Transformer Full Load Current (amps) Calculation Calculator

Transformer Full Load Current Calculator:

Enter the voltage, kVA rating then press the calculate button. You can choose the single or three-phase as well as line to line or line to neutral option to find the full load amps. After changing press calculates button to get the current in Amps. Reset button clears all the value in the field.

Choose the phase #:  
Enter kilovolt-amps: kVA
Enter Voltage: V
Result in amps: A

What is full load current:

Full load current is nothing but a the maximum allowable current to the winding and which is used to design the protection system for the transformer.

Transformer current calculations:

Transformer current can be calculated from two ways such as

  • Using Power calculation method
  • Using the turns ratio method

Using Power equation method:

Power equation can be classified into two types one is single-phase and another is three-phase. If the input of the transformer has single phase (R or Y or B) with neutral (N) means those transformers are called a single-phase transformer. If the transformer has three-phase input means those transformers are called as a three-phase transformer.

Single-phase transformer current calculations

Transformer full load current I(A) in amps for single-phase transformer is equal to 1000 times of transformer rating S(kVA) in kVA (kilo Volt-Amp) divided by the primary V(P-V) or secondary voltage V(S-V) in volts of the transformer. In general, the full load current is equal to

I(A) = S(kVA) *1000 / V(V)

If the transformer is rated in MVA means, the formula will be

I(A) = S(MVA) *1000000 / V(V)

The transformer has two current one is primary current and another one is secondary current.

If you want to calculate primary current we should consider primary voltage only, then the formula will be

Primary current in Amps I(P-A) = S(kVA) * 1000 / V(P-V)

If you want to calculate secondary current, then we need to take secondary voltage only; Then the formula will be

Secondary current in Amps I(S-A) = S(kVA) *1000 / V(S-V)


Calculate the full load current of the single-phase transformer rating of 25kVA, 230 volts.

Full load current in amps = 25 *1000 / 230 = 108.696 A

Three-phase transformer current calculations

The full load current I(A) in amps is equal to 1000 times of transformer rating S(kVA) in kVA divided by the multiplication of root 3 times of line to line voltage V(V) in volts.

I(A) = S(kVA) *1000 / (1.732 * V(V))

if you take the phase to neutral voltage V(L-N) in Volts means the current formula will be

I(A) = S(kVA) *1000 / (3 * V(L-N))

Hence for calculating primary current I(P-A) in Amps will be

I(P-A) = S(kVA) *1000 / (1.732 * V(P-V))

V(P-V) is the primary voltage in Volts

Hence, the formula for secondary current I(S-A) in amps will be

I(S-A) = S(kVA) *1000 / (1.732 * V(S-V))

V(S-V) = Secondary voltage in volts.

Turns ratio method:

As you know, the ratio between the primary voltage V(P-V) in volts to the secondary voltage V(S-V) in volts is equal to the ratio between the secondary current I(S-A) in Amps to the primary current I(S-A) in amps. The relation can be written as,

(V(P-V)/V(S-V)) = (I(S-A)/I(P-A)) = (N(P)/ N(S)

Np = Primary Turns

Ns = Secondary Turns

If you know any three parameters of the above, you can calculate the full load current in amps of the transformer from turns ratio.

Let rewrite the formula for secondary current,

I(S-A) = (V(P-V) * I(P-A) / V(S-V))

I(S-A) = N(P) * I(P-A) / N(S)

Let rewrite the formula for primary current

I(P-A) = V(S-V) * I(S-A) / V(P-V) 

I(P-A) = I(S-A) * N(S)­ / N(P)

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