How to calculate KVA size of transformer

What is kva rating of Transformer

To figure out, how to calculate KVA size of transformer kVA size, we should know what is kva rating. kva stand for kilo volt- amperes. You’ll sometimes see transformers, especially smaller ones, sized in units of VA. VA stands for volt-amperes. A transformer with a 100 VA rating, for instance, can handle 100 volts at one ampere (amp) of current.

The kVA unit represents kilovolt-amperes or 1,000 volt-amperes. A transformer with a 1.0 kVA rating is the same as a transformer with a 1,000 VA rating and can handle 100 volts at 10 amps of current.

How to calculate KVA size of transformer

For calculating transformer KVA rating we need two-term first one is voltage and the second one is the load to be connected through transformer secondary side. As load increases amperage will also increase hence insulation of winding and core are required of high quality so the size of the transformer also gets increased.

Load (kw) inceases ⬆ ___ Ampere (A)increase ⬆

⬆ ___ Core insulation size ⬆

Transformer size ⬆

The electrical load that connects to the secondary winding requires a particular input voltage or load voltage. Let’s call that voltage V. You’ll need to know what this voltage is — you can find it by looking at the electrical schematic. We could say that an example load voltage V must be 150 volts.

You’ll then need to determine the particular current flow your electrical load requires. You can look at the electrical schematic to determine this number as well. If you can’t locate the required current flow, you can calculate it by dividing the input voltage by the input resistance. Let’s say the required load phase current, which we’ll call l, is 50 amperes.

Once you’ve located or calculated these two figures, you can use them to figure out the load’s power requirements in kilowatts. To do that, you’ll need to multiply the required input voltage (V) by the required current load in amperes (l) and then divide that number by 1,000:

KVA rating = V * l / 1,000

In the example above, you would multiply 150 by 50 to get 7,500 and then divide that number by 1,000 to get 7.5 kilowatts.

The last step is to convert the figure in kilowatts to kilovolt-amperes. When you do that, you’ll need to divide by 0.8, which represents the typical power factor of a load. In the example above, you’d divide 7.5 by 0.8 to get 9.375 kVA.

When you’re choosing a transformer, though, you won’t find one rated 9.375 kVA. Most kVA ratings are whole numbers, and many, especially in the higher ranges, come in multiples of five or 10 — 15 kVA, 150 kVA, 1,000 kVA and so on. In most cases, you’ll want to select a transformer with a rating slightly higher than the kVA you calculated — in this case, probably 10 or 15 kVA.

You can also work backward and use the known kVA of a transformer to calculate the amperage you can use. If your transformer is rated at 1.5 kVA, and you want to operate it at 25 volts, multiply 1.5 by 1,000 to get 1,500 and then divide 1,500 by 25 to get 60. Your transformer will allow you to run it with up to 60 amperes of current. How to calculate KVA size of the transformer

How to Convert Amps to kVA

Amps are a measure of electrical current in an electrical circuit. kVA, or kilovolt-amps, are a measure of apparent power in a circuit, and 1 kVA is equal to 1,000 volt-amperes.

Because amps and kVA are different things in an electrical circuit, voltage is also needed for the conversion. By also using the voltage, it’s possible to convert amps to kVA using the electrical power formula.

Using the electric power formula the formula to convert amps to kVA can be derived:
Power_(kVA) = Current_(A) × Voltage_(V)1,000

Thus, power in kVA is equal to the current in amps times the voltage, divided by 1,000.

How to Convert Amps to kVA in a Three-Phase Circuit

Three-phase circuits require a slightly different formula for the conversion. To convert A to kVA in a three-phase circuit use the following formula.

Power_(kVA) = √3 × Current_(A) × Voltage_(V)1,000

Example

What is the apparent power in kVA when the phase current is 12A and the RMS voltage supply is 110V?

Solution:

S = 12A × 110V / 1000 = 1.32kVA

3 phase amps to kVA calculation formula

Calculation with line to line voltage

The apparent power S in kilovolt-amps (with balanced loads) is equal to the square root of 3 times the phase current I in amps, times the line to line RMS voltage V_L-L in volts, divided by 1000:

S_(kVA) = √3 × I_(A)× V_L-L(V)/ 1000

So kilovolt-amps are equal to √3 times amps times volts divided by 1000.

kilovolt-amps = √3 × amps × volts / 1000

or

kVA = √3 × A ⋅ V / 1000

Example

What is the apparent power in kVA when the phase current is 12A and the line to line RMS voltage supply is 190V?

Solution:

S = √3 × 12A × 190V / 1000 = 3.949kVA

Calculation with line to neutral voltage

The apparent power S in kilovolt-amps (with balanced loads) is equal to 3 times the phase current I in amps, times the line to neutral RMS voltage V_L-N in volts, divided by 1000:

S_(kVA) = 3 × I_(A)× V_L-N(V)/ 1000

So kilovolt-amps are equal to 3 times amps times volts divided by 1000.

kilovolt-amps = 3 × amps × volts / 1000

or

kVA = 3 × A ⋅ V / 1000

Example

What is the apparent power in kVA when the phase current is 12A and the line to neutral RMS voltage supply is 120V?

Solution:

S = 3 × 12A × 120V / 1000 = 4.32kVA

Copper loss in transformer

Calculating transformer KVA rating we need two-term first one is voltage and the second one is the load to be connected through transformer secondary side. as ampere increases, the losses also increase so the size of the transformer also increases to counter the heating effect.

Let us see what is copper loss due to which heating occur that is a factor in knowing to how to calculate KVA size of the transformer :

Copper losses result from Joule heating and so are also referred to as “I squared R losses”, in reference to Joule’s First Law. This states that the energy lost each second, or power, increases as the square of the current through the windings and in proportion to the electrical resistance of the conductors.

where I is the current flowing in the conductor and R is the resistance of the conductor. With I in amperes and R in ohms, the calculated power loss is given in watts.

Joule heating has a  coefficient of performance of 1.0, meaning that every 1 watt of electrical power is converted to 1 Joule of heat. Therefore, the energy lost due to copper loss is: