Electrical Unit Conversions

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11/26/11 Electrical Unit Conversions

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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) IXEXPF IXEX 1.73XPF

KILOWATTS (kW)E._X_b_XPF E X IX 1.73 X PF

1000 1000

AMPERES (I)kWX 1000 kWX 1000

EXPF EX1.73XPF

KILOVOLT AMPERES lX_E LX_E_X 1.73

(kVA) 1000 1000

FREQUENCY (Hertz or f)Rotor Poles X RPM Rotor Poles X RPM

120 120

RPM (n)Hertz X 120 Hertz X 120

Rotor Poles Rotor Poles

NUMBER OF ROTOR Hertz X 120 Hertz X 120

POLES (P) RPM RPM

POWER FACTOR (PF)Actual Watts Actual Watts

IXE IX 1.73X E

HORSEPOWER (HP) IvE X PF lX_E_X 1.73 X PF746 X EFF 746 X EFF

AMPERES (when kW is kWX 1000 kWX 1000

known) EXPF EX1.73XPF

AMPERES (when kVA is kVAX 1000 kVAX 1000

known) E EX 1.73

I I

I = current in amperes

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E = voltage in volts

W = watts

kW = power in ki lowatts

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 kWand kVA

To Find kBTUs from Electrical Values

Background

It is often necessary to tum \Ultage, 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: Dewlop 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

extenslve electrical tests on each piece of equipment. If you must lower your estimates, make sure you

haw a good reason. In years to come you will want ewry 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 haw a appliance with a nameplate shows 2.5 amps. Giwn a normal 120 Volt, 60 hz power source an

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 11000 = .300 kVA

2. TWO-PHASE

• Giwn: We haw 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

• Giwn: We haw 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






11/26/11

To Find Kilowatts

Electrical Unit Conversions

• Finding Kilowatts is a bit more complicated in that the formula includes a value for the "power factor". T

power factor is a nebulous but required value that is different for each electrical device. It in\{)lws 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 haw a

power factor of .80. Whatewr the number, it places a slight inaccuracy into the numbers. Its OK and it

gets us wry close for the work you need to do.

1. SINGLE PHASE

Giwn: We haw 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

• Giwn: We haw 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

3. THREE-PHASE

• Giwn: We haw a wry 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 o

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 kWand 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 generatoruse. The kVA value is always higher than the value for kW.

kW To kVA

kVA To kW

kW 1 .80 = SAME VALUE EXPRESSED INkVA

kVA * .80 = SAME VALUE EXPRESSED IN kW

To Find BTUs From Electrical Values

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







• The above is a generally known value for conwrting electrical values to BTUs. Many manufacturers publis

kW, kVA and BTU in their equipment specifications. Often, di\liding the BTU value by 3413 does not equa

their published kW value. So much for knowns and giwns. Where the information is pro\lided by the

manufacturer, use it. Where it is not, use the above formula.

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