two 4 pin peripheral cables

into a PCI Express cable. If you use an adapter then be sure to plug the 4 pin peripheral connectors into separate

cables coming from the power supply. If you plug them both into the same power supply cable then you are

drawing all the power of the PCI Express connector through a single 18 gauge wire. You can usually get away

with that but there's no reason to do it. The PCI Express 6 pin connector is polarized so it can only be plugged in

pointing in the correct direction. But as with connectors of this type, you can sometimes force them into the

wrong kind of socket if you try hard enough. If it doesn't slide in easily then you're probably plugging it into the

wrong place.

Some video cards come with the 8 pin PCI Express power connector to support higher wattage than the 6 pin

PCI Express connectors. It's okay to plug a 6 pin PCI Express power cable into an 8 pin PCI Express connector.

It's designed to work that way but will be limited to the lower wattage provided by the 6 pin version of the cable.

The 6 pin cable only fits into one end of the 8 pin connector so you can't insert it incorrectly but you can

sometimes force the 6 pin cable in the wrong way if you try hard enough. Video cards can sense whether you

have plugged a 6 pin or 8 pin cable into an 8 pin connector so the video card can impose some kind of restriction

when running with only a 6 pin power cable. Some cards will refuse to run with only a 6 pin cable in an 8 pin

socket. Others will work with a 6 pin cable at normal speeds but will not allow overclocking. Check the video

card documentation to get the rules. But if you don't have any other information then just assume that if your

video card has an 8 pin connector then you must plug in an 8 pin cable.

Pinout



+12 volts yellow 1 4 black ground

+12 volts or not connected yellow or not connected 2 5 black ground


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Video card connector Cable connector Terminals Maximum current per circuit

Molex 45558-0002 Molex 45559-0002Molex 39-00-0168,




+12 volts 3 2.083 amps 75 watts

The PCI Express specification is, unfortunately, not a free, public specification. So most people have never seen

it. Including me. ATX specification: freely available to all. PCI Express specification: expensive so hardly

anyone has seen it. ATX: good. PCI Express: bad. It's a shame when a widely used standard isn't freely available

to the public. Nonetheless, information leaks out from the specification and the 6 pin PCI Express power cable is

actually rated at an extremely conservative 75 watts. I have no idea why the wattage is rated so low because the

specifications from Molex clearly allow substantially more power. Part of the reason may be that pin 2 (listed

above as a 12 volt line) may be listed as not connected in the specification. I've never seen a 6 pin PCI Express

power cable with pin 2 not connected. They've all had a 12 volt line connected to pin 2. I've also seen claims

that there may be unimplemented sense lines in the specification. Welcome to the uncertainty which happenswhen you don't have freely available specifications. Even with only two 12 volt lines the standard

implementation of PCI Express power cables use large enough gauge wire and a good enough connector to

provide much more than the three amps per wire required to provide 75 watts. Nonetheless, the 6 pin PCI

Express power cable officially provides only 75 watts. In all likelihood, however, real implementations of this

power cable can provide far more than 75 watts.

8 pin PCI Express power cable

The PCI Express 2.0 specification released in January 2007 added an 8 pin PCI Express power cable. It's just an

8 pin version of the 6 Pin PCI Express power cable. Both are primarily used to provide supplemental power to

video cards. The older 6 pin version officially provides a maximum of 75 watts (although unofficially it can


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usually provide much more) whereas the new 8 pin version provides a maximum of 150 watts. It is very easy to

confuse the 8 pin version with the very similar-looking EPS 8 pin 12 volt cable.

The 8 pin PCI Express and the EPS 8 pin 12 volt connectors are polarized differently so you won't be able to

plug one kind of cable into the other kind of connector. That is, you won't be able to plug the wrong kind of

cable in unless you try really hard. Unfortunately, the Molex Mini-fit Jr. connectors used by both kinds of power

cables can sometimes be forced into a differently-polarized connector if they only have a few pins and you push

hard enough. If the cable won't slide in easily then you're probably trying to insert the wrong kind of cable. The

8 pin PCI Express connector does have a small plastic bridge which prevents it from being plugged into an EPS

8 pin 12 volt motherboard connector. You can see the bridge in the image above between the rightmost two pinsin the top row of the connector. But there's no such protection to prevent EPS 8 pin 12 volt cables from being

plugged into an 8 pin PCI Express connector on a video card. That combination may fit if you shove hard

enough. And if you plug in the wrong kind of cable then expect fireworks. Some of the grounds and 12 volts

wires for an EPS 8 pin 12 volt are reversed compared to an 8 pin PCI Express. Fortunately, most 8 pin PCI

Express connectors are labeled "PCI-E" so people won't confuse them with EPS 8 pin 12 volt cables. If the

connectors aren't labeled then you can tell an 8 pin PCI Express power cable from an EPS 8 pin 12 volt cable by

checking the color of the wires which plug into the clip side of the connector. On the EPS 8 pin cable, the yellow

wires (the 12 volt wires) go into the clip side of the connector. On the 8 pin PCI Express cable, the wires on the

clip side are all black (grounds). That's the same as it is with the 6 Pin PCI Express power cable. Of course, none

of this helps you if your cable uses the trendy all-the-same-color-wires design which is popular with high-fashion

power supplies. In that case you'll just have to be very careful or hope the connectors are labeled.

Pinout







Connector part numbersVideo card connector Cable connector Terminals Maximum current per circuit

? ? ? ?



+12 volts 3 4.167 150 watts

6+2 pin PCI Express power cable


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Some video cards have 6 Pin PCI Express power connectors and others have 8 Pin PCI Express power

connectors. Many power supplies come with a 6+2 PCI Express power cable which is compatible with bothkinds of video cards. The 6+2 PCI Express power cable is made up of two pieces: a 6 pin piece, and a 2 pin

piece. If you put the two pieces together then you have a full 8 pin PCI Express power cable. But if you split the

connector into two parts then you can plug the 6 pin part into the older 6 pin PCI Express connector and leave

the 2 pin part unplugged. That way, your power supply only needs to have one 6+2 cable to be compatible with

both 6 pin and 8 pin PCI Express connectors.

Video card information

How to select a video card

How to install a video card

How to troubleshoot video card problems

The big fat table of video cards

AGP compatibility for sticklers

DVI compatibility for sticklers

Troubleshooting AGP

Troubleshoot your video card by underclocking

Diagnose your video card problems by comparing with example corrupted screens

Troubleshooting DVI problems

The basics of 3D graphics without making your eyes glaze over - just enough to get you started

How to uninstall your current display drivers

How to install display drivers for your video card

Understanding video RAM memory bandwidth

What kind of expansion slot should you use for your video card?VGA video card outputs

Useful technical information

How to install your motherboard chipset drivers

Test your motherboard memory with Memtest86

Torture test your CPU with Prime95

How to find specifications for your computer

Create a system restore point in case something goes wrong

Getting administrator privileges


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Fiddling with your BIOS

How to boot into safe mode or VGA mode

Rate your CPU speed at games

Glossary of technical terms

Random stuff

My favorite RollerCoaster Tycoon 3 coasters

PeepFactory peep generator for RollerCoaster Tycoon 3The view from my old backyard

Copyright 2005 - 2008 by Mark Allen


all about the various pc power supply cables and connectors

Documents