# Will 400W power supply handle 4770? 12v@18a x2

#### lsc04361

##### Distinguished
I know that questions have been asked alike.

People always saying better to have 26 amps on the 12V rail.

Quote:
[The HD4770 draws a maximum of 80 watts. 80 watts / 12 volts = 6.7 Amps. If the rest of your system is typical (a 100-watt CPU, 1HDD, 1ODD, mobo + ram), they will pull another 13 amps or so on the +12V under full load. So look at your PSU's label and see if it can deliver at least 20 Amps on the +12V. If it can, it should work.]

But what if i got 2 12v rails? 2 and each supply max 18a. Can i took it as 32a?

CPU: Q6600 95W
RAM: 2GB
1 hd and dvd drive
Motherboard: MSI p45 platinum

Power supply: SPI ATX 400-PN
Details:
+3.3V 0.5A 10.0A 28.0A
+5V 0.3A 7.0A 30.0A
+12V1 1.0A 7.0A 18.0A
+12V2 1.0A 8.5A 18.0A
-12V 0.0A 0.4A 0.5A
+5Vsb 0.0A 1.25A 2.5A
*+3.3V and +5V total output not exceed 180W
*+12V1 and +12V2 total output not exceed 348W
*+3.3V & +5V & +12V1 & +12V2 total output not exceed 385W
* Total output for this subject power supply is 400W

Efficiency: 70% minimum measured at normal AC mains
voltage and frequency with full load

Sorry for the rough data presentation.

#### 4745454b

##### Titan
Moderator
Bring up the calculator program or pick up a calculator. Lets do some math. You obviously can't add the two 18A together to equal 36A. If you did, you would come up with (36A * 12V) 432W just on the 12V rail, which is higher then the 400W the unit is rated at. This is a perfect example of why I tell people you can't add the two numbers together. Each rail INDIVIDUALLY can run 18A, but the source providing the power can't do both at the same time.

So how much power is avaiable? "*+12V1 and +12V2 total output not exceed 348W" 384W / 12V = 32A. This means your PSU can output 32A between both rails.

I'm not convinced of this number btw. Thats seems very high for a 400W PSU, more so if its only 70% efficient. You might want to look for a review of your PSU online somewhere and see if its any good.

#### szymek

##### Distinguished
It will be ok. Believe me. If 3870 can work on mine 350wat having 14amp on one 12v rail and 16 on the second one with e4700@3.46ghz then your 4770 will be really ok.

#### lsc04361

##### Distinguished

Really thanks for the calculation. I start to understand what's happening.
Is it just like, the power supply provide 348W for 12V rails, and there are 2 rails carrying it. For each rail, the max amps it can suffer is 18a, but the actual max amps can be provided is 32A and running throw 2 rails?