Basic Electrical Calculations

BASIC ELECTRICAL CALCULATIONS

Computing Watts When Volts and Amps are known                                                                               

POWER (WATTS) = Volts x Amps

For example, a small computer has a nameplate that 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) = 2.5Amps x 120Volts = 300 WATTS

Generally: P=IE

P= Power(WATTS)

I = Current(Amps)

E =

Voltage(Volts)

So: I = P/E and E = P/I

Therefore: 1 Watt = 1 Ampere x 1 Volt

Computing Kilovolt-Amps (kVA)                                                                             

kVA stands for "Thousand Volt-Amps".
A 2-Pole Single Phase 208-240 power source requires 2 hot wires from 2 different circuits (referred to as poles) from a power distribution panel.

SINGLE PHASE

KILOVOLT-Amps (kVA) = Volts x Amps / 1000

Using the previous example: 120 x 2.5 = 300 VA 300 Va / 1000 = .3 kVA
208-240 SINGLE-PHASE (2-POLE SINGLE-PHASE)
    • Example: An enterprise computer bvserver with a 4.7 amp rating and requiring a 208-
    240 power source. Use 220 volts for our calculations.

kilovolt-Amps (kVA) = Volts x Amps /1000

220 x 4.7 = 1034 1034 / 1000 = 1.034 kVA

THREE-PHASE

    • Example: A large disk storage system loaded with disks. The equipment documentation
    shows a requirement for a 50-amp 208-240 VAC power source. Do not calculate any
    value for the plug or receptacle. Use 220 volts for the calculation.
    

kilovolt-Amps (kVA) = Volts x Amps x 1.73 / 1000

220 x 50 x 1.73 = 19,030 19,030 / 1000 = 19.030 kVA This would be rounded to 19

To Convert Between kW and kVA                                                                            

The only difference between kW and kVA is the power factor. The power factor, unless taken from the manufacturer's specifications, is an approximation. For this example, we use a power factor of .95. The kVA value will always be larger than the value for kW.

kW to kVA kW / .95 = kVA

kVA TO kW kVA x .95 = kW

To Convert Between kW-Hours and kVA                                                                

There is NO conversion from kWH to kVA. These are two different measures. kWH is energy and kVA is power (not necessarily dissipated). If you look at kW (power) and kVA (power), then there is a relationship. That relationship is the power factor of the load.

Computing BTUs                                                                                                       

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

• If you divide the electrical nameplate BTU value by 3413 you may not get the published
kW value. If the BTU information is provided by the manufacturer, use it, otherwise use
the above formula.

Shotgun Section                                                                                                        

Here are conversions, short and sweet:

To convert kVA to Amps:

Multiply kVA by 1000/voltage [ (kVA x 1000) / E ]

For 3 Phase power divide by 1.73 [ (kVA x 1000) / E x 1.73 ]

• To convert Watts to Volts when amps are known:

Voltage = Watts / Amps

E = P / I

• To convert Watts to Amps when volts are known:

Amps = Watts / Voltage I = P / E

For 3 Phase power divide by 1.73

• To convert Amps to Watts when volts are known:

Watts = Voltage x Amps P = E x I

For 3 Phase power multiply by 1.73

• To convert Horsepower to Amps:

Horsepower = (E x I x EFF) / 746

Efficiency = (746 x HP) / (V x A)

Multiply Horsepower by 746W (1 HP = 746 Watts)

Find Circuit Voltage and Phase
Example: 40 HP at 480 (3 Phase) 746 multiplied by 40 = 29,840

                    29,840 divided by 480 (3 Phase) = 62.2

                    62.2 divided by 1.73 = 35.95Amps

• To convert kW to Amps: Multiply kW by 1000/voltage and then by Power Factor [ (kW x 1000) / E x PF ]

For 3 Phase power divide by 1.73 [ ( kW x 1000) / E x PF x 1.73 ]

Basic Horsepower Calculations                                                                            

Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute.
When work is done by a source of torque (T) to produce (M) rotations about an axis, the work
done is: radius x 2 x rpm x lb. or 2 TM 
 

When rotation is at the rate N rpm, the HP delivered is:
           HP = radius x 2 x rpm x lb. / 33,000 = TN / 5,250
 

For vertical or hoisting motion:
            HP = W x S / 33,000 x E
                   

 Where:
W = total weight in lbs. to be raised by motor
S = hoisting speed in feet per minute
E = overall mechanical efficiency of hoist and gearing. For purposes of estimating
E = .65 for eff. of hoist and "connected gear."

Mechanical                                                                                                                

General Approximations - RULES OF THUMB

Use these in the field for fast approximations:

At 3600 rpm, a motor develops a 1.5 lb-ft of torque per HP at rated HP output

At 1800 rpm, a motor develops a 3 lb-ft of torque per HP at rated HP output

At 1200 rpm, a motor develops a 4.5 lb-ft of torque per HP at rated HP output

At 900 rpm, a motor develops a 6 lb-ft of torque per HP at rated HP output

At 575 volts, a 3-phase motor draws 1 AMP per HP at rated HP output

At 460 volts, a 3-phase motor draws 1.25 AMP per HP at rated HP output

At 230 volts a 3-phase motor draws 2.5 AMP per HP at rated HP output

At 230 volts, a single-phase motor draws 5 AMP per HP at rated HP output

At 115 volts, a single-phase motor draws 10 AMP per HP at rated HP output