kVA to Amps Calculator

The formula#

For a three-phase transformer or generator:

I = (S × 1000) / (√3 × V)

And for single-phase:

I = (S × 1000) / V

Where S is apparent power in kVA, V is the line-to-line voltage in volts, and I is the full load current in amps.

Notice what is missing: power factor. It appears nowhere. kVA is the volt-amp product itself, so converting it to current is pure arithmetic. This is exactly why transformers are rated this way — the manufacturer has no idea what power factor you will connect, and does not need to.

A worked example#

A 100 kVA transformer with a 415 V three-phase secondary:

I = (100 × 1000) / (1.732 × 415)
I = 100,000 / 718.8
I = 139.1 A

So the secondary full load current is 139 A. On the primary side at 11 kV, the same 100 kVA gives:

I = (100 × 1000) / (1.732 × 11,000) = 5.25 A

The kVA is the same on both sides — that is what a transformer does — but the current differs by the turns ratio. It is always worth calculating both: the HV side current decides the HT fuse or relay setting, and it is small enough that people misjudge it by an order of magnitude.

kVA to amps at 415 V, three-phase#

The chart every panel shop has taped to a wall:

The 433 V column is not decoration. Indian distribution transformers are frequently specified with a 433 V no-load secondary so that the voltage at the far end of the network still sits near 415 V under load. If your nameplate says 433 V, the full load current is the figure the manufacturer used, and it is about 4 % lower than the 415 V column.

kVA to kW: what the rating actually delivers#

A 100 kVA source does not deliver 100 kW. It delivers 100 kVA, and how much of that is real power depends entirely on the load:

kW = kVA × PF

Generators are the place this bites. A DG set is sold as "100 kVA / 80 kW" — two ratings for one machine. The alternator is limited to 100 kVA by winding heating; the engine is limited to 80 kW by the fuel it can burn. Run that set on a poor power factor load and you hit the alternator's kVA limit while the engine loafs along well below its capability. Correct the power factor and the same set delivers more useful work.

The kVA to kW calculator does that conversion, and the DG set sizing calculator covers the motor starting case, which usually governs the rating rather than the running load.

Sizing the breaker and the cable#

Full load current is the input to protection, not the answer:

Transformer secondary breaker. Common practice is to select the incomer at 125 % of full load current, then confirm the setting against the transformer's withstand curve. For the 100 kVA example: 139 A × 1.25 = 174 A, so a 200 A frame set near 175 A.

Cable. The cable has to carry full load current after derating for ambient temperature, grouping and installation method. A 139 A load in a hot cable trench is not a 139 A cable — the cable sizing calculator applies the derating factors properly.

Fault level. The current that flows in a fault is set by the transformer's impedance, not its rating:

Fault current ≈ Full load current / (Z% / 100)

At 4.5 % impedance, our 139 A transformer can deliver about 3,090 A into a secondary fault, so every device downstream needs a breaking capacity above that. The short circuit current calculator works this through, including the cable's own contribution to limiting it.

Single-phase, and why the current is so much higher#

The same kVA on single-phase draws far more current:

• 10 kVA three-phase at 415 V: 13.9 A
• 10 kVA single-phase at 230 V: 43.5 A

Three times the current for the same apparent power. Both the lower voltage and the absent √3 contribute. It is the reason single-phase supplies are capped at modest ratings — beyond about 10 kVA the conductor sizes stop making economic sense.

Common mistakes#

Applying power factor to a kVA figure. kVA already includes it. Dividing by √3 × V gives the current, full stop. Multiplying by 0.8 somewhere in the middle is the single most common error on this conversion.

Using the primary voltage with the secondary rating. Same kVA, different voltage, wildly different current. Be explicit about which winding you are sizing for.

Sizing a generator on running kVA alone. Motor starting can demand three to five times the running kVA for a few seconds, and the voltage dip it causes is what trips the rest of the site.

Ignoring the 433 V nameplate. If the transformer is specified at 433 V, using 415 V overstates the full load current by about 4 %.