In this post we can learn about Basic Electrical calculations for fresh engineering students. this Pdf very helpful to students who are preparing for their first dream electrical job.

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

Kilovolt-Amps Calculation (kVA)

kVA is an abbreviation for "Thousand Volt-Amps."
A 2-Pole Single Phase 208-240 power source necessitates the use of two hot wires from two different circuits.

(also known as poles) from a power distribution panel


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

Continuing with the previous example: 300 VA = 120 x 2.5 300 volts / 1000 volts equals.3 kVA


• As an example, consider a 4.7 amp enterprise computer bvserver that requires a 208-volt power supply.

240 volt power supply For our calculations, we'll use 220 volts.

Kilovolt-Amps (kVA) = Volts x Amps divided by 1000

220 times 4.7 equals 1034 1034 / 1000 equals 1.034 kVA


• As an example, consider a large disk storage system loaded with disks. According to the equipment documentation, a 50-amp 208-240 VAC power source is required. Make no calculations for the plug or receptacle. For the calculation, use 220 volts.

Kilovolt-Amps (kVA) = Volts multiplied by Amps multiplied by 1.73 / 1000
220 multiplied by 50 multiplied by 1.73 equals 19,030 19 kVA = 19,030 / 1000 This is rounded to 19.

Converting The difference between kW and kVA

• The power factor is the only distinction between kW and kVA. Unless taken from the manufacturer's specifications, the power factor is an approximation. We'll use a power factor of.95 in this example. The kVA value will always be greater than the kW value.

kW to kVA kW / .95 = kVA
kVA TO kW kVA x .95 = kW

How to Convert kW-Hours to kVA

• There is NO kWH-to-kVA conversion. These are two distinct measurements. kWH stands for energy, and kVA stands for power (not necessarily dissipated). There is a relationship between kW (power) and kVA (power). The power factor of the load is the result of this relationship.

• Known Standard: 1 kW = 3413 BTUs (or 3.413 kBTUs) • Dividing the electrical nameplate BTU value by 3413 may not yield the published kW value. If the manufacturer provides BTU information, use it; otherwise, use the above formula.

Horsepower Calculations for Beginners

Horsepower is defined as work done per unit of time. One horsepower equals 33,000 ft-lb of torque per minute.

The work done by a torque source (T) to produce (M) rotations about an axis is: radius x 2 x rpm x lb. or 2 TM

When the rotation rate is N rpm, the HP delivered is:

33,000 = TN / 5,250 HP = radius x 2 x rpm x lb.

For hoisting or vertical motion:

W x S / 33,000 x E = HP

W = total weight in pounds to be lifted by motor S = hoisting speed in feet per minute

E denotes the overall mechanical efficiency of the hoist and gearing. For the purpose of calculating E =.65 for the eff. of the hoist and "connected gear."

Mechanical General Approximations - THUMB RULES

For quick approximations in the field, use these:

At rated HP output, a motor develops 1.5 lb-ft of torque per HP at 3600 rpm.
At rated HP output, a motor produces 3 lb-ft of torque per HP at 1800 rpm.
At rated HP output, a motor produces 4.5 lb-ft of torque per HP at 1200 rpm.
At rated HP output, a motor develops 6 lb-ft of torque per HP at 900 rpm.
A 3-phase motor draws 1 amp per horsepower at 575 volts.
A 3-phase motor draws 1.25 amps per horsepower at 460 volts.
A three-phase motor draws 2.5 amps per horsepower at 230 volts.

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