PSU - ampere?

I have absolutely no clue when it comes to PSUs. I know what watt represents but that's about it. :pfff:
Anyway, I am about to buy/upgrade a/the computer and I was wondering if the PSU will be compatible. If my memory doesn't fail me, I did have some troubles with the 12v rail the last time.

INTEL Core i5 "Lynnfield" 750, 4x 2.66GHz, Socket 1156
MSI R4890 Cyclone, Radeon HD 4890, 512MB GDDR5
CORSAIR, 2x 2.0GB, DDR3-DIMM, PC3-12800 (1600MHz), CL7-7-7-20
LITEON iHAS324, DVD±RW, Retail

All powered by
CORSAIR VX450, 450 Watts

It has a single +12V rail with a maximum of 33A.
Now comes the part where I don't know what to do with this information. :D
13 answers Last reply
More about ampere
  1. If you are asking if the VX450 will run that system it should be fine.
  2. It will (JUST) but (is that two 4890's there ?? jeez Louise!! ).

    I would take a look at the new 650 to 850 Watt units - there was a recent PSU shootout at THG so take a look.

    A 450 Watt unit is running too close to the wire under load at present ... especially if you plan to overclock and chuck anything else into the box.

    Excellent case - regularly clean the front dust filter so it continues to allow cool air to get into the case.

    Good choice of components - -well done.

  3. Just 1 4890 I think......if its 2 I would get the 750TX.
  4. Yes, it's only one 4890 and I don't think I will add another in the near future. Though, if I get the 750W would it use more power than the 450W with the same system? Just recently I've read that it wouldn't but I was always under the impression it would.

    Anyway, thank you and thanks for the compliment reynod!
  5. Weird, can't edit my post anymore. Got a last question:

    The PSU has to deliver enough W and A but how do I figure out how much A my system needs?
  6. I am not sure of an exact way to calculate the amperage a system requires. For my peace of mind, I'll use an online wattage calculator (this one works well: to see the maximum wattage draw of my system. I will then match the wattage needed with a name brand PSU (Corsair, Antec, PC Power & Cooling) that is rated for such wattage. Name brand PSU's generally have high amperage ratings, and as a result they easily suffice the amperage required of the build.

    In my opinion, I think the brand of your selected PSU is great. But, I think it's wattage is cutting it too close for comfort. I would recommend getting a little stronger PSU.
  7. That's the calculator I used and I think it gave me something around 360W. Ah well, will do it again, hang on.
    - 346W Capacitor Aging 20%, TDP 85%, System Load 90%
    - 432W TDP, System Load and Capacitor Aging at maximum

    But if a stronger PSU doesn't consume more watt than my system needs I wouldn't mind buying one.
  8. That's correct, your PSU will only draw the power needed by your system.
  9. Alright, didn't know that.
    750 then, thanks!
  10. Watts = Amps x Volts or Amps = Watts/Volts

    Your PSU with 33 amps on 12v rails can put out 33 x 12 = 396 watts on those 12v rails. Note that the max stated for the 12v rails assumes certain limits are not exceeded on the 3v and 5v rails. Or put another way, if you look at what the PSU says it can put out each on the 3, 5 nad 12v rails independently, and add the wattages up they will usually exceed the total capacity of the PSU - so you can't be running all the legs at maximum at the same time. Some PSUs show, on the label, the maximum watts that can be delivered for a combination of legs - such as 12v and 5v -depending on how the PSU is configured.

    Total system requirements are calculated in watts not amps because it is the common denominator for amps running with different voltages. You can’t speak of amps for the total system except in its components – eg 28 amps at 12v, 15 amps at 5v, etc.

    It is helpful to have an idea of your power requirements for each power leg. The “lite” calculator linked above only gives total system wattage. The same site has a tool that calculates each leg for a nominal charge – starting at $1.99 for 3 days access to $9.99 for lifetime access. I used it when I build my computer – but the longest time they offered it for then was one year and my subscription has expired. With this tool you can easily compare the power requirements for each of the legs with what your PSU provides – and model what you might need for later upgrades. Crossfire and overclocking can both have a major impact on requirements.

    Using a larger PSU, does not in itself increase the power used by the system, and in some cased may marginally decrease it. The power used at the outlet is a function of your system requirements and the efficiency of the PSU. If you look at PSU reviews, they often show the power curves - how efficient it is over the power range of the PSU based on watts consumed by the system. The sweet spot is generally at 50% to 70% of capacity. The page on the following link shows the curve for the VX450.

    Note that if you are using it at 80% capacity (or 360w) it drops to 84% efficiency (from 85% peak) and at 90% (414w) to a little over 82%. Not too bad. However you lose a little more because the reduced efficiency means the power is lost in the form of heat, maybe making your fans run a little faster and using a small amount of additional power. While the VX450 maintains efficiency better than most, it is still something to be considered.

    The bigger issue may be shown on the curve above it – noise level. Above 400w the decibel level (fan noise) starts taking off – so I would hate to operate it there.
  11. 1898 said:
    Alright, didn't know that.
    750 then, thanks!

    Keep in mind it is not 450w or 750w only - you can get 500w, 550w, 700w, and even some others. 750w might be a safe way to go if you plan to go Crossfire, or want to leave that option open, but otherwise that much is not needed. Run the calculator - then figure you don't want to be at the top end. Although I beleive the calculator includes some reserve to be safe - but the amount is unknown. Also when you think you have the PSU identified, google for reviews to look at the power curve, noise curve, and other comments. It took me less than a minute to google "corsair VX450 reviews" and find the one I linked above.
  12. Thank you, great explanation. It's a bit clearer now.. I think. :D
    Yea, the 750W is an investment for the future. I guess it would be too much for CF alone but rather for my next system. And believe me, I've read tons of reviews that's why I chose Corsair! One of the main reasons was the noise level at idle.
  13. The Corsair is the Cadillac - along with PC Power & Cooling but they don't offer as quiet a ride.
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