How Many Amps for This Power Supply?

Bulldog17

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The published specifications for my computer's power supply read as follows:

Input voltage: 115/230 VAC
Input frequency: 50/60HZ
Rated output current: 8 A/4 A

My household current is 115 VAC @ 60Hz - is the output current for my power supply 4 A or 8 A?
 

Bulldog17

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Hello fellow Dell user.

I also thought they meant to write 'input current'...but they wrote instead "Rated output current".

I'm sure there are plenty of smart people at Dell. But they don't work in documentation, and they don't work in technical support either. ("It means you can choose either 8A or 4A, as you wish.")

All I want is an estimate for the volt-amp rating of my system. A ballpark number. Just to get a feel for what kind of output I need from a UPS. I get 115 volts from the wall socket. Roughly many amps can my power supply draw at 115 volts? According to Dell's documentation, it's either 8 amps or 4 amps, depending on the voltage. Does the 8 amp figure correspond with 115 volts or 230 volts?

Sorry to rant. I've been struggling with Dell technical support for hours.
 
Somewhere on the PSU label, it will say "400 watts" some other number of watts. That's what you use to estimate UPS capacity.

Now, the "8 amps" corresponds to the 115 VAC setting. But, frankly, that makes no sense because that corresponds to about a 900 watt power level.

"Rated output current" does not correlate with the 8 amp figure.
 
Watts = amps x volts.

That would indicate a 800 watt psu(which I doubt). But, a psu does not draw it's maximum power, only what is demanded of it which will be much less.

Look at the psu itself to find out exactly what you have. A UL psu will be required to have a data plate attached which will say how many watts or amps capacity is available at several of the voltages used in a PC

All that does not help you to size a UPS. You could buy a device like a KILLAWATT meter for about $20:
http://www.amazon.com/Kill-a-Watt-Meter/dp/B001JHGY2Q

If your need is for simple electrical conditioning, or for temporary spikes, then a minimum unit should suffice. If you want to provide for several hours of uptime, then you need to more accurately find your current usage.

Go to the APC UPS selector to find what you need:
http://www.apc.com/tools/ups_selector/index.cfm
 

Never saw one that said "output" in regards to the number's you posted.
Still don't know which one you have so here's a random Dell psu label, take notice of the " input " just below the model number

14y6ck5.jpg
 
"Input voltage: 115/230 VAC Rated output current: 8 A/4 A "

As you quessed @ 115 VAC the max rated current drawn from the mains is 8A (for 230 VAC is 4 AMPS) In amost all elecrical systems the V times A is higher than the "delievered" power. This rating takes into condieration the INRUSH current which can be much higher than the nominal current. It is also why "slow blow" fuses are required in some applications.

The "real" current is dependent on what the PSU delievers to the Load divided by the effeincy of the PSU.

EXample: 400 Watt PSU with an eff rating of 80% delievering a 300 Watts of power to the Computer.
300 / .8 = 375 Watts 375/115 = 3.26 Amps The 3.26 Amps is the nominal current drawn from the wall outlet.
However when the Computer is first powered on the current could be as high as 6 amps for several milliseconds- Which is why they have a 8 Amp Max listing.



 

Bulldog17

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Many thanks to everyone for your help.

In fact, I have a Dell Studio XPS 8100 desktop. This computer probably has a power factor correcting (PFC) power supply. I say 'probably' because no one I have corresponded with at Dell seems to know; however, the Studio XPS 9000 does have a PFC power supply.

My computer's power supply is rated at 350 watts, which is certainly far less than its maximum capacity, since 115V times 8A is 920 VA. As many of you have observed, it doesn't seem correct that the maximum current is as high as 920 watts...that's with a power factor of unity. Even with 80% efficiency...736 watts?

Of course, the PSU delivers far less during typical service. But a PFC PSU can draw its full capacity when first started. At least, that's what I understand.

How this relates to a UPS: If the transfer time (to battery power) is long enough - as little as 8 ms, according to what I've read - a PFC power supply will draw its full inrush current from the UPS, or attempt to. This can cause the UPS to drop its load (to say nothing of the computer user.)
 
When an UPS is switched from Line to battery (and yes it can take several millisectons) the Inrush current is not a factor. The reason is that durning this switch over time the computer Power is not switched off. The Capacitors inside the PSU will supply the power for this duration. You might see some perabations on the output lines but they would be small. Inrush currents are normally due to the capacitance in the load charging being at 0 Volts and appears as a short circuit decreasing to an open in 5 time constants.

Even with a PFC circuit, the inruch current amplitute and duration is depentent on the load characteristics. Setting the MAX rating to 3 to 4 times rated is not abnormal.
 

Bulldog17

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"Setting the MAX rating to 3 to 4 times rated is not abnormal."

Surely you're not suggesting a UPS capacity of 3-to-4 X 350 watts in my case? I figured 2 X 350 watts would be a reasonable amount to look for.
 
No, Select UPS based on Nominal system requirements. Normally your load must be less than the rated max for the System. More than just the Computer. IE computer + Monitor+ any thing else plugged into the "On battery" supplied power. The 2nd thing then is the desired run time when on battery. Most UPS will provide a chart indicating the run time for typical systems. You choice on this one.

I was refering to The reason the PSU has a Much Higher AC Input rating as compared to the Max output load rating. (115 x 8 = 920 Watts vs output max of 400W.
 

Bulldog17

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A mute point: I checked my existing UPS by pulling the plug (of the UPS) out of the wall socket. My computer continued to run, no problem. I could probably do with a more properly sized unit, anyway, but at least there's no big rush.

Thanks to all. This has been an interesting learning experience for me.
 


Did I miss something? I understood that you were shopping for a UPS. If your UPS is from APC, you can download Their power chute personal edition software for free. It will estimate how much longer your system will run on battery power.

Glad you got it sorted out.
 

Bulldog17

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Sorry if I confused you, and others. In addition to the power rating, I have also been learning about whether or not I need a type of UPS with an inverter that produces current in a sine wave output format. That's a separate discussion that I mixed up with this thread. Sorry!.
 

debianos

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This was hilarious to read casually. :lol:


But yes, in this case they're right, because the amperage depends on what part of the world you're plugging into, i.e., in the U.S. 115V/8A and most everywhere else 230V/4A.

Glad you got this resolved, and it reminds me to price some UPS myself.
 

Maniac56

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