The common rule was 120MM for each component.  That was before we ended up with 300W devices.  I would suggest a triple radiator of 120mm or 140mm if youre doing cpu and gpu at a minimum.  So a triple in any form should be enough for your GPU.  This coming from the guy who runs a mora3 420 for his current 3950x and plans to include a 3080ti.  

spiffy_1
The common rule was 120MM for each component.  That was before we ended up with 300W devices.  I would suggest a triple radiator of 120mm or 140mm if youre doing cpu and gpu at a minimum.  So a triple in any form should be enough for your GPU.  This coming from the guy who runs a mora3 420 for his current 3950x and plans to include a 3080ti.  

Thank you, very helpful.

spiffy_1
The common rule was 120MM for each component.  That was before we ended up with 300W devices.  I would suggest a triple radiator of 120mm or 140mm if youre doing cpu and gpu at a minimum.  So a triple in any form should be enough for your GPU.  This coming from the guy who runs a mora3 420 for his current 3950x and plans to include a 3080ti.  

I agree, A 360mm radiator probably is enough for a 3090 Kingpin with the 520 watt BIOS.  Most slim 360mm radiators can effectively dissipate about 500 watts of heat with good fans.  Some of the thicker 360mm radiators are effective for 600-800+ watts of heat.

talon951
This really depends  a lot on what you consider to be acceptable temps.  If you are ok with core temps in the 50's, then a single 360mm will work if you are ok with some fan noise.  If you're shooting for temps in the 40's and minimal fan noise, then you'll either need a very large single rad (probably almost have to be external), or multiple rads like two thick 360's or more.  

 
My Kingpin Hybrid ran in the 40s in most benchmarks and benchmark loops with a mild overclock with the stock 360mm radiator hybrid cooling.  It didn't hit the 51C until it was overclocked on the LN2 BIOS.  That was with the auto fan speed and not 100% fan speed.
 

sunt88126
I'm new to custom loop and plan to get a Radiator for the 3090 KHC, not sure if hardware labs GTR/GTX 420mm enough for the 3090 GPU only?
I read some thread said hardware labs GTS is good, not sure for the thicker one, which brand is the best?
My current pump-reservoir choice is EK-Quantum Kinetic TBE 300 D5 PWM D-RGB, is it good?
Could you share your experience? thanks in advance!
I already have AIO for CPU, will make a custom loop for GPU only. 
 

 
As talon951 points out it's about Temps and noise. The more radiator surface area you have the lower your fan speed will need to be achieve the same water temps. This raises the point of fan selection too, make sure your fans are designed for high static pressure, not all fans are optimized for moving air through a radiator, which will cause increased air resistance. 
 
You'll find numerous comparisons for radiators. You'll want to look at how restrictive a radiator is along with how efficient it is in cooling. The Hardware labs are a good balance, but i've read they are not completely standard size so you'll need to check mounting points and width to ensure they fit in your case. Be sure to measure rad thickness and fan thickness when checking if you have sufficient clearance, especially if mounting in the top of the case. If you can fit a 45mm radiator, do so over a 30mm, you'll be thankful you did at some future point.
 
You might also want to consider replacing your  current AIO and including the CPU in the loop, depending on your requirements and CPU. A high end cpu (say 11900k) will not be pushed hard in games, this will effectively mean you'll have additional radiator capacity vs a single radiator for cpu and gpu setup. A D5 pump should be adequate for 2 radiators/cpu/gpu, but again check resistance of the components you select and  design your loop with the least possible 90 degree bends.  I  doubt you'll find resistance values for the HC water block, but looking at it, it doesn't  appear to have a jet plate design, so i would expect the flow resistance to be quite low.

mdb983

sunt88126
I'm new to custom loop and plan to get a Radiator for the 3090 KHC, not sure if hardware labs GTR/GTX 420mm enough for the 3090 GPU only?
I read some thread said hardware labs GTS is good, not sure for the thicker one, which brand is the best?
My current pump-reservoir choice is EK-Quantum Kinetic TBE 300 D5 PWM D-RGB, is it good?
Could you share your experience? thanks in advance!
I already have AIO for CPU, will make a custom loop for GPU only. 
 

 
As talon951 points out it's about Temps and noise. The more radiator surface area you have the lower your fan speed will need to be achieve the same water temps. This raises the point of fan selection too, make sure your fans are designed for high static pressure, not all fans are optimized for moving air through a radiator, which will cause increased air resistance. 
 
You'll find numerous comparisons for radiators. You'll want to look at how restrictive a radiator is along with how efficient it is in cooling. The Hardware labs are a good balance, but i've read they are not completely standard size so you'll need to check mounting points and width to ensure they fit in your case. Be sure to measure rad thickness and fan thickness when checking if you have sufficient clearance, especially if mounting in the top of the case. If you can fit a 45mm radiator, do so over a 30mm, you'll be thankful you did at some future point.
 
You might also want to consider replacing your  current AIO and including the CPU in the loop, depending on your requirements and CPU. A high end cpu (say 11900k) will not be pushed hard in games, this will effectively mean you'll have additional radiator capacity vs a single radiator for cpu and gpu setup. A D5 pump should be adequate for 2 radiators/cpu/gpu, but again check resistance of the components you select and  design your loop with the least possible 90 degree bends.  I  doubt you'll find resistance values for the HC water block, but looking at it, it doesn't  appear to have a jet plate design, so i would expect the flow resistance to be quite low.

Thickness on radiators is not everything. A HWL GTS will outperform many thicker radiators. When picking radiator sizes you want to first decide how fast you want to run your fans and then pick the best radiator rated for that fan speeds. Going push/pull also plays a difference. Be counter productive to get a 60mm over a GTS if not going push/pull and running low fan speeds. 
 

I use the same pump/res myself and very happy with it.