Basic Electrical Calculations

                                              BASIC  ELECTRICAL  CALCULATIONS

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..

To Find Watts 

To Find Volt-Amperes 

To Find Kilovolt-Amperes 

1.To Find Watts -

 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

2.To Find Volt-Amperes (VA) 

 Same as above. VOLT-AMPERES (VA) = VOLTS x AMPERES ANS: 300 VA

3.To Find kilovolt-Amperes (kVA) 

 A) SINGLE PHASE

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

 Using the previous example: 120 * 2.5 = 300 VA 300 VA / 1000 = .3 kVA 

 B) 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

C) 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

4.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.

A) 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

 B) 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 

C) 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