PowerEdge Power Requirements

Question

I've been estimating power distribution for server racks for +10 years based on what I've read in the NEC.  I assign one (or two) power strip (PDU) per power drop (one drop usually connects to a 20A circuit breaker.  I base the proposed load on the Dell Power Technical Specifications (e.g., PE 2650 Wattage = 500W (AC).  So if I have a rack with (10) PE 2650s I'll divide 120 VAC into the 500 W for a load of 4.17 Amps.  For (10) PE 2650s that would give me 41.7 Amps.  The NEC recommends not loading a circuit breaker more than 80% of it's rated capacity so I would design the rack with (4) 20A drops to (4) 20A Power Strips.  I would spread the PE 2650s out (3) on Pwr Strip #1, (3) on Pwr Strip # 2, (3) on Pwr Strip #3 and (1) on Pwr Strip #4.  The highest load at 3 x 4.17 = 12.51 Amps.   Now I have an engineer telling me that I'm not considering power factor and losses due to power supply efficiency and heat dissapation.  So my question is:   What factors are included in the determination of the Power Supply Wattage?  Do you take into account the power supply's efficiency, power factor and heat dissapation?  When Dell is planning rack installations of servers How do they calculate the power requirements for a given rack configuration?

RackSolutions · Answer

The power supply spec is 500W under ideal conditions. This could vary by +/- 20% depending on the efficiency of the device. Efficiency is affected by temperature and host of other things.

If you use this in your calculation the PE2650 could draw from 3.33A to 5A.

500W + 20% = 600W
500W - 20% = 400W

600W/120VAC = 5A (max)
500W/120VAC = 4.17A (spec)
400W/120VAC = 3.33A (min)

20A * 80% = 16A
20A * 85% = 17A
20A * 90% = 18A

5A * 3 = 15A (maximum load)
4.17A * 4 = 16.7A (based on spec)
3.33A * 5 = 16.6A (possible load)

All this is just a guideline. If you really want to know how much power the servers are going to draw you need a clamp meter.

Remove the outer insulation of the Left power cord and use the clamp meter to record the Current from any wire other than ground. You can also check the Voltage from the power source if your clamp meter has probes to verify 120V.

V*C=VA

The Power Factor constant can be 1 or 0.9 don't ask me where PF comes from it's insignificant in most circumstances. I just wanted to put it in here for reference.

VA*PF=W

Record the power usage every 30-60 minutes over a given period of time under normal operating conditions and you will know exactly how many servers you can plug into that powerstrip.

If you have any questions please don't hesitate to contact me.

Will Beene
903-453-0835
Will@RackSolutions.com

ematus · Answer

Your explanation applied to the NEC recommendation of 80% max load on a circuit breaker makes a lot of sense then.   BTW, Power Factor, as I understand it, is the energy reflected back toward the source.  It is based on the phase differences in the power supply and the AC source.  But with active Power Factor Correction circuits in use by nearly all PC Power Supply manufacturers it has become inconsequential.   Thanks Will!

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PowerEdge Power Requirements

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