Primary current

Secondary power Ps = Is x Vs (Secondary current x secondary voltage)

Primary power Pp = Ps ÷ ε (Secondary power ÷ efficiency)

Primary current Ip = Pp ÷ Vp (Primary power ÷ primary voltage)

Just substitute in the known values.

How to know the effiiciency?

Ask the manufacturer. If you can't get anywhere that way, look at data for transformers of similar size & construction for which you can find the data, and take the average (or lowest, if you want to be on the safe side).

Hi,

If we know the turns of both the windings & the current in secondary, then it is much easier to calculate the current in primary.

Ip/Is = Ns/Np

Further the above eqn =>

Is =(Np/Ns) x Ip

Where

Np->Turns of primary Winding

Ns->Turns on secondary winding

Ip->Primary current

Is->Secondary current

Hari

But this is for an ideal transformer. It ignores losses (resistance in both windings + magnetic coupling losses).

What is your transformer KVA? (Power)

What is your transformer primary voltage?

What is your transformer secondary voltage?

You need all these information to do the primary current calculation.

I don't know the power (KVA), The primary voltage is 415V, Secondary Voltages are 24V 2.5Amps, 24.5V 5Amps, 220V 2.5Amps I know only these informations from my transformer.

Output: (24 x 2.5) + (24 x 5) + (220 * 2.5) = 60 + 120 + 550 = 730VA

Assume 80% efficiency, so input is ≈ 912VA. That gives input current ≈ 912/415 ≈ 2.2A.

MR. JohnDC has given you the answer to your problem.

Since you dont have the transformer KVA, you can calculate the transformer primary current from the secondary side of the transformer.

Secondary Output: (24(V) x 2.5(I)) + (24(V) x 5(I)) + (220(V) * 2.5(I)) = 60 + 120 + 550 = 730VA (Secondary output KVA)

Assume 80% efficiency, so input is ≈ 912VA. That gives input current ≈ 912/415 ≈ 2.2A. (Primary current = KVA/V)