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Even the humblest of components, such as the LEDs that light up when a disk is active, will need to be plugged in somewhere on your computer’s motherboard or into its Power Supply Unit (PSU) to work correctly. It can be a little confusing to figure out where the different connectors go. For instance, what to do about the two cables on a PC fan?

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How PC Fans With Two Cables Work

PC fans with two cables allow you to plug your fan into either your computer’s motherboard or the PSU. The extra cable is useful because motherboards have a limited number of fan slots. Once they’re all occupied, you can use the PSU to power additional fans.

However, if you choose to plug a fan cable into your system’s PSU, you won’t tap into all its capabilities. Also, you should never plug in both cables at the same time as this can short-circuit the fan header and potentially damage other components, too.

What Are the Two Cables on a PC Fan?

The cables on a PC fan provide the (relatively minor amounts of) power it needs to run smoothly. Depending on the fan and connection, they may also allow control over RPM and relay information about the fan’s status to your software.

Many fans come with two separate cables, with different types of connectors. And they have slightly different forms and capabilities.

The female (hollow receptacle) Molex connector (3-pin or 4-pin) connects to the fan headers on the motherboard of your computer. On the other hand, the male (protruding pin) Molex (2-pin) cable connects to your machine’s PSU.

The next few sections will outline the specific functions each cable serves and explain which one you should use to power your fans.

Motherboard Fan Connectors Explained

The 3 or 4-pin connector from your PC fan goes into the 4-pin fan header on your computer’s motherboard. These are usually labeled “SYSFAN” or “CPUFAN,” depending on whether they power case or CPU fans.

Technically there\s no difference between the two. The distinct labels are a way to keep track of multiple connections in a crowded cabinet space more than anything else.

3 and 4-pin cables are also interchangeable. That is, both 3-pin cables and 4-pin cables can go into the motherboard’s 4-pin fan slots without any adverse consequences.

Each pin on a cable corresponds to a separate wire and a separate function. The coloring of the wires conveys this information.

  • Black wires: provide grounding at 0 V
  • Yellow wires: supply power at 12 V
  • Green wires: take a tachometer reading (of the speed at which the fan is turning)
  • Blue wires: enable Pulse Width Modulation (PWM)

The PWM function allows you to control the speed of a fan by rapidly cycling its power supply between on and off states.

For instance, if you instruct your software to run fans at 50% speed, then the motherboard will only supply the fan with power 50% of the time. However, it will cycle on and off so frequently and smoothly that the fan will appear to spin at half speed.

Fans with only 3-pin cables aren’t PWM capable, so you can’t control the fan speed using this protocol. However, this doesn’t mean they offer no fan speed regulation capability.

On 3-pin fans, the speed is controlled by modulating the amount of power supplied in volts. When you set a fan’s speed lower, the motherboard reduces the amount of power supplied to the fan, causing it to spin more slowly.

That said, there are limits to how low the power can be, placing a lower limit on the fine-tuning capability of a 3-pin cable-powered fan. That’s why 4-pin cables are preferred for the most advanced fan speed control capabilities they offer.

The Fan-PSU Cable

The larger (and older style) male Molex cables on a PC fan only provide power and grounding. They don’t enable tachometer readings or fan speed control. So, the fans will run at 100 percent speed all the time.

Because each fan uses a 2-pin connector, you can connect three of them to a typical 6-pin slot on a PSU at one time. The minimal power draw of PC fans allows you to run more fans off your motherboard.

Which Cables Should You Use To Connect a PC Fan?

If you have free slots, you should plug your fan into one of the fan headers on your motherboard. As mentioned earlier, the 3 and 4-pin fan headers on a computer’s motherboard enable critical functionalities that aren’t available to a PSU-powered fan.

You can adjust the speed of a motherboard-powered fan using PWM protocols or by varying the power supplied. In contrast, PSU-powered fans don’t have tachometer readings and PWM functions enabled. As a result, they always run at 100 percent speed.

Running a fan at full speed all the time consumes more power, produces more noise, and may not create the best air circulation within your computer cabinet.

Depending on their placement, the fans inside your computer need to have different speed settings to optimize air circulation and cool your machine more efficiently.

Running fans at full speed also unnecessarily taxes them and can shorten their lifetime.

What Is the Purpose of Having Two Cables on PC Fans?

Since I’ve recommended always plugging your fans into a header on your motherboard, you may be wondering why a second cable exists in the first place.

The PSU cable on your fan is a holdout from an earlier era when fans lacked their current functions, such as speed control and fancy lighting. Because they only needed power and grounding, they didn’t need to be plugged into a motherboard.

But why do fans today retain this cable?

Well, the reason most fans have two cables is the limited number of fan headers on motherboards. While more expensive boards can have six or more fan headers, many budget options only come with two.

Considering many people use 3-4 fans or more, this leaves them with a limited power supply directly from the motherboard.

This is where the PSU cable on your fan comes in. Once you’ve used all the fan headers on your motherboard, you may have no other option than to plug any remaining fans into the PSU. This way, you can run more fans, even if the additional fans have limited functionality.

Note that most newer power supplies come with SATA sockets and may have limited or no Molex connections left once you’ve connected other critical components to your computer. In this case, you may have to use an adapter to connect your fans to the PSU in this case.

Enabling PWM on PSU-Powered Fans

If you have no remaining free motherboard fan headers and would still like to use your fan’s PWM capability, you can use a PWM hub to connect multiple PWM-capable fans to your computer’s PSU.

This device lets you enjoy similar functionalities as if the fans were connected to your motherboard headers. However, you won’t have control over the speeds of individual fans this way. Instead, a universal speed setting will be applied to all the fans connected via a single hub.

Also note that, depending on your hub’s features, you might not get support for RGB lighting. So, if you’re particular about your fan’s extra features, stick to the motherboard fan header.

If you’re looking for a PWM hub, check out the Deep Cool FH-10 Integrated Fan Hub on Amazon.com. It can connect up to 10 fans via SATA, and comes with velcro and screws for proper mounting.

Why You Might Need So Many Fans

Now that we’ve looked at the various fan connectors and understood why they differ, let us consider the question of why you might need so many fans to begin with.

More Powerful Computers Generate More Heat

On average, systems with higher-end components pushed further more consistently will need more sophisticated cooling apparatuses than modest builds used on light computing tasks. Intensive processing generates excess heat that can permanently damage delicate electronic components.

The more the number of high-end components in a machine, the likelier it is to generate more heat. Many gaming computers and workstations will have high-end processors and graphics cards (sometimes more than one). Each of these will come with its own fan or need you to install one to keep it cool.

More powerful computers are also likely to be installed in larger cases. Inevitably, they will produce more heat and require more fans to push the hot air outside the case.

Fans Create Unidirectional Airflows

The direction a fan faces affects the direction in which it moves air. So a fan fixed in one orientation can only push air away from a case or component or pull air toward them; it cannot do both. For effective cooling, you will need both intake fans and exhaust fans.

Cooling Requires Exchanging Air Inside a PC Case For Outside Air

One set of fans will draw cool air into your computer’s case. Another set will push hot air outside. Together, the two sets of fans will create an airflow pattern within your case that exchanges the hot air generated inside it for cooler air from outside.

This circulatory airflow pattern is essential to keeping your components cool as they persistently generate heat and do not remain cool unless continuously cooled.

Final Thoughts

PC fans have two cables to allow users maximum flexibility on the number of fans they want to install and how to power them.

Since many budget motherboards only have two fan headers, you can power additional fans using the cable that connects to your PSU.