PSU load levels?

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Is 272w too much continuous draw from a 300W power supply?

Would 272w be too much draw from a 350w PSU?

What percentage load is 'best' for a PC PSU?

Intel Core 2 e6400
motherboard - asrock (4coredual)
2 sticks DDR2
2 sata hard drives
1 DVD writer (rarely used)
hd 4830
PCI Sata card
PCI FireWire card
USB - mouse and keyboard

No case fans at the moment - running with side off
2 case fans (undervolted) soon.

Currently running a Nexus 3000 silent PSU. Considering an ebay 2nd hand
Nexus 3500 (350w) as I know they are good and definitely silent.

Online calculator suggested this would be 272w



Re: PSU load levels?

GT wrote:
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E6400 65W, 65W/0.90 = 72W or 12V @ 6A.

HD4830 - 85W (mostly from +12V). Say 12V @ 7A

Allocate 50W for motherboard and RAM (less in this case,
since it is a VIA chipset). 12W for each hard drive.
25W for the optical, but only when media is present
will it get close to that. 10W for USB devices or +5VSB
standby current.


And that number is not realistic. My E4700 for example, a
65W processor, only draws 36W when running Prime95. (It draws
more when overvolted.) So that shaves 30W off the number right
away. The 4coredual plus RAM doesn't burn up 50W, so that
estimate could well be 20W over the real situation. Now
we're down to 216W. If you're not using USB, and only the
normal motherboard standby loads are present, you're saving
another 5W. The optical drive, with the motor not using any
power, is going to drop to 5W for the logic board. We could
be under 200W by now.

When you do an actual measurement, it'll be less than
the estimates. The estimates assume everything goes
to hell at the same time. And real systems don't work
that way. For example, I've measured my current optical
drive, and with media present, the motor draws 12W.
So even when using my drive, it doesn't hit 25W.
The 25W number, is a "boiler plate" value used by
drive manufacturers, so they don't have to tell you
the truth.

Some high quality supplies, can be run at a significant
percentage of their rating.

If the supply only cost you $10 on the other hand, then
"super-size" it. Shoot for double the rating if shopping
in the $10 category. 500W should cover it.

This is an example of a supply someone posted about earlier
today. It is a 420W supply, but notice that there isn't
enough 12V amps. Which rail(s) are the fat rails, makes
a difference. I think you'd be unhappy if you happened
to own this.

RAIDMAX KY-520ATX 420W ATX Power Supply - Retail
+3.3V @ 26A, +5V @ 32A, +12V @ 13A, -5V @ 0.5A, -12V @ 0.8A, +5VSB @ 2.0A


Re: PSU load levels?

GT wrote:

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With second hand PSU's, you have no idea what your getting. Why is he
selling it? Better of going for some thing new and that has more watts than
you need that way it will never be pushed to it's limit. In the end it's
your choice and you have to live with it.

Re: PSU load levels?

GT wrote:

Quoted text here. Click to load it

Online calculators are nearly worthless.  They make way too many guesses
based on generic power specs for typical devices rather than those you
actually have or plan to have and you don't know if they are showing
results for a constant load or (less likely) the peak load which must be
supported when starting your host with all devices (internally or
externally connected) switched on.  They're like throwing a boat anchor
over the side trying to catch fish.  

You would have to use an ammeter to measure how much AC current is
getting sucked into your computer.  Then figure the wattage being
consumed.  Then multiply by the efficiency of the PSU; for example, if
200W were going in through an 80% efficient PSU then the computer is
consuming 160W.  Most PSUs are much less than 80% efficiency and many
don't list their efficiency (because they are so poor that they don't
want you to know).  This would measure only the working load on the PSU
and not the peak load.  You are not likely to have everything running at
its peak load (usually when a device starts) when you measure the input
AC current to the PSU.  If the online calculator only computed the
working load on the PSU, figure on at least doubling that to cover the
peak load (doubling would be excessive but than many PSUs really can't
support better than two-thirds of their claimed rated peak load).

The wattage listed for most PSUs are their *peak* load, not their
/sustained/ load.  Figure you can run only 65% to 75% of the rated peak
load as the constant load.  Some high-end (and pricey) PSUs actually
underrate their peak wattage so you can actually run them at their rated
load.  A high-quality PSU will also handle more than the rated load
during the surge upon startup of the host with all devices switched on
(because they underrate their peak load, they can handle a higher peak
than specified).  The "maximum load" specification is the peak load but
few PSUs mention how long that peak can be sustained.  I remember
reading several tests at the Tom's Hardware site where PSUs would burn
up (sometimes as a protection mechanism but often due to failure) when
ran constantly at their maximum rated load.

I haven't used any Nexus PSUs to know if they are a high-end quality
PSU.  Not familiar with or heard of that brand.  Could be they simply
rebrand another manufacturer's PSU, like how Sparkle is Fortron under a
different name (i.e., both from FSP Source).  This, for example, is like
Crucial Memory who doesn't manufacturer any memory modules but simply
slaps their name on components from any manufacturer that meets their
specs. has no "About Us" page for me
to even start to find out who they are, if they have any manufacturing
facilities, or who they might be getting the PSUs and then rebrand them
as Nexus.  I found them at /.  From the About page
there, it appears highly likely that they don't manufacture the PSUs but
rebrand those that meet their specs or have someone make the PSUs for

If you are going to fork out the money for a new PSU (whether you really
know whether or not it will handle the constant and peak loads), why
would you only increment from a 300W to a 350W PSU?  Quiet is nice but
the PSU supplies the lifeblood for your computer so think quality first
and then noise level.

I only saw one Nexus 3500 listed at eBay.  The auction price is
extremely cheap but then it is for a *used* item.  Also, if you aren't
in the UK, the shipping cost is 17.50 GBP (over $27 USD) - and this eBay
seller doesn't even offer insurance!  This seller also refuses any
returns on his used item as the auction says "The seller will not accept
returns for this item.".  You could end up paying shipping on a DOA item
or one that burns up soon after you start using it.  I have no idea what
their "SATA ready airflow" marketspeak means.  Airflow is airflow and
has nothing to do with whether or not there are SATA devices inside the
case.  Maybe they meant it has SATA power connectors but from their
picture I could only see one black SATA plug.  More specs are available
at the Nexus site
( ) which
says there are 2 SATA connectors.  Not a lot but maybe sufficient for

Just remember that "quiet" might not be that quiet.  20dB is often
equated to a whisper.  Well, do you want someone constantly whispering
in your hear?  Yeah, it's better than them constantly talking in your
ear.  Also, some PSUs will have their fans generate noise at a frequency
that you hear better than a louder fan generating noise at a different
frequency.  One that generates noise at 1024 Hz is perceived louder than
one that generates noise at 200 Hz or 10 kHz.  It depends on what
frequencies your ear hears best.  Fans generate something akin to white
noise (a range of frequencies) but they can have predominate
frequencies.  If you put the computer under your desk (which collect
more lint inside the case from the carpet on the floor) and off to the
side then it may seem quiet.  That 20dB won't seem so quiet if the only
place to put the case is on the desk next to your monitor and right in
front of your ears.  This PSU is "quiet" because it uses a thermally
controlled fan speed.  Any fan makes less noise when spinning slower
hence the purpose of fan speed controllers (either in hardware or
software).  If you don't provide enough breathing room between operating
and peak loads, the fan will be spinning at max speed and you've lost
all that "quiet" promise.

Since you intend on operating this PSU near its peak level, you might
want to look at their spec chart at the URL that I gave above.  Their
fan is temperature controlled but running under a higher constant load
will result in the fan spinning faster.  The 19.2dB rating is for a load
under around 150W.  By going with a PSU with little breathing space
between the working and peak loads, this PSU's fan will run at its max
speed and be generating around 30dB of noise so you've gained nothing
compared to other "noisier" PSUs.

Power supplies are not on what you go cheap for your computer.  A weak
or unreliable PSU means a flaky or inoperative computer.  

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