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ELECTRICAL UNIT CONVERSIONS

The purpose of this document is to provide information, formulas and documentation to take certain electrical values and convert them into other electrical values.  The formulas below are known and used universally but we use them here in association with computer, network, telecom and other IT equipment.  

To Find Watts
To Find Volt-Amperes
To Find Kilovolt-Amperes
To Find Kilowatts
To Convert Between kW and kVA
TO Find kBTUs from Electrical Values

Background

It is often necessary to turn voltage, amperage and electrical "nameplate" values from computer, network and telecom equipment into kW, KVA and BTU information that can be used to calculate overall power and HVAC loads for IT spaces.  The following describes how to take basic electrical values and convert them into other types of electrical values.  

  • NOTE #1:
    The informational nameplates on most pieces of computer or network equipment usually display electrical values.  These values can be expressed in volts, amperes, kilovolt-amperes, watts or some combination of the foregoing.  

  • NOTE #2:
    If you are using equipment nameplate information to develop a power and cooling profile for architects and engineers, the total power and cooling values will exceed the actual output of the equipment.  Reason:  the nameplate value is designed to ensure that the equipment will energize and run safely.  Manufacturers build in a "safety factor" when developing their nameplate data.  Some nameplates display information that is higher than the equipment will ever need - often up to 20% higher.  The result is that, in total, your profile will "over engineer" the power and cooling equipment.  Electrical and mechanical engineers may challenge your figures citing that nameplates require more power than necessary.  

  • NOTE #3:
    Our advice:  Develop the power and cooling profile using the nameplate information and the formulas below and use the resultant documentation as your baseline.  Reasons:  (1) it's the best information available without doing extensive electrical tests on each piece of equipment.  Besides, for most projects, you are being asked to predict equipment requirements 3-5 years out when much of the equipment you will need hasn't been invented yet.  (2) the engineers will not duplicate your work; they do not know what goes into a data center.  They will only challenge the findings if they appear to be to high.  If the engineers want to challenge your figures, it's OK but have them do it in writing and let them take full responsibility for any modifications.  If you must lower your estimates, do so.  But, document everything.  There will come a day in 3-5 years when you will need every amp of power you predicted.  We've had projects where it was very evident within six months that what we predicted would come true - sometimes even earlier than we estimated.  

  • NOTE #4
    If you are designing a very high-density server room where you will have racks and racks (or cabinets and cabinets) of 1U and 2U servers tightly packed, you need to read our article entitled "IT Pros - Don't be Left in the Dust on IT Server Room Design".  

To Find Watts

1.     When Volts and Amperes are Known

        POWER (WATTS) = VOLTS x AMPERES

  • We have a small server with a nameplate shows 2.5 amps.  Given a normal 120 Volt, 60 hz power source and the ampere reading from equipment, make the following calculation:  

        POWER (WATTS) = 120 * 2.5                   ANSWER:  300 WATTS

To Find Volt-Amperes (VA)

1.     Same as above.  VOLT-AMPERES (VA) = VOLTS x AMPERES    ANS:  300 VA

To Find kilovolt-Amperes (kVA)

1.     SINGLE PHASE

        KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES
                                                                1000

        Using the previous example:  120 * 2.5 = 300 VA       300 VA / 1000 = .3 kVA

2.     208-240 SINGLE-PHASE (2-POLE SINGLE-PHASE)

  • Given:  We have a Sun server with an amp rating of 4.7 and requiring a 208-240 power source.  We'll use 220 volts for our calculations.

        KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES
                                                                  1000

        220 x 4.7 = 1034          1034 / 1000 = 1.034 kVA

3.     THREE-PHASE

  • Given:  We have a large EMC Symmetrix 3930-18/-36 storage system with 192 physical volumes.  EMC's website shows a requirement for a 50-amp 208 VAC receptacle.  For this calculation, we will use 21 amps. Do not calculate any value for the plug or receptacle. 

        KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x 1.73
                                                                    1000

        208 x 21 x 1.73 = 7,556.64          7,556.64 / 1000 =  7.556 kVA

To Find Kilowatts

  • Finding Kilowatts is a bit more complicated in that the formula includes a value for the "power factor".  The power factor is a nebulous but required value that is different for each electrical device.  It involves the efficiency in the use of of the electricity supplied to the system.  This factor can vary widely from 60% to 95% and is never published on the equipment nameplate and further, is not often supplied with product information.  For purposes of these calculations, we use a power factor of .85.  This arbitrary number places a slight inaccuracy into the numbers.  Its OK and it gets us very close for the work we need to do.  

1.     SINGLE PHASE

        Given:  We have a medium-sized Compaq server that draws 6.0 amps.  

        KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR
                                                                             1000

        120 * 6.0 = 720 VA       720 VA * .85 = 612      612 / 1000 = .612 kW

2.     TWO-PHASE

  • Given:  We have a Sun server with an amp rating of 4.7 and requiring a 208-240 power source.  We'll use 220 volts for our calculations.

        KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR x 2
                                                                                  1000

        220 x 4.7 x 2 = 2068          2068 x .85 = 1757.8      1757.8 / 1000 = 1.76 kW

3.     THREE-PHASE

  • Given:  We have a large EMC Symmetrix 3930-18/-36 storage system with 192 physical volumes.  EMC's website shows a requirement for a 50-amp 208 VAC receptacle.  For this calculation, we will use 22 amps. Do not calculate the value of the plug or receptacle.  Use the value on nameplate.  

        KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR x 1.73
                                                                                  1000

        208x22x1.73 = 7,916.48    7,916.48 * .85 = 6,729.008    6,729.008/1000=6.729 kW

To Convert Between kW and kVA

  • The only difference between kW and kVA is the power factor.  Once again, the power factor, unless known, is an approximation.  For purposes of our calculations, we use a power factor of .85.  The kVA value is always higher than the value for kW.  

        kW to kVA         kW / .85 = SAME VALUE EXPRESSED IN kVA
        kVA TO kW       kVA * .85 = SAME VALUE EXPRESSED IN kW

To Find BTUs From Electrical Values

  • Known and Given:   1 kW = 3413 BTUs (or 3.413 kBTUs)

  • The above is a generally known value for converting electrical values to BTUs.  Many manufacturers publish kW, kVA and BTU in their equipment specifications.  Often, dividing the BTU value by 3413 does not equal their published kW value.  So much for knowns and givens.  Where the information is provided by the manufacturer, use it.  Where it is not, use the above formula. 

 

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