# How Many Amps for This Power Supply?

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April 9, 2010 1:26:53 AM

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?

More about : amps power supply

a c 243 ) Power supply
April 9, 2010 1:48:26 AM

Are you sure it doesn't say " input current " ?
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April 9, 2010 4:13:38 AM

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.
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a c 144 ) Power supply
April 9, 2010 6:12:19 AM

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.
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a c 147 ) Power supply
April 9, 2010 6:30:05 AM

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

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
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a c 243 ) Power supply
April 9, 2010 10:59:45 AM

Bulldog17 said:
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.

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

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April 9, 2010 2:51:03 PM

Please state the make of your psu, rather than the input coltage/curent/whatever else. That might help alot.
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a b ) Power supply
April 9, 2010 6:58:29 PM

"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.

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April 9, 2010 7:49:08 PM

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.)
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a b ) Power supply
April 9, 2010 8:12:27 PM

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.
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April 12, 2010 8:13:05 PM

RetiredChief said:
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.

"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.
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a b ) Power supply
April 13, 2010 4:36:19 PM

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.
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April 14, 2010 5:35:48 AM

geofelt said:
Don't make this harder than it has to be. Go to the APC configurator and answer a few simple questions: http://www.apc.com/tools/ups_selector/index.cfm

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.
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a c 147 ) Power supply
April 14, 2010 2:10:44 PM

Bulldog17 said:
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.
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April 14, 2010 5:55:19 PM

geofelt said:
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.

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!.
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April 16, 2010 1:23:20 PM

Bulldog17 said:
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.")

This was hilarious to read casually.

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.
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December 31, 2013 10:58:05 PM

RetiredChief said:
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.

Dear RetiredChief,

I have a inrush related questions and would like to have your professional opinion.

When I was checking my external hdd, I found the startup current of Seagate ST2000DM001 (aka Barracuda or Desktop HDD) in my Buffalo HD-LX2TU3 is 2.5A and output current of the adapter is 1.5A. It requires 67% higher than the adapter could offer. Is it safe use of electricity usage and okay to my hard disk?

I collect all related information for this issue on Match Hard Disk Startup Current with Power Adapter Output Current and will update in the future.

Any help would be appreciated!

Best regards,

Amigo
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January 25, 2014 7:34:25 AM

I have answer my own questions after weeks.

Here is a list of what I have learned:

1. Adapter is responsible for steady voltage within its designed capacity (watt).

2. Inrush current will lower voltage and increase temperature suddenly.

3. High temperature is bad to capacitor.

4. Low than minim voltage will shutdown the component.

5. Most adapter only provide guarantee current which is different from inrush current.

6. Most vendor didn't provide inrush current information.

7. Inrush current has its limit which can be calculated by formula:

capacitor inrush current = 1.44 âˆš(short circuit current * normal capacitor Rated Current)

8. "The high inrush currents can cause damage to the capacitors of the capacitors bank and to weld the contacts of the switch together" according to S.J. Kulas

9. Capacitor is used to make sure there is steady voltage.

My conclusion: Inrush current is bad to power adapter. It is a better to shorten the gap of inrush current and output current to help capacitor live longer. If there is huge gap between them, inrush current generate much more heat than usual which will shorten the capacitor life and your power adapter might not be able to provide steady voltage.

If the power adapter becomes unstable, it is dangerous.

Detail information are explained in Match Hard Disk Startup Current with Power Adapter Output Current. The reference sections provides many useful links and helps me to know more about electricity.
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July 31, 2014 8:12:02 AM

Bulldog17 said:
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.

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July 31, 2014 8:13:09 AM

The ampearage goes with the voltage 115 volts is 8 amps and 230 volts goes with the 4 amps.
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