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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.
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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.
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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.
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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.
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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
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)
KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES
1000
220 x 4.7 = 1034 1034 / 1000 =
1.034 kVA
3. THREE-PHASE
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
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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
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
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
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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
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Known and Given: 1 kW = 3413 BTUs (or 3.413
kBTUs)
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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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