Transformer Sizing Calculator

Enter the transformer's rating in kilovolt-amperes, the primary and secondary voltages and whether it is single-phase or three-phase. The full-load current is the kilovolt-amperes times 1,000 divided by the voltage, and for three-phase also divided by the square root of 3. Take the rating from the nameplate, and use the line-to-line voltage for three-phase systems.

The loads on the secondary determine the size you need.

Overcurrent protection and conductor sizing for transformers follow specific code articles that allow ratings above full-load current. Impedance, efficiency and temperature rise are not covered. Harmonic loads can overheat a transformer.

Installation, grounding and clearance must be done by qualified electricians.

Primary full-load current (amps) from transformer rating (KVA)

Transformer rating (KVA)	75
Primary voltage	480
Secondary voltage	208
Phase	Three-phase
Primary full-load current (amps)	90.2
Secondary full-load current (amps)	208.2

Common error: Sizing a transformer to the connected load exactly. Motors, future growth and starting current all argue for a margin above the calculated need.

Transformer Sizing: formula

I = KVA × 1000 ÷ (V × √3) for three-phase, or KVA × 1000 ÷ V for single-phase

NFPA 70: National Electrical Code

Transformer Sizing: changing Transformer rating (KVA)

Primary full-load current (amps): assumed Transformer rating (KVA) scenarios.
Transformer rating (KVA)	Primary full-load current (amps)	Secondary full-load current (amps)
60	72.2	166.5
68	81.8	188.7
75 (example)	90.2	208.2
83	99.8	230.4
90	108.3	249.8

Transformer Sizing: compare input ranges

Transformer Sizing Calculator: Primary full-load current (amps) with each input moved on its own, every other input held at the worked-example value.
Input	Tested from	To	Primary full-load current (amps): low input	Primary full-load current (amps): high input	Swing
Phase	Single-phase	Three-phase	156.3	90.2	66 (73%)
Primary voltage	432	528	100.2	82	18.2 (20%)
Transformer rating (KVA)	68	83	81.8	99.8	18 (20%)
Secondary voltage	187	229	90.2	90.2	0

Transformer Sizing: individual input comparisons

Transformer Sizing Calculator: change one input at a time; other values remain at the stated example.
Input	Value	Primary full-load current (amps)	Secondary full-load current (amps)
Transformer rating (KVA)	60	72.2	166.5
	68	81.8	188.7
	75	90.2	208.2
	83	99.8	230.4
	90	108.3	249.8
Primary voltage	384	112.8	208.2
	432	100.2	208.2
	480	90.2	208.2
	528	82	208.2
	576	75.2	208.2
Secondary voltage	166	90.2	260.9
	187	90.2	231.6
	208	90.2	208.2
	229	90.2	189.1
	250	90.2	173.2
Phase	Single-phase	156.3	360.6
Phase	Three-phase	90.2	208.2

Primary full-load current (amps): Transformer rating (KVA) against Primary voltage

Transformer Sizing: Primary full-load current (amps) at combined Transformer rating (KVA) and Primary voltage settings.
Transformer rating (KVA) / Primary voltage	384	480	576
60	90.2	72.2	60.1
68	102.2	81.8	68.2
75	112.8	90.2	75.2
83	124.8	99.8	83.2
90	135.3	108.3	90.2

Transformer Sizing: values and definitions

Worked-example inputs and the results they produce for the Transformer Sizing Calculator.
Quantity and definition	Value used
Transformer rating (KVA) — Transformer nameplate rating, in KVA.	75
Primary voltage — Primary-side line voltage, in volts.	480
Secondary voltage — Secondary-side line voltage, in volts.	208
Phase — Single-phase or three-phase transformer.	Three-phase
Primary full-load current (amps)	90.2
Secondary full-load current (amps)	208.2

A transformer's kilovolt-amp rating sets how much current it can supply on each side. A 75 kVA three-phase transformer stepping 480 volts down to 208 draws about 90.2 amps on the primary and can deliver about 208.2 amps on the secondary. The secondary current is higher because the voltage is lower for the same power.

Overcurrent protection and conductors on each side must be sized from those currents, using the code rules for transformers.

Size the protection and wiring from the code rules for the primary and secondary, and allow for the starting current of large motors on the load.

See three-phase power on the three-phase power calculator, wire ratings on the wire ampacity page and voltage drop on the voltage drop calculator. The conduit fill and watts to amps pages support the installation, and the power converter changes units.

How do I find a transformer's full-load current?

Multiply the kVA rating by 1,000 and divide by the voltage, and by the square root of 3 as well for three-phase.

What does kVA mean?

Kilovolt-amperes, a measure of apparent power, which for a transformer sets its capacity regardless of the power factor of the load.

Should I size a transformer to exactly match my load?

No. Leave margin for growth and startup surges, and follow code rules for protection and conductors.

Transformer Sizing Calculator