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. 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.
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.
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. U.S. rules set minimum efficiencies of 97.70% to 99.28% for low-voltage dry-type units of 15 to 1,000 kVA made before April 23, 2029, measured at 35% load.9 Units made from that date must reach 98.31% to 99.42%.9 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.
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 to 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.2 If the secondary voltage is lower than the primary, it is a step-down transformer.2 If it is higher, it is a step-up transformer.2 If both are the same, the voltage does not change, but the transformer can still serve as an isolation transformer, used to separate circuits, for example to keep noise out of electronic circuits.2
kVA and Full Load Amps
Transformer nameplates use kVA (kilovolt-amperes). This is the apparent power the unit can deliver.1 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.
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.1
Why Primary Amps Are Lower
Power stays about the same on both sides. So when voltage goes down, current goes up.2 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.2 U.S. minimum efficiencies for low-voltage dry-type units of 15 to 1,000 kVA run from 97.70% to 99.28% at 35% load, and from 98.31% to 99.42% for units made from April 23, 2029.9 So the primary must supply a little more kVA than the load actually uses.2
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).5 The breaker or fuse size is a percent of the transformer's rated current.5 The rated current comes from the transformer's kVA rating and the winding voltage, using the formulas above.2 The limits are:
- Primary protection only: 125% if the rated primary current is 9A or more, 167% if it is 2A to 9A, and 300% if it is under 2A.5
- Primary and secondary protection: the primary may go up to 250%, and the secondary uses 125% (9A and up) or 167% (under 9A).5
Every percentage in the table is a maximum. Where 125% of the current does not match a standard device size, the next size up is allowed.5 The larger percentages are ceilings, so you round down. Standard fuse and breaker ratings are 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A and on up to 6000A, and fuses also come in 1, 3, 6 and 601A.6 When the secondary is also protected, a transformer with coordinated thermal overload protection from its maker that cuts off the primary current may have primary protection up to six times its rated current at 6% impedance or less, or four times at over 6% up to 10%.5 A transformer installed as a motor control circuit transformer under NEC 430.72(C) is protected under that section instead of the table.5
Conductor Sizing
Wires feeding and leaving a transformer are normally sized for at least 125% of the full load amps. Feeder conductors must carry at least the noncontinuous load plus 125% of the continuous load.8 The breaker or fuse ahead of a conductor may not be rated above the conductor's ampacity, except that up to 800 A, where the ampacity falls between two standard ratings, the next standard rating up is allowed.12 The conductors on a transformer's secondary get no such allowance.12 Where they end at a breaker or fuse, their ampacity may not be less than its rating.12 This calculator uses 75°C copper ampacity from NEC Chapter 3 tables. Equipment for circuits rated 100 A or less, or marked for 14 AWG through 1 AWG wire, uses the 60°C ampacity unless it is listed and identified for higher-rated conductors.11 Real jobs may need more adjustment for high heat, many wires in one conduit, long runs with voltage drop, or aluminum conductors.4 Small copper wires also carry a protection limit: the overcurrent protection may not exceed 15 A for 14 AWG, 20 A for 12 AWG or 30 A for 10 AWG.4
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 250.30.7
These results are a planning tool. A licensed electrical engineer or electrician should confirm the final design and check your local code.