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Author Topic: PSU Ampere Calculation (on the wall with 230V) ??  (Read 6176 times)
antirack
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May 31, 2012, 10:51:20 PM
 #1

I am a bit puzzled again. Apologies if this has been answered before.

I am using 230V.

The PSU has 850W, I am currently using around 640W at the 12V rail. Lets assume for a moment I'd be drawing the complete 850W.

The PSU has 88% efficiency typical at this load (= 12% inefficiency).

850*1.12 = 952W.

At 230V that means the PSU should require a current of 4.14A (952/230). Why does the PSU say the following on the box:

AC Input:  100-240V~ 11-5.5A  60-50Hz

I know it's 240V not 230V, but 10V difference make 1.36A???

How am I supposed to estimate how much power it draws, how many of them I can connect to a single 13A or 16A outlet, etc? If I go by the 5.5A number, I can only connect two PSUs :/

Does anybody feel like teaching a dork like me some basic electrical skills?
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May 31, 2012, 10:56:38 PM
 #2

PSUs that allow a range of voltage without a switch tend to become more efficient as the voltage rises, due to how they work internally. Likely, that 5.5 is just an absolute max, and not what it will be pulling. Try it and see.

Additionally, the efficiency also varies with load - so with a different load and a different voltage, you could get a very different efficiency.

11 Amps at 100 volts is also a worst case scenario. All those are based on full load, and running at full load often reduces the efficiency a lot in less-expensive PSUs.

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June 01, 2012, 01:01:28 AM
 #3

I am a bit puzzled again. Apologies if this has been answered before.

I am using 230V.

The PSU has 850W, I am currently using around 640W at the 12V rail. Lets assume for a moment I'd be drawing the complete 850W.

The PSU has 88% efficiency typical at this load (= 12% inefficiency).

850*1.12 = 952W.

At 230V that means the PSU should require a current of 4.14A (952/230). Why does the PSU say the following on the box:

AC Input:  100-240V~ 11-5.5A  60-50Hz

I know it's 240V not 230V, but 10V difference make 1.36A???

How am I supposed to estimate how much power it draws, how many of them I can connect to a single 13A or 16A outlet, etc? If I go by the 5.5A number, I can only connect two PSUs :/

Does anybody feel like teaching a dork like me some basic electrical skills?


It's more about the total wattage it can provide, the voltage and amperage are just the two parts to that.  120 Volts at 11 Amps is ...  1320 Watts.  240 Volts at 5.5 Amps is...  1320 Watts still.  So if your actual voltage is 230 volts, you max amperage would be...  5.74 Amps for the PSU max and 6.5 amps at the wall.

Your total amperage is dependent on what your actual voltage is.  Which can fluctuate quite a bit, so always be on the safe side and error on the side of caution.  Electrical fires are a real bitch.

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June 01, 2012, 06:17:28 AM
 #4

Could you post what PSU you actually have?

Does anybody feel like teaching a dork like me some basic electrical skills?

The PSU has 88% efficiency typical at this load (= 12% inefficiency).

850*1.12 = 952W.

Nit-picking here, but you need to divide by 0.88, not multiply by 1.12.

850/0.88 = 966W

At 230V that means the PSU should require a current of 4.2A (952/230). Why does the PSU say the following on the box:

AC Input:  100-240V~ 11-5.5A  60-50Hz

I know it's 240V not 230V, but 10V difference make 1.36A???

Upping the voltage lowers the amperage required, so actually 240V actually requires 3.97A.

How am I supposed to estimate how much power it draws, how many of them I can connect to a single 13A or 16A outlet, etc? If I go by the 5.5A number, I can only connect two PSUs :/

Actually you really shouldn't even connect to 13A output with 3 power supplies rated for 3.97A. Most breakers are only rated for 80% load continuous operation, so dont connect more than 10.4A continuous load to it.


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June 03, 2012, 05:06:49 PM
 #5

wattage=ampere*voltage

So if you have 10 ampere with 110volt you have 1100 watt. With 240 volt you have 2400watt. And that's why europe is better  Cheesy
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June 04, 2012, 01:19:45 AM
 #6

wattage=ampere*voltage

So if you have 10 ampere with 110volt you have 1100 watt. With 240 volt you have 2400watt.

So how do you explain that the 850W power supply with 88%+ efficiency at full load says 5.5A on the box with 240V?

Here is the PSU:
http://www.coolermaster.com/product.php?product_id=6806
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June 04, 2012, 01:40:03 AM
 #7

wattage=ampere*voltage

So if you have 10 ampere with 110volt you have 1100 watt. With 240 volt you have 2400watt.

So how do you explain that the 850W power supply with 88%+ efficiency at full load says 5.5A on the box with 240V?

Here is the PSU:
http://www.coolermaster.com/product.php?product_id=6806

Normally, I would explain it away as 100% load being less than the stated efficiency, since efficiency is always pegged at 50% load, and drops off at 100%. However, I went and looked at the curves in your link, and although it does drop off, it only goes down one point to 87% at full load, which doesn't explain it. Therefore, all I can assume is that the rating is a "worst case scenario" rating, and not indicative of what it will draw even under 100% load.

EDIT: Calculations: 850 watts divided by 0.87 for the efficiency rating gives me 977 watts, and that divided by your voltage gives amps. It could be that the PSU is rated for 850 watts continuous, and 950 watts burst, and if that were the case the numbers at 950 watts would be very close to what is shown for the amperage on the label.

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June 04, 2012, 03:50:15 AM
 #8

So how do you explain that the 850W power supply with 88%+ efficiency at full load says 5.5A on the box with 240V?
Please read up about power factor and the difference between watts (W) and volt-amperes (VA).

http://en.wikipedia.org/wiki/Power_factor

Please comment, critique, criticize or ridicule BIP 2112: https://bitcointalk.org/index.php?topic=54382.0
Long-term mining prognosis: https://bitcointalk.org/index.php?topic=91101.0
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June 04, 2012, 05:18:06 AM
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So how do you explain that the 850W power supply with 88%+ efficiency at full load says 5.5A on the box with 240V?
Please read up about power factor and the difference between watts (W) and volt-amperes (VA).

http://en.wikipedia.org/wiki/Power_factor

That's an Active PFC power supply, which means it's going to be at 0.99PF for pretty much all normal operating conditions. Also, even if you use 0.9PF, it at most explains 4.52A, not 5.5A.

I was looking at some of the reviews for said power supply and noticed that it has 6A listed for 220V like this. Which has the same rating as its 1000W older brother. Notice the identical product no.

My guess is that the 850W is a 1000W that has been relabeled and maybe didn't pass some quality testing criterion at 1000W.

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June 04, 2012, 01:06:53 PM
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My guess is that the 850W is a 1000W that has been relabeled and maybe didn't pass some quality testing criterion at 1000W.
I never heard of anyone bin-sorting power devices. If it fails QA then it cannot be sold, period.

Some countries have the electrical code demanding the worst-case labeling for the devices that exhibit negative dynamic impedance.

For the usual case of positive dynamic impedance the device draws more current if the input voltage rises and vice-versa draws less current if the input voltage sags.

In the case of an internally compensating device that exhibits negative dynamic impedance it will draw less current if the input voltage rises and more current if the input voltage sags.

I know that old German industrial regulations (DIN) demanded such a labeling. And the input voltage ranges for testing were asymmetric, something like -20%/+10% or -25%/+12%. I remember reading that those regulations were scheduled to be obsoleted by the pan-European regulations, but the consultants advised us to follow the older regulations to assure speedy regulatory approvals.

Edit: I just wanted to add that I wasn't talking about "regular electrical code". I was talking about "special electrical code for health care facilities".

Please comment, critique, criticize or ridicule BIP 2112: https://bitcointalk.org/index.php?topic=54382.0
Long-term mining prognosis: https://bitcointalk.org/index.php?topic=91101.0
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June 04, 2012, 02:24:44 PM
 #11

I never heard of anyone bin-sorting power devices. If it fails QA then it cannot be sold, period.

I was thinking if the power supply doesn't meet internal product qualification targets, like <3% ripple or efficiency numbers or what have you. Not electrical code violations.

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June 04, 2012, 03:04:42 PM
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I was thinking if the power supply doesn't meet internal product qualification targets, like <3% ripple or efficiency numbers or what have you. Not electrical code violations.
Me neither. As far as I know any discrepancy from the approved design targets must be considered a potential failure. For example: ripple too high -> capacitors already faulty or with shortened expected lifespan. This is one of the first rules of safety engineering. Basically if any component or assembly procedure can be shown to be out of specification the whole device is considered faulty and potentially dangerous.

The above is a theory and the broad market practice. There may be other practices applied in the extreme low-end and extreme high-end. For example ultra-high-end handmade audio equipment is sometimes bin-sorted.

At the ultra low-end side anything goes.

Please comment, critique, criticize or ridicule BIP 2112: https://bitcointalk.org/index.php?topic=54382.0
Long-term mining prognosis: https://bitcointalk.org/index.php?topic=91101.0
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June 04, 2012, 06:05:03 PM
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For example: ripple too high -> capacitors already faulty or with shortened expected lifespan. This is one of the first rules of safety engineering. Basically if any component or assembly procedure can be shown to be out of specification the whole device is considered faulty and potentially dangerous.
Your design only is considered unsafe when you exceed ATX specification. If your design targets are significantly more stringent than the ATX specification, then it would not be a safety violation at all to sell slightly out-of-spec components at reduced power levels which meet relaxed (but still within ATX specification) secondary targets.

Another way to think of this is that your design targets ARE the ATX specification, and you're simply charging a premium for better quality.

Besides, I think we both know of more than a few companies that are a bit unscrupulous when labeling their power supplies. See Raidmax claiming 80 PLUS Gold on their RX-1000AE units for example. It would not be a big surprise to me if other manufacturers were less... unethical.



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June 04, 2012, 06:59:23 PM
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Your design only is considered unsafe when you exceed ATX specification.
I'm sorry I wasn't specific enough.

I'm pretty sure nobody is testing every power supply comming off the assembly line for the ATX conformance. Only a statistical sampling is done on the already-certified products.

However every power supply has to be tested for the UL,CE and other local electrical product safety specifications. The full UL (and equivalent) tests are destructive. Therefore only specific non-destructive subset of the tests is run, which must include verification of certain nominal parameters. The only results allowed are PASS and FAIL; grading isn't done and is not allowed.

I apologise for derailing this thread with my imprecise answers.

Please comment, critique, criticize or ridicule BIP 2112: https://bitcointalk.org/index.php?topic=54382.0
Long-term mining prognosis: https://bitcointalk.org/index.php?topic=91101.0
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June 05, 2012, 04:15:21 AM
 #15

Thanks a lot, I really appreciate all your help.

I did some measuring using my Brennenstuhl Wattage and Current Meter (Kill-A-Watt not available in my Area).
 
Max 3.393A (seems to stay at 3.351A)
Max 715W (usually between 700W-710W)
Power Factor 0.94 (cycles between 0.94 and 0.95, sometimes 0.96)
222V to 224V

I also cut open a power cord and used my clamp meter to double check and it basically confirms the above numbers (the clamp meter actually reads slightly lower current but I'll stick with the higher numbers for safety).

I've been looking at it for a half day and I couldn't get it any higher than that (no 5.5A as stated on the box), not at system startup, not when power cycling, not after hours of running.

Can I safely connect 4 of them to a 16A circuit?
How about 13A?  Grin (not seriously considering this, just asking what if)

I am still working on my cluster and now is the time to really understand all this.
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June 05, 2012, 04:50:09 AM
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Can I safely connect 4 of them to a 16A circuit?
Safely? Yes. The worst that would happen will be a circuit breaker trip during a prolonged brownout. Or if you have a brief outage then the breaker will trip right after the power is restored because of the startup surge current.

If your measurement device has a "MAX" function for the "A" values you can measure the startup surge current on your own. You could also simply try it

Additionally you could try to borrow a variac/autotransformer from some nearby electrical shop and test a single power supply simulating the brownout by adjusting the variac.

On the other hand, if you live in a locale where brownouts are a rarity then just don't worry about them.

Please comment, critique, criticize or ridicule BIP 2112: https://bitcointalk.org/index.php?topic=54382.0
Long-term mining prognosis: https://bitcointalk.org/index.php?topic=91101.0
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June 05, 2012, 05:08:51 AM
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We don't have brownouts as far as I know (I had to Google this).

The measuring device has a MAX function, that's what I used to get the above numbers.

Since I have your attention, let me ask another question. I am meeting an electrician to get the power circuit and breaker box upgraded.

Say my requirement is 100A at 220V (used for ease of calculation). I have 3 phase power in the building and the guy asked me how many Ampere I need. It's then 55A @ 400V, is that correct?

Just trying to understand at least the basics.
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June 05, 2012, 07:17:16 AM
 #18

Can I safely connect 4 of them to a 16A circuit?

Safely, yes. Recommended? No.

You are exceeding the breaker 80% continuous load recommendation. You may get spurious trips, especially if the temperature goes up.

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June 05, 2012, 02:04:03 PM
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This is really a bit confusing. Why don't they call them 12.8A circuit breakers if you can only use them for 12.8A continues load?

So I need the electrician to install higher current circuit breakers (ie. 20A) and make sure the wiring is proper for this purpose, in order to pull 14A load from each socket?

He's gonna be very happy, more parts more work more money Wink
 
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June 05, 2012, 02:10:22 PM
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I would say it's because you need the extra amp for when some appliance reach their peak in electrical consumption. An AC motor will use more energy to start than maintaining speed. Same thing apply to ballast.
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