208VAC Watts and Current Calculation Help

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Paul P

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Hi,

I have a rack-mounted power distribution unit that accepts a 3-phase, 60-amp input (3-phase + GND, no neutral) and provides (12) 208VAC single-phase outputs.

I plug one output into a server/computer that uses 540watts.

Now I want to calculate amp draw for the server/computer.

Is the answer:
a) 540 / 208 = 2.596 amps (a single phase calculation)

or

b) 540 / (208 * 1.73) = 1.5 amps (a 3-phase calculation)

or

c) something else!

I'm confused about which calculation to use because 208VAC is referred to as single phase, but the input to the racks is certainly 3-phase.

Thanks for any help!
 
Paul P said:
Hi,

I have a rack-mounted power distribution unit that accepts a 3-phase, 60-amp input (3-phase + GND, no neutral) and provides (12) 208VAC single-phase outputs.

I plug one output into a server/computer that uses 540watts.

Now I want to calculate amp draw for the server/computer.

Is the answer:
a) 540 / 208 = 2.596 amps (a single phase calculation)

or

b) 540 / (208 * 1.73) = 1.5 amps (a 3-phase calculation)

or

c) something else!

I'm confused about which calculation to use because 208VAC is referred to as single phase, but the input to the racks is certainly 3-phase.

Thanks for any help!
Click to expand...

Assuming the server is single phase the answer is 'a'.

But can I ask why you want to know the amp load?

It looks like your power supply, 3-phase, 60-amp input has 21,590 watts of capacity non-continuously or 17,272 watts continuously.

I would figure my loading based on watts not amps.
 
Paul P said:
...I have a rack-mounted power distribution unit that accepts a 3-phase, 60-amp input (3-phase + GND, no neutral) and provides (12) 208VAC single-phase outputs. ...
Click to expand...

Paul -
I'm curious about the "rack mouned distribution unit". What does it consist of? Is this an input CB abd several output CB? Or is it a UPS with several output cbs?

carl
 
iwire said:
Assuming the server is single phase the answer is 'a'.
Click to expand...
That's true for a linear a purely resistive load.
But much commercial and domestic stuff use switch mode power supplies.
They are neither linear nor resistive.
 
Besoeker said:
That's true for a linear a purely resistive load.
But much commercial and domestic stuff use switch mode power supplies.
They are neither linear nor resistive.
Click to expand...

I just figured it is easier to learn to walk before trying to run. :smile:
 
Thanks for the quick answers.

The rack is designed to hold up to (4) large IBM servers that can consume about 4KW each, thus the high density power distribution unit. It is not an UPS. It splits the 3-phase 60-A input into 12, 16-amp breakered outputs, apparently splitting the phases: AB, AC, BC. There is no input CB, just a CB for each of the 12 outputs.

The PDU:
http://www-304.ibm.com/shop/americas/webapp/wcs/stores/servlet/default/ProductDisplay?catalogId=-840&storeId=1&langId=-1&dualCurrId=73&categoryId=4611686018425093831&productId=8667826

Our customer is asking for both total watts and total amps. Some of the hardware going into the rack does not have an amp spec, just the watts.
 
Paul P said:
I have a rack-mounted power distribution unit that accepts a 3-phase, 60-amp input (3-phase + GND, no neutral) and provides (12) 208VAC single-phase outputs.
...
Click to expand...

Paul P said:
...It is not an UPS. It splits the 3-phase 60-A input into 12, 16-amp breakered outputs, apparently splitting the phases: AB, AC, BC. There is no input CB, just a CB for each of the 12 outputs. ...
Click to expand...

paul -
This is not what you asked about, but unless I'm mis-understanding, you have a problem. The unit needs a neutral with the feeder comming in.

carl
 
coulter said:
paul -
This is not what you asked about, but unless I'm mis-understanding, you have a problem. The unit needs a neutral with the feeder comming in.

carl
Click to expand...

Carl, it is entirely possible the servers run at 208 volts, this is something I am staring to see more of.

I believe one of the advantages is not loading the neutral with harmonic currents.
 
I dont think that a neutral is required in this case.
I presume that we are talking about a 3 phase, 4 wire 120/208 volt supply, of which only the three phases and the EGC are used.
The servers or other loads are designed for a 208 volt input and may therefore be connected between any phase and any other.

A neutral would be required if it was desired to serve 120 volt loads, but if all loads are 208 volts, as is believed to be the case, then the neutral would not be required.

The switch mode power supplies used in servers etc are liable to draw a third harmonic current from the supply. If numerous such loads are connected between phase and neutral on a three phase supply, then the third harmonic currents add up in the neutral, sometimes to more than the phase current. this can overheat the neutral.
To avoid this it is desirable to use a 208 volt input to the units, the absence of any connection to the neutral means that there is no question of excessive neutral current.
 
That's right, there is no neutral to the rack -- no 120VAC provided in the rack, just 208. The connector is a 3P +G model IEC309.

So to be clear, the PDU divides up the 3 phases and provides (12) outputs of 208VAC (2-hot +GND). And that is called single phase, and that means we use the single phase calculation to go from watts to amps, right?

Paul
 
Paul P said:
So to be clear, the PDU divides up the 3 phases and provides (12) outputs of 208VAC (2-hot +GND). And that is called single phase, and that means we use the single phase calculation to go from watts to amps, right?
Click to expand...

Yes, that is correct, but you can not add up the single phase 'amps' to figure out the loading on the three phase feeder.
 
Ah, and that is why you recommended just adding up the watts in the rack, right?

Ultimately, the current per phase is going to differ depending on what gets connected the various (12) PDU outputs. Do you think it worthwhile to analyze the current per phase so as to balance the load across the (12) outputs. Sounds like just balancing the 17,272 total constant watts will be easier.

Note the (12) PDU outputs are actually split into (3) domains, and each domain has a 16A breaker -- not a breaker per output as I indicated earlier.

The available constant watts per domain = 208 * 16 * 1.73.
 
Paul P said:
That's right, there is no neutral to the rack -- no 120VAC provided in the rack, just 208. The connector is a 3P +G model IEC309. ...
Click to expand...

Of course your right. You even said several times, "single phase 208". What was I thiking?

carl
 
I agree with iwire that it's easier to deal with watts than amps in determining capacity. But if you want amps, let's say you have three 540 W PDU's (assume pf=1), connected AB, BC, CA.

Current in each PDU = 540/208 = 2.6A
Current input = 540?3/208/sqrt(3) = 4.5A

You have two PDUs connected to each phase, but the currents are 120? apart, so they don't add arithmetically. They add vectorially

Itotal = 2.6?sqrt(3) = 4.5A
 
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