Question How many Noctua NF-P12 redux 1700 PWM can i use in 1 splitter ?

[finalygotit]

Hi,
I'm struggling with the ( only) 4 fan headers the B550 m plus provides and would like to use a cable splitter ( not a hub ) on 1 header.
I've been on a few forums and the answer is always that you can put 2 , and maybe 3 fans on 1 header.
But according to the technical specs of the Noctua NF-P12 redux 1700 fans and the ASUS mobo i find that you can easily put more than 5 and stay way lower than the max amps allowed.
Could someone ( who is a bit more into tech than i am confirm that i can use at least 5 NF-P12 redux 1700 PWM on just 1 header)
The Asus manual states that each header is good for 1 Amp and the NF-P12 fan is rated for max 1.08 W ( x 5 = 5.4 W in total ) .
On a 12 volt board that would mean only 0.45 A .
Which would mean a 55 % margin when used at full RPM .

So where does the usually accepted maximum of 2 or 3 fans come from , has it got anything to do whit the section of the splitter cable ?
Are the fan specs (max current & power) not taking in account the full spin when the system is powered on ?
What am i missing ?

 

[Phaaze88]

Input current.
https://noctua.at/en/nf-p12-redux-1700-pwm/specification
Max. input current: 0.09A
With a motherboard header rated at 1A, that means you could theoretically chain 11(0.99A) of these fans to a single header without risk of burnout.

Take this other fan as an example: https://noctua.at/en/nf-f12-industrialppc-3000-pwm/specification
Max. input current: 0.3 A
3 of these is the most that can be safely connected to a 1A header.

https://noctua.at/en/nf-a14-industrialppc-3000-pwm/specification
Max. input current: 0.55 A
... and just one of these per 1A header.

 

[Paperdoc]

OP, you are exactly right. So why those common recommendations? Two reasons. The first is that quite some time ago the commonly-available fans consumed at max 0.25 A, sometimes more. IF you did NOT understand and use the current specs properly, using more that 3 MIGHT overload the header. So this was a cautious "rule" anyone could use without caring about details. Second reason is as Phaaze88 above shows. SOME of today's fans (the Noctua iPPC line of high-performance units is a good example) do have max current use over 0.25 A, so you MUST take that into account. Again, for the people who don't do this, a "rule" a 3 max is safe. Even 2 max would apply for fans that can use 0.35 A or more.

There used to be concern expressed for the brief surge current at fan start-up, which can be twice the typical max running current. But that is already taken into account in these specs. The key thing here is heat generation in the output stage transistor of the header chip. Sustained high current will cause it to overheat and damage it. But brief high current lasting less than 1 sec will not cause its temperature to rise that high.