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.

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.