Free tool screening estimate

Transformer Calculator: kVA and Full-Load Amps

Full-load current on both sides of a single-phase or three-phase transformer, and the standard kVA rating a load needs.

Tool referenceDE-TOOL-8978

Systems: Single-phase and three-phase
Start from: kVA, load current or kW
Output: Primary and secondary full-load current and the standard kVA

This transformer calculator gives the full-load current on the primary and secondary of a single-phase or three-phase transformer, or the standard kVA rating a load needs. Three-phase current is kVA × 1,000 ÷ (1.732 × volts); single-phase current is kVA × 1,000 ÷ volts. A 75 kVA three-phase transformer from 480 V to 208Y/120 V draws 90.2 A on the primary and delivers 208.2 A on the secondary at full load.

Transformer calculator: kVA and full-load amps

Work out the full-load current on the primary and secondary of a single-phase or three-phase transformer, or the standard kVA rating a load needs. Pick a quick start or enter your own transformer; the answer updates as you type.

The calculator needs JavaScript. The formulas and the standard ratings it uses are below.

Quick start Fills in the form. Change anything after.

The transformer

System

Three-phaseSingle-phase

Start from

Pick "kVA rating" when you know the transformer and want its currents. Pick "Load current" or "Load in kW" to find the smallest standard rating that carries a load.

kVA ratingLoad currentLoad in kW

Transformer rating (kVA)

The kVA is on the transformer nameplate. Common three-phase sizes are 45, 75, 112.5, 150, 225, 300 and 500 kVA; single-phase pole and pad transformers are often 25, 37.5, 50, 75 or 100 kVA.

The voltages

Primary voltage, line to line (V)

The voltage the transformer is fed at, line to line, for example 480 V for a dry-type unit in a building or 12,470 V for a utility pad-mounted unit. For a single-phase transformer on one phase of a wye line, use the line-to-neutral voltage, such as 7,200 V.

Secondary voltage, line to line (V)

Line to line: for 208Y/120 V enter 208, for 480Y/277 V enter 480.

Result

75 kVA three-phase transformer

At full load, a three-phase transformer of 75 kVA draws 90.21 A on the 480 V primary and delivers 208.2 A on the 208 V secondary.

Transformer rating: 75 kVA
Primary full-load current at 480 V: 90.21 A
Secondary full-load current at 208 V: 208.2 A
Load: kVA (%)

Left: primary. Right: secondary. Values are full-load currents at the nameplate rating. Below, in load mode: the standard ratings around the answer and the load (dashed).

Screening estimate. Full-load current is not the overcurrent protection rating, which NEC 450.3 sets, and transformer sizing also weighs inrush, harmonics, motor starting and fault current. A load study gives the load to size from, or ask an engineer.

Full results and steps

Transformer rating	75 kVA
Primary full-load current (480 V)	90.21 A
Secondary full-load current (208 V)	208.2 A
Voltage ratio	2.308 : 1

Primary: 75 kVA × 1,000 ÷ (1.732 × 480 V) = 90.21 A.
Secondary: 75 kVA × 1,000 ÷ (1.732 × 208 V) = 208.2 A.

Adding load to a transformer, or sizing a new one?

Dreiym's licensed Professional Engineers size transformers from measured or calculated loads and check the fault current, protection and voltage drop that come with them, PE-stamped where required. Free quote; we typically respond within one business day.

Safety and limits

What this does not check: overcurrent protection of the primary and secondary (NEC 450.3), secondary conductors (240.21(C)), the available fault current (see the fault current calculator), K-factor for harmonic loads, inrush, temperature, altitude and the ratings of the specific unit. A transformer can carry more than its nameplate kVA for short periods; that loading is an engineering decision.

Screening tool: Screening estimate for education and planning. Not an engineering study or a design, and not a substitute for a licensed engineer's review where one is required.

How the calculation works

Three-phase full-load current: I = kVA × 1,000 ÷ (1.732 × V), with V the line-to-line voltage of that winding.
Single-phase full-load current: I = kVA × 1,000 ÷ V. For a 120/240 V three-wire secondary, V is 240 V.
Load from current: kVA = 1.732 × V × I ÷ 1,000 (three-phase) or V × I ÷ 1,000 (single-phase). Load from power: kVA = kW ÷ power factor.
With spare capacity, the load is multiplied by (1 + spare ÷ 100) before the smallest standard rating at or above it is picked.
Loading = load kVA ÷ the standard rating × 100.

Standard ratings used

Common standard ratings in kVA. Liquid-filled: IEEE C57.12.00 preferred ratings. Dry-type: the usual NEMA ST 20 ratings.
System	Ratings, kVA
Single-phase	1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500
Three-phase	3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000, 1,500, 2,000, 2,500, 3,750, 5,000, 7,500, 10,000

Sources

Sources: IEEE C57.12.00, Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (preferred kVA ratings); NEMA ST 20, Dry-Type Transformers for General Applications; NFPA 70, National Electrical Code, 2023 edition, 220.87 and 450.3; ANSI C84.1 nominal system voltages.

CopiedResult copied, with a link that keeps your inputs.Could not copy. Copy the address from the address bar instead.Reset to the starting example.InputsScreening estimate, not an engineering study.Line to line: for 208Y/120 V enter 208, for 480Y/277 V enter 480.For a 120/240 V three-wire secondary enter 240.{k} kVA{v} V{a} Aload {k} kVA{k} kVA transformer: {vp} V primary at {ip} A, {vs} V secondary at {is} A.Check {list}., Check the highlighted fieldsCheck the highlighted fields.Larger than the biggest standard rating listed ({m} kVA)The load needs {k} kVA, more than the largest rating in the list ({m} kVA). Use several transformers or a unit built for the job.three-phasesingle-phase{k} kVA {ph} transformerUse a {k} kVA transformerPrimary full-load current at {v} VSecondary full-load current at {v} VAt full load, a {ph} transformer of {k} kVA draws {ip} A on the {vp} V primary and delivers {is} A on the {vs} V secondary.A load of {a} A at {vs} V is {l} kVA.{kw} kW at power factor {pf} is {l} kVA.With {s}% spare capacity: {n} kVA.The next standard rating is {k} kVA, loaded to {p}%. At full load it draws {ip} A at {vp} V and delivers {is} A at {vs} V.{k} kVA: {ip} A primary, {is} A secondary.Transformer ratingPrimary full-load current ({v} V)Secondary full-load current ({v} V)Voltage ratioLoadLoad current at {v} VLoading at the standard ratingNext smaller standard ratingNext larger standard ratingTransformer currentsLoad: 1.732 × {v} V × {a} A ÷ 1,000 = {l} kVA.Load: {v} V × {a} A ÷ 1,000 = {l} kVA.Load: {kw} kW ÷ {pf} = {l} kVA.Spare capacity: {l} kVA × (1 + {s}%) = {n} kVA.Smallest standard rating at or above {n} kVA: {k} kVA.Primary: {k} kVA × 1,000 ÷ (1.732 × {v} V) = {a} A.Primary: {k} kVA × 1,000 ÷ {v} V = {a} A.Secondary: {k} kVA × 1,000 ÷ (1.732 × {v} V) = {a} A.Secondary: {k} kVA × 1,000 ÷ {v} V = {a} A.{v}. {ip} A primary, {is} A secondary.the kVA ratingthe load currentthe load in kWthe power factorthe primary voltagethe secondary voltagethe spare capacityEnter the kVA rating.Enter the load current.Enter the load in kW.Enter the power factor, for example 0.9.Enter the primary voltage.Enter the secondary voltage.Enter a number, for example 75.Enter more than 0 and up to 100,000 kVA.Enter more than 0 and up to 100,000 A.Enter more than 0 and up to 100,000 kW.Enter more than 0 and up to 1.Enter a voltage above 0, line to line, in volts.Enter 0 to 200%.

Cite or share this tool

Dreiym Engineering. Transformer kVA and full-load current calculator. Method: I = kVA × 1,000 ÷ (1.732 × V) three-phase, kVA × 1,000 ÷ V single-phase. https://www.dreiym.com/transformer-calculator/

Instructors and shops are welcome to link to this tool or put its address on a handout. It is free, needs no sign-up and works on a phone.

How to use the calculator

Choose single-phase or three-phase, then start from the kVA rating, the load current or the load in kW. Enter the line-to-line primary and secondary voltages: 208 for a 208Y/120 V secondary, 480 for 480Y/277 V, and 240 for a 120/240 V single-phase secondary. When you start from a load, the calculator picks the smallest standard rating at or above it, with optional spare capacity, and shows how heavily that rating is loaded.

How to find transformer full-load current

Get the kVA. Read it from the nameplate, or work it out from the load: 1.732 × volts × amps ÷ 1,000 for three-phase, volts × amps ÷ 1,000 for single-phase, or kW ÷ power factor.
Use the winding voltage. Use the line-to-line voltage of each winding. For a single-phase transformer on one phase of a wye line, use the line-to-neutral voltage, such as 7,200 V.
Divide. Three-phase: amps = kVA × 1,000 ÷ (1.732 × volts). Single-phase: amps = kVA × 1,000 ÷ volts.
Repeat for the other winding. The currents are in the inverse ratio of the voltages.

Worked examples

75 kVA, 480 V to 208Y/120 V

Primary: 75 × 1,000 ÷ (1.732 × 480) = 90.2 A. Secondary: 75 × 1,000 ÷ (1.732 × 208) = 208.2 A.

Sizing for a 180 A load at 208 V

The load is 1.732 × 208 × 180 ÷ 1,000 = 64.8 kVA. The next standard three-phase rating is 75 kVA, loaded to 86%. With 25% spare capacity the load becomes 81.1 kVA and the answer moves up to 112.5 kVA.

50 kVA pole transformer, 7,200 V to 120/240 V

Primary: 50,000 ÷ 7,200 = 6.94 A. Secondary: 50,000 ÷ 240 = 208.3 A on each line of the 120/240 V service.

Full-load current of common three-phase transformers

kVA	Amps at 208 V	Amps at 480 V
15	41.6	18.0
30	83.3	36.1
45	124.9	54.1
75	208.2	90.2
112.5	312.3	135.3
150	416.4	180.4
225	624.5	270.6
300	832.7	360.8
500	1,387.9	601.4
750	2,081.8	902.1
1,000	2,775.7	1,202.8
1,500	4,163.6	1,804.2

Method and sources

Full-load current is the rated kVA divided by the winding voltage, and by 1.732 (the square root of 3) for three-phase. It is the current at nameplate load, not the inrush current or the fault current. The standard ratings follow the preferred kVA ratings of IEEE C57.12.00 for liquid-filled transformers and the usual NEMA ST 20 ratings for dry-type units. Overcurrent protection is a separate step under NEC 450.3. The formulas and the list of ratings are under the calculator.

Frequently asked questions

How many amps is a 75 kVA transformer?

At 208 V three-phase, 208.2 A; at 480 V three-phase, 90.2 A. A 75 kVA single-phase transformer at 240 V carries 312.5 A.

Can a transformer run at 100% of its rating?

A transformer is rated to carry its nameplate kVA continuously in its rated ambient. Many designs leave room for growth and for the extra heating of harmonic loads. Loading above the nameplate is a judgment for the specific unit and its temperature rise.

Why is the secondary current higher than the primary?

The power is the same on both sides, less a small loss, so the lower-voltage side carries proportionally more current: 480 ÷ 208 = 2.31 times as much in the 75 kVA example.

Does the calculator size the breaker?

No. NEC 450.3 sets the largest primary and secondary protection, which depends on the voltage, the impedance and whether there is secondary protection. The secondary conductors follow 240.21(C).

When to bring in an engineer

A transformer is sized from a reliable load, and a new transformer changes the fault current and the protection downstream. Dreiym’s licensed Professional Engineers measure and calculate loads and size transformers, feeders and protection together, PE-stamped where required. Related tools: the available fault current calculator, which starts from the kVA and %Z, and the wire size calculator. See our electrical load analysis and short circuit analysis services, or contact us for a free quote. We typically respond within one business day.

Disclaimer: general information, not engineering advice

This article is published for general educational purposes only. It is not engineering advice, not a professional opinion on any specific project, and not a substitute for a site-specific investigation; reading it creates no engineer-client relationship. Your project may be different, and will need its own investigation and review. Site conditions, equipment, installation history, and the codes and standards in force (including which editions your jurisdiction has adopted) vary from project to project and change over time, so an approach that is correct in one case may be unsafe or non-compliant in another. Nothing here is a sealed or stamped engineering document. Before relying on any of it, consult a professional engineer licensed in your jurisdiction to evaluate your specific facts and conditions.

More engineering tools

All calculators

Arc flash calculatorIEEE 1584-2018 Fault current calculatorPoint-to-point Voltage drop calculatorNEC Chapter 9 Conduit fill calculatorNEC Chapter 9 Galvanic corrosion chartAnodic index Circuit simulatorIn the browser Wire size calculatorNEC 310.16 Motor circuit calculatorNEC Article 430 Cathodic protection calculatorCurrent and anode life

Need more than a screening estimate?

We typically respond within one business day.

Request a free quoteor call (866) 621-6920