Full-Load Current of a 1600 kVA Transformer at 400, 415 and 11 000 V

The full-load (rated) current of a three-phase transformer follows from its rating and the line-to-line voltage of each winding: I = S / (√3 × V). For 1600 kVA that is 2,309 A on a 400 V secondary and 83.98 A on an 11 kV primary.

Ratings of 1600 to 2500 kVA are found at large industrial and commercial sites. At 400 V a 1600 kVA unit delivers 2,309 A, so the link to the main switchboard is normally busbar trunking or several cables in parallel per phase.

Last reviewed 10 November 2026

Line current in amperes, three-phase, at 100, 75 and 50 % of the rating. 433 V is quoted on some nameplates as the no-load secondary voltage of transformers for 400 V systems.
Voltage (line-to-line)	Winding	Full load (A)	75 % load (A)	50 % load (A)
400 V	LV secondary	2,309	1,732	1,155
415 V	LV secondary	2,226	1,669	1,113
433 V	LV secondary	2,133	1,600	1,067
6.6 kV	MV primary	140	105	69.98
11 kV	MV primary	83.98	62.98	41.99
33 kV	MV primary	27.99	20.99	14

Line current in amperes, three-phase, at 100, 75 and 50 % of the rating. 433 V is quoted on some nameplates as the no-load secondary voltage of transformers for 400 V systems.

Voltage (line-to-line)

Winding

Full load (A)

75 % load (A)

50 % load (A)

400 V

LV secondary

2,309

1,732

1,155

415 V

LV secondary

2,226

1,669

1,113

433 V

LV secondary

2,133

1,600

1,067

6.6 kV

MV primary

140

105

69.98

11 kV

MV primary

83.98

62.98

41.99

33 kV

MV primary

27.99

20.99

Worked example

1600 kVA, 11 kV / 400 V, three-phase:

I_LV = 1600 × 1000 / (1.73205 × 400) = 2,309.4 A I_HV = 1600 × 1000 / (1.73205 × 11 000) = 83.98 A Check: I_LV / I_HV = 11 000 / 400 = 27.5

The currents on the two sides are in the inverse ratio of the voltages, because the same kVA passes through the transformer (losses neglected).

Single-phase equivalent

A single-phase transformer of the same 1600 kVA at 230 V would carry 1600 × 1000 / 230 = 6,957 A. In the three-phase unit each phase winding handles one third of the rating, 533.3 kVA; on a star-connected 400 V secondary the phase voltage is 231 V and the phase current equals the line current in the table.

What the full-load current is used for

Cables or busbar trunking from the LV terminals to the main switchboard are sized for at least 2,309 A after applying the correction factors of their installation; the cable size calculator handles the ambient and grouping factors.
The LV main breaker is chosen with a rated current at or above the full-load current. 2,309 A is above the MCCB ratings listed on this site; at this current air circuit breakers (ACBs) are normally used, with the setting chosen to suit the cable or busbar and discrimination.
Current transformers for metering and protection have standard primary ratings in IEC 61869-2 (10, 12.5, 15, 20, 25, 30, 40, 50, 60 and 75 A and their decimal multiples). A common practice is to take the first one at or above the full-load current, here 2,500 A on the 400 V side; the metering accuracy class and the protection requirements decide the final ratio.
On the primary side, HV fuses and relay settings start from the primary full-load current but must also ride through the magnetising inrush when the transformer is energised; they are selected from the fuse or relay maker's tables.
Measured current divided by full-load current gives the transformer loading; the transformer loading calculator adds losses and efficiency.

Example: fault current at the LV terminals

Assume, for illustration only, an impedance voltage of uk = 6 %. IEC 60076-5 lists minimum short-circuit impedances by rating, and the actual value is on the nameplate; it governs. With an infinite upstream network:

Isc ≈ I_LV / (uk / 100) = 2,309.4 / 0.06 = 38.5 kA

A real source impedance lowers this figure and the voltage factor of IEC 60909-0 raises it slightly; the short-circuit calculator takes both into account and continues to the end of a cable. Transformer Short-Circuit Current Calculator

Full-Load Current of a 1600 kVA Transformer at 400, 415 and 11 000 V

Rated current of a 1600 kVA transformer

Worked example

Single-phase equivalent

What the full-load current is used for

Example: fault current at the LV terminals

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