Having trouble with calculating amperage. Different Answers.

[wnetwork]

I'm trying to know how to calculate amperage so I can find the right power supply. Is any of them correct?

Example: Builder Series™ CX430 — 430W Power Supply

Specs given on the page:

Voltage +3.3V +5V +12V -12V +5Vsb
Max. Current 20A 20A 28A 0.8A 3A
Max Combined Wattage 120W 336W 9.6W 15W

Some says:
1) 28A (+12V)
2) 336W (+12V)-120W=216W (+3.3V+5V) /12V (rails)=18A
3) 430W/12V= 35.83A (suggested from here )
4) Some also say 12V1+12V2=??A (for psu with multiples V rails)


On many recent threads here and elsewhere, users (even experienced ones) give different answers/methods to calculate psu amperage.
Some say its right, some say its wrong.

 

[Rugger]

Amperage = Watts / Volts
or
Watts = Volts * Amps
or
Volts = Watts / Amps (not so useful for us since we know the voltage for all three PSU rails: 3.3V, 5V, and 12V.)

The easiest way to tell amperage in your example is:
1. Follow the link for the CX430 on the Corsair site.
2. Click "Tech Specs" on the left hand bar
3. Scroll Down to the specs box that lists voltage, watts, and amperage for each rail on the PSU. The 12V rail can provide up to 28 amps which is equal to 336W.

Edit: When determining PSU requirements, you have to separate the requirements by voltage rails. The vast majority of modern, good quality PSUs have the majority of capacity on the 12V because the two largest power consumers in a modern PC (the CPU and GPU) get their power from the 12V Rail.

 

[wnetwork]

3. Scroll Down to the specs box that lists voltage, watts, and amperage for each rail on the PSU. The 12V rail can provide up to 28 amps which is equal to 336W.

Right. However some users say it might not be the actual amperage and then mention ways to calculate to get the *true* amperage.

 

[ko888]

Problems arise when the label lies and the power supply can't even deliver the power claimed on its label so no matter what calculations you do it doesn't mean anything.

Your example power supply CMPSU-430CX isn't made anymore. It has been replaced by the CMPSU-430CXV2.

It will deliver 28 Amps on the +12V rail if the +3.3V and +5V rails are not fully loaded to its combined 120W maximum.

If the +3.3V and +5V rails are loaded to their combined 120W maximum then the +12V rail will only be able to provide up to 24 Amps maximum.

 

[wnetwork]

Best answer selected by wnetwork.