electrical unit conversion

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ELECTRICAL UNIT CONVERSIONS

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)E X L X PF

1000E X I X 1.73 X PF

1000

AMPERES (I)kW X 1000

E X PFkW X 1000

E X 1.73 X PF

KILOVOLT AMPERES (kVA)

I X E1000

I X E X 1.73 1000

FREQUENCY (Hertz or f)

Rotor Poles X RPM120

Rotor Poles X RPM120

RPM (n)Hertz X 120Rotor Poles

Hertz X 120Rotor Poles

NUMBER OF ROTOR POLES

(P)

Hertz X 120RPM

Hertz X 120RPM

POWER FACTOR Actual Watts Actual Watts

of 7Electrical Unit Conversions

4/10/2010http://www.generatorjoe.net/html/electricunit.html

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

(PF) I X E I X 1.73 X E

HORSEPOWER (HP)

I v E X PF746 X EFF

I X E X 1.73 X PF746 X EFF

AMPERES (when kW is known)

kW X 1000E X PF

kW X 1000E X 1.73 X PF

AMPERES (when kVA is known)

kVA X 1000E

kVA X 1000E X 1.73

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



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

� We have a appliance 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 = .300 kVA

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.

KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x 2 1000

220 x 4.7 x 2 = 2068 2068 / 1000 = 2.068 kVA

3. THREE-PHASE

� Given: We have a large appliance with nameplate data of 50-amp 208 VAC receptacle. For this calculation, we will use 21 amps. Do not



calculate the value of the plug or receptacle. Use the value on

nameplate.

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

� Given: We have a larger appliance with a running 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 x POWER FACTOR x 2 1000

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



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3. THREE-PHASE

� Given: We have a very large appliance that shows a requirement for a 50-amp 208 VAC receptacle. For this calculation, we will use 21 amps. Do not calculate the value of the plug or receptacle. Use the value on

nameplate.

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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