Transformer Sizing Calculator

Q: Why is one of my input boxes shaded and marked Calculated?

That box is the one the tool is solving. Enter any two of secondary voltage , FLA , and kVA , and the third one fills itself in. If you type in the shaded box, the tool switches and solves a different one instead.

Q: Should I size the transformer from the load kVA or the input kVA?

Use the load kVA on the secondary side. That is what the transformer must deliver. The input kVA is bigger only because of losses, and it is used to find primary amps and primary breaker size.

Q: For three-phase, do I enter line-to-line or line-to-neutral voltage?

Enter line-to-line voltage, like 480 V or 208 V. The 1.732 factor in the formula already accounts for three-phase. Using 277 V or 120 V by mistake will give wrong amps.

Transformer Sizing Calculator

Updated Aug 6, 2026 By Jehan Wadia

Introduction

This transformer sizing calculator helps you pick the right transformer for your job. Enter the primary voltage, the secondary voltage, and either the load kVA or the full load amps (FLA). The tool solves for the missing value and shows you the transformer size you need.

You can size single-phase (1Ø) or three-phase (3Ø) transformers. The calculator gives you:

Load kVA and the next standard transformer size
Primary and secondary full load amps
Turns ratio and transformer type (step-down, step-up, or isolation)
Fuse or breaker sizes using NEC 450.3(B)
Copper wire sizes at 75°C

It also adjusts for transformer losses, so the primary side numbers match real-world use. Every answer comes with the math, step by step, so you can check the work. Charts at the bottom compare primary and secondary current and show how the kVA numbers stack up.

Use this tool to plan a job, check a design, or study for a test. If you need to work the voltage ratio and winding turns by themselves, try the Transformer Calculator. For quick current conversions, the kVA to Amps Calculator and Amps to kVA Calculator handle those in one step. Always have a licensed engineer or electrician review the final design before you build.

How to use our Transformer Sizing Calculator

Enter your transformer voltages plus either the load kVA or the full load amps. The calculator gives you the transformer type, turns ratio, primary and secondary current, the right standard kVA size, NEC 450.3(B) breaker or fuse sizes, copper wire sizes, and a step-by-step solution.

Phase Selection: Pick Single-Phase (1Ø) or Three-Phase (3Ø) to match your power system. Each tab saves its own numbers. If you are working out balanced three-phase loads first, the 3 Phase Power Calculator is a good starting point.

Size From: Choose "Known kVA" if you know the load in kVA. Choose "Known FLA + V" if you only know the amps and voltage. This tells the tool which value to solve for you. The kVA Calculator can help you build that kVA figure from a panel schedule.

OCPD Protection Scheme: Pick "Primary-Side Protection Only" if one breaker or fuse feeds the transformer. Pick "Primary & Secondary Protection" if both sides have a device.

Transformer Efficiency (%): Type how efficient your transformer is. Most units run between 95% and 99%. Leave it at 98% if you are not sure.

Primary Voltage (V): Enter the supply voltage feeding the transformer, like 480 V or 240 V. This is the utility or panel side.

Secondary Voltage (V): Enter the output voltage your equipment needs, like 208 V or 120 V. Leave it blank to let the tool find it from kVA and amps.

Full Load Amps (FLA): Enter the amps your load pulls on the secondary side. Small decimal amps are fine for control transformers. Leave it blank to have it solved for you. If you only know the wattage, the Watts to Amps Calculator converts it fast.

Load kVA: Enter the total load in kVA. Fill in any two of secondary voltage, FLA, and kVA, and the tool solves the third one.

Calculate and Reset: Results update as you type, but you can click Calculate to refresh them. Click Reset Calculator to clear your entries and start over.

Transformer Sizing: What It Means

A transformer changes voltage. Power comes in one side and leaves the other side at a different voltage. Sizing a transformer means picking one big enough to carry your load without overheating, and then picking the right breakers and wires for it.

Primary and Secondary Sides

The primary side is the input. It connects to the utility or an upstream panel. The secondary side is the output. It feeds your equipment. If the secondary voltage is lower than the primary, it is a step-down transformer. If it is higher, it is a step-up transformer. If both are the same, it is an isolation transformer, used to separate circuits for safety or noise control.

kVA and Full Load Amps

Transformer nameplates use kVA (kilovolt-amperes). This is the apparent power the unit can deliver. FLA (full load amps) is the current your load pulls when it runs at full power. Voltage, current, and kVA are tied together, so if you know two of them you can find the third. The Amp Calculator and Ohms Law Calculator cover the same relationships for simpler circuits.

Single-phase: kVA = (Volts × Amps) ÷ 1000
Three-phase: kVA = (Volts × Amps × 1.732) ÷ 1000

Three-phase power uses the √3 factor (about 1.732). That is why a three-phase transformer at the same voltage and current carries more kVA than a single-phase one. If your load is listed in kilowatts instead, the kW to Amps Calculator and Power Factor Calculator bridge the gap between kW and kVA.

Why Primary Amps Are Lower

Power stays about the same on both sides. So when voltage goes down, current goes up. A 480V primary feeding a 208V secondary will show low amps on the primary and high amps on the secondary. Transformers are not perfect, though. A few percent of the power turns into heat. Typical units run at 95% to 99% efficiency, so the primary must supply a little more kVA than the load actually uses.

Standard Transformer Sizes

You cannot buy any kVA number you want. Transformers come in set sizes, and you round up to the next one. Common ratings include:

1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 45, 50, 75, 100, 112.5, 150, 167, 225, 300, 500, 750, and 1000 kVA.

Overcurrent Protection (NEC 450.3(B))

Transformers rated 1000V or less follow NEC Table 450.3(B). The breaker or fuse size is a percent of the winding's full load current:

Primary protection only: 125% if primary FLA is 9A or more, 167% if FLA is 2A to 9A, and 300% if FLA is under 2A.
Primary and secondary protection: the primary may go up to 250%, and the secondary uses 125% (9A and up) or 167% (under 9A).

The 125% number is a minimum, so you round up to the next standard device. All the larger percentages are ceilings, so you round down. Standard breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A and on up.

Conductor Sizing

Wires feeding and leaving a transformer are normally sized for at least 125% of the full load amps. This calculator uses 75°C copper ampacity from NEC Chapter 3 tables. Real jobs may need more adjustment for high heat, many wires in one conduit, long runs with voltage drop, or aluminum conductors. Check your feeders with the Wire Size Calculator, confirm the run length with the Voltage Drop Calculator, and verify raceway capacity with the Conduit Fill Calculator.

Things to Check Before You Buy

Match the nameplate voltage to the real system voltage, including 208V versus 240V and 480V versus 600V.
Add room for future load. Many designers leave 20% to 25% spare capacity.
Large motors draw big starting current, so check inrush.
Computers, LED drivers, and VFDs make harmonics. These loads may need a K-rated transformer.
Grounding and bonding of a separately derived system follow NEC 250.30.
If a backup source feeds the same load, size it with the Generator Sizing Calculator, and price the running load with the Electricity Cost Calculator.

These results are a planning tool. A licensed electrical engineer or electrician should confirm the final design and check your local code.

Formulas used

Phase multiplier

m = \begin{cases} \sqrt{3} \approx 1.7321 & \text{three-phase} \\ 1 & \text{single-phase} \end{cases}

Load kVA from secondary FLA and voltage

\text{kVA} = \frac{I_{sec} \times V_{sec} \times m}{1000}

Secondary full load amps

I_{sec} = \frac{\text{kVA} \times 1000}{V_{sec} \times m}

Required input kVA adjusted for efficiency

\text{kVA}_{in} = \frac{\text{kVA}}{\eta / 100}

Primary full load amps

I_{pri} = \frac{\text{kVA}_{in} \times 1000}{V_{pri} \times m}

Turns ratio

\frac{N_p}{N_s} = \frac{V_{pri}}{V_{sec}}

Overcurrent device rating per NEC 450.3(B)

\text{OCPD} = I_{FLA} \times \frac{\%}{100}, \quad \% \in \{125,\ 167,\ 250,\ 300\}

Minimum conductor ampacity at 125% of FLA

I_{cond} = I_{FLA} \times 1.25

Frequently asked questions

Why does the calculator show a bigger transformer than my load kVA?

Transformers are only built in set sizes. The tool rounds your load up to the next standard rating. If your load is 38 kVA, you get a 45 kVA unit, because a 37.5 kVA unit would be too small.

Why is one of my input boxes shaded and marked Calculated?

That box is the one the tool is solving. Enter any two of secondary voltage, FLA, and kVA, and the third one fills itself in. If you type in the shaded box, the tool switches and solves a different one instead.

What happens if I fill in all three of voltage, FLA, and kVA?

You get a warning. The tool keeps solving one field, so your typed value there gets replaced. Clear one box so the math stays correct.

Should I size the transformer from the load kVA or the input kVA?

Use the load kVA on the secondary side. That is what the transformer must deliver. The input kVA is bigger only because of losses, and it is used to find primary amps and primary breaker size.

For three-phase, do I enter line-to-line or line-to-neutral voltage?

Enter line-to-line voltage, like 480 V or 208 V. The 1.732 factor in the formula already accounts for three-phase. Using 277 V or 120 V by mistake will give wrong amps.

Can I use this for medium-voltage transformers like 4160 V or 13.8 kV?

The kVA and amp math still works. But the breaker and fuse rules shown here come from NEC 450.3(B) for transformers 1000 V and under. Higher voltage units follow NEC 450.3(A) and need an engineer.

Does this work for autotransformers or buck-boost units?

No. This tool assumes separate primary and secondary windings. Autotransformers share windings and are sized by the amount of voltage change, not the full load kVA.

Can I use it for a small control transformer?

Yes. Fractional amps work fine, like 0.5 A. Small units often fall under the 167% or 300% breaker rules because primary amps are low.

Why is my primary breaker allowed to be 300% of the amps?

Very small primary currents, under 2 A, need room for inrush when the transformer energizes. NEC lets you go up to 300% so the breaker does not trip on start-up. That 300% is a ceiling, so you round down.

The result says no listed device. What does that mean?

It means every standard breaker size is larger than your allowed ceiling. You need a smaller special fuse or a different protection plan. Ask a licensed electrician.

Do I still need a secondary breaker if I pick primary-only protection?

NEC 450.3(B) lets you protect the transformer from the primary side alone. But the panel or feeder on the secondary still needs its own overcurrent protection under NEC 240 and 408. Transformer protection and feeder protection are two different jobs.

Are the wire sizes for aluminum too?

No. The tool uses 75°C copper ampacity. Aluminum carries less current, so you need a larger size. Check aluminum runs with the Wire Size Calculator.

Does the calculator check voltage drop?

No. It sizes wire on ampacity only. Long runs may need a bigger wire. Use the Voltage Drop Calculator to check the run length.

Why does the 1-phase and 3-phase reference box show two answers?

It shows the same volts and amps worked both ways so you can compare. Only the row marked ACTIVE is used in your results. The other row is just for reference.

My load is in kW, not kVA. What do I enter?

Divide kW by the power factor to get kVA. For example, 40 kW at 0.9 power factor is about 44.4 kVA. The Power Factor Calculator and kW to Amps Calculator can help.

What is the turns ratio used for?

It shows how the voltage changes between windings. A 480 V to 240 V unit has a 2:1 ratio. It also tells you the current ratio, flipped: the low-voltage side carries twice the amps.

My load is over 1000 kVA. Why is there no standard size?

The list in this tool stops at 1000 kVA. Larger units are still made, but they are ordered from a manufacturer. You can also split the load between two transformers.

Should I add spare capacity to my entry?

Yes, if you plan to grow. Enter your future load, not just today's load. Many designers add 20% to 25% so the transformer is not maxed out later.

Does the tool handle motor starting current?

No. It sizes for running load only. Big motors can pull six times their run amps when starting, which can dip the voltage. Have an engineer check inrush on motor-heavy jobs.

Why do my numbers reset when I switch between 1-phase and 3-phase?

They do not reset. Each phase tab keeps its own values while you stay on the page. Switch back and your old entries are still there.

Can I use these results for a permit or final design?

Use them for planning, checking, or studying. A licensed electrical engineer or electrician must confirm the final design and your local code before you build.

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