Do I need both the 8 pin and 4 pin cpu power plug for my MSI B450 Pro Carbon?

Solution 1:

From experience, it suffices to install the 8 pin connector on the left.

The right 4-pin-connector is basically a redundancy that the CPU may draw upon under extreme load most likely only relevant when you do extreme overclocking.

(The board manufacturers most likely also build in some more extra room in case the next CPU generation needs more power.)

So only connect the 2 ports (CPU_PWR_1) on the left, and leave the right one (CPU_PWR_2) empty. If your system won't boot or doesn't run stable, only then you should consider buying the extra cable or upgrading your PSU.

Solution 2:

I’ve bought a Ryzen 3700X (65 watts TDP), a MSI X570-A PRO (with also both connectors) and a Seasonic FOCUS Plus Gold 550 as PSU. As you, I discovered the two CPU connectors on the motherboard and that the PSU only had a single CPU 8 pin (2x4) connector.

I tried with that single connector and the system (Windows 10) froze after several minutes of normal use (CPU was not under stress). My system also hosts a Geforce 1070 and 3 SATA SSDs. It draws around 100~150 watt of power without screen, far from PSU 550 watts maximum power.

I’ve replaced the PSU with my 10 year’s old Corsair HX520 (which had both connectors) and the system is perfectly stable since.

So, at least for Ryzen 3000, I think you need to connect both CPU connectors to avoid Windows 10 freeze.

Also, the MSI x570 A PRO manual (page 32) explicitly says « Important : Make sure that all the power cables are securely connected to a proper ATX power supply to ensure stable operation of the motherboard » on the Power Connectors page

Solution 3:

To provide another example per paulgreg's answer, I am currently running a Ryzen 7 3700X with an MSI X570 Gaming PLUS. I just installed everything this morning and have been running benchmarks and stress tests all afternoon. My system is running perfectly stable with only the 8-pin CPU connector.

At this point, I'm inclined to believe everyone that the extra 4-pin connector is only there for redundancy in heavy overclocking.