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This information
page provides formulas and documentation to take certain electrical values
and convert them into other electrical values. The formulas below
are known and used universally in the Generator Industry but you can use
them for computer, network, telecom and powered equipment
|
VALUE |
1-PHASE |
3-PHASE |
|
WATTS (W) |
I
X
E
X
PF |
I
X
E
X
1.73
X
PF |
|
KILOWATTS (kW) |
|
|
|
AMPERES (I) |
|
|
|
KILOVOLT AMPERES
(kVA) |
|
|
|
FREQUENCY (Hertz
or f) |
|
|
|
RPM (n) |
|
|
|
NUMBER OF ROTOR
POLES (P) |
|
|
|
POWER FACTOR (PF) |
|
|
Actual Watts |
|
I
X 1.73
X E |
|
|
HORSEPOWER (HP) |
|
|
I
X E
X
1.73
X PF |
|
746
X EFF |
|
|
AMPERES (when kW
is known) |
|
|
|
AMPERES (when kVA
is known) |
|
|
|
I |
= |
current in amperes |
|
E |
= |
voltage in volts |
|
W |
= |
watts |
|
kW |
= |
power in kilowatts |
|
kVA |
= |
apparent power in
kilo-volt-amperes |
|
HP |
= |
output power in
horsepower |
|
RPM (n) |
= |
motor speed in revolutions
per minute (RPM) |
|
ns |
= |
synchronous speed
in revolutions per minute (RPM) |
|
Rotor Poles (P) |
= |
number of poles |
|
Hertz (f) |
= |
frequency in cycles
per second (CPS) |
|
T |
= |
torque in pound-feet |
|
EFF |
= |
efficiency as a
decimal |
|
PF |
= |
power factor as
a decimal |
|
HP |
= |
horsepower |
For a detailed explanation
of each formula, Click on the links below to go right to it.
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 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
profile for use in selecting a generator, the total power 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 requirements of the equipment. This is not generally bad, you
should just be aware of it.
-
NOTE #3:
We advise: Develop the power profile using the nameplate information
and the formulas below and use the resultant documentation as your baseline.
Why? Because it's the best information available without doing extensive
electrical tests on each piece of equipment. If you must
lower your estimates, make sure you have a good reason. In years to
come you will want every watt you can get. Better to be "oversized"
then "undersized".
The Formulas
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 = .300 kVA
2.
TWO-PHASE
KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x
2
1000
220 x 4.7 x 2 = 2068
2068 / 1000 = 2.068 kVA
3.
THREE-PHASE
KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x
1.73
1000
208 x 20.5 x 1.73 = 7,376.72
7,376.72 / 1000 = 7.377 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 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. Most generators have
a power factor of .80. Whatever the number, it places a slight inaccuracy
into the numbers. Its OK and it gets us very close for the work
you need to do.
1.
SINGLE PHASE
Given: We have a medium-sized appliance that draws 6.0 amps.
KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x
POWER FACTOR
1000
120 * 6.0 = 720 VA 720 VA * .85 = 612
612 / 1000 = .612 kW
2.
TWO-PHASE
KILOVOLT-AMPERES (kVA) = 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
KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x
POWER FACTOR x 1.73
1000
208x20.5x1.73 = 7,376.72 7,376.72 * .85 = 6,720.21
6,720.21/1000=6.27 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 .80 which
most generators use. The kVA value is always higher than the value
for kW.
kW To kVA kW / .80
= SAME VALUE EXPRESSED IN kVA
kVA To kW
kVA * .80 = 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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