CPU FAN vs. CPU OPT (When to use which)

CG Director Author Alex Glawionby Alex Glawion   /  Updated 

Wondering what the difference between all those fan headers on your motherboard is? Wonder no longer.

Imagine this.

You’re building your PC, You have your CPU, your cooler, and the motherboard in your PC-Case.

All you need to do now is to plug that little connector from your CPU Cooler into the Motherboard. You take the wire and go to plug it in.

And then you see this:

Motherboard CPU OPT and CPU FAN Headers

Motherboard CPU_OPT and CPU_FAN Headers

Which one do you choose? CPU_FAN or CPU_OPT?

What is CPU FAN (CPU_FAN)?

CPU_FAN, standing for “CPU Fan”, is the main header used by the motherboard, and the BIOS and any other software (SpeedFan, HWMonitor, NZXT CAM, etc), to regulate and control the CPU’s thermals depending on the load by controlling the fan(s) speed (if you’re using some sort of splitter).

It is critical that you make sure to plug your CPU cooler fan into this header as most motherboards worth the name would not allow you to boot the system otherwise.

Also, if the system is already running somehow, it will safely shut it down if it detects that the CPU has no cooler or that the fans are defective and/or not spinning (when they are supposed to).

What is CPU OPT (CPU_OPT)?

CPU_OPT, standing for “CPU Optional”, is mostly used when you have more than one fan on your cooler/radiator and you need someplace to plug it in.

It’s basically CPU_FAN without the “not booting if nothing is connected to it” part.

It can also be used by some AIO/liquid cooling solutions as a power source for things like water-cooling pumps and other things that might require power but is not important or power-hungry enough to reroute power directly from the power supply.


System Fan Header on Motherboard

System Fan Header on Motherboard

And while we’re at it, we might as well address these as well.

I’m sure you’ve already figured it out from the name. These are the headers you connect your case fans to and are meant for differentiating fan-control.

They’re like CPU_OPT, but not connected to the thermals of the CPU. So you’re free to tinker with them and increase or decrease them according to your GPU thermals, CPU thermals, Case thermals, or whatever you wish.

And there are a wide variety of such headers on modern motherboards—mostly focused on helping water cooling. Let’s rejoice that the days of plugging in 20 different things to one or two headers are thankfully over.

In Summary

Use cases for a fan hub

If you want to connect fans from your CPU cooler or radiator, use the CPU_FAN header (With a splitter/hub if you have a lot of fans).

If you only have a two fan cooler, for example, you can just use CPU_FAN for one fan and CPU_OPT for the other. 

If you have a radiator with a large number of fans or just a large number of fans in general, you can look into getting a fan splitter/fan hub to accommodate them.

For example, you can use something like that to plug all three fans (or six if you have fans on both sides) of a 360mm radiator to the CPU_FAN header.

Or you can just get a Y splitter (two female connectors, one male) and plug one of the fans into CPU_FAN and the other two into the splitter, and then that into CPU_OPT.

You can do the same thing with your case fans as well. Case fan into hub/splitter, hub/splitter into case fan header.


Can I plug a fan hub into CPU_OPT or CPU_FAN?

Yes. But I’d recommend you use CPU_OPT or a case fan header and keep CPU_FAN just for the CPU.

And make sure that you’re not powering a lot of fans from one header. Headers can run hot when there’s a lot of power going through them.

Get one that takes in external power from the power supply instead.

Where do you plug in PWM fans?

If you want to use any of the PWM functionality, you need to plug the 4-pin PWM connector into another 4-pin header or else you will lose that functionality.

Automatic PWM Fan Control

Can you plug a 4-pin fan into a 3-pin? And vice versa?

Yes. 4-pin fans can also be connected to your motherboard’s 3-pin fan headers just fine. But you have to keep in mind that when connected to a 3-pin fan header, the fan will run at full speed unless your motherboard has voltage-based speed control.

What is the difference between DC and PWM fans, and which is better?

It depends. DC is old and has been used since the times of the dinosaurs. It has a 3-pin connector and it works well, but if you want more control over how fast your fan spins, you want PWM.

PWM allows you to have finer control over your speeds and allows you to reliably go to lower speeds.

Mostly any fan worth its weight in salt comes with a 4-pin PWM connector these days.

Over to you

That’s about it from us! Need help with assembling your PC? Let us know in the comments or our Forum.

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Alex Glawion - post author

Hi, I’m Alex, a Freelance 3D Generalist, Motion Designer and Compositor.

I’ve built a multitude of Computers, Workstations and Renderfarms and love to optimize them as much as possible.

Feel free to comment and ask for suggestions on your PC-Build or 3D-related Problem, I’ll do my best to help out!


Also check out our Forum for feedback from our Expert Community.

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  • M kris

    Hi Alex i am Kris here from an architect firm we use rendering systems here just want know which is the best processor with more core and threads