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# Estimating Power Requirements

## Estimating Power Requirements

(OP)
I've got several Compaq servers with dual power supplies which I'm going to split across 2 different power sources.

I am trying to determine the amount of power required for these servers so I don't overload my UPS and/or the electrical circuits.

Since the electrician here talks about the circuit load in terms of amps, that's how I've been trying to rate the load that the servers will need.

The power rating on the back of the server has 2 numbers for amps (e.g., 7.6 and 3.8)--I'm assuming that the 2 numbers represent the amperage at 120 volts and 240 volts. What I need to determine...

(1) At 120 volts, is the 7.6 amps the total load that the server pulls or the load per power supply?
(2) When both power supplies are operational, is each one pulling half of the amp load or is 1 power supply the master and pulls the full load?
(3) At 120 volts, is 7.6 amps the peak load that will be pulled? It seems that I've read somewhere that equipmdnt typically pulls more power during a power on cycle. Is this 7.6 amp rating the most that the server will ever pull? If so, what's a ballpark pct for what it requires during normal operation?

Thanks!

Ken Scoville

### RE: Estimating Power Requirements

Most of your assumptions are correct...

If the rating plate is on each power supply, the rating is for each supply. If their is a single power rating plate, it is for the whole server.

The 7.6 amp rating should refer to the highest sustained power draw this power supply system will require from an A/C line ( at a specific voltage) without tripping a fuse or overload protection circuit and or causing power distortion, give or take a small tolerance. When a computer, or just about any electrical device starts, a power surge is created for a very brief time which can greatly exceed the devices maximum power rating. Rarely is this a factor with UPSs unless you attach something like a laser printer which can surge at more than twice the rating plate.

Ball park useage varies by the server design and number of devices in the machine, I would say an average machine will draw roughly 50% of the rating sustained. Total wattage for the system is very close to the sum of voltage times the amperage of all devices connected to a power supply, as measured with an ampmeter.

As far as how the power supplies supply the juice....
Some redundant supplies run on more than one  power supply, in parallel; some will allow a second power supply to take over only if the first fails.

As far as the maximum power use, generally a well designed system will not come close to the maximum draw. Keep in mind some manufacturer do not correctly rate their units properly, power tolerances vary considerably ( they should not, but they do)

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