How do I calculate transformer size?

Find the load in kVA: single-phase kVA = V × I ÷ 1000, three-phase kVA = 1.732 × V × I ÷ 1000, or kVA = kW ÷ power factor. Multiply by 1.25 if the load is continuous (3 hours or more), add a growth allowance, and choose the next standard size. Example: 100 A at 480 V three-phase is 83.1 kVA; with 25% growth it is 103.9 kVA, so a 112.5 kVA transformer is selected.

What size transformer do I need for a kW load?

Divide kW by the power factor to get kVA. A 60 kW load at 0.85 PF needs 70.6 kVA; with 25% growth that is 88.2 kVA, so the next standard three-phase size is 112.5 kVA. If the PF is unknown, 0.8 is a conservative assumption for mixed motor loads.

What are the standard transformer kVA sizes?

Common single-phase ratings are 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 and 833 kVA. Common three-phase ratings are 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3750 and 5000 kVA (ANSI C57.12 preferred ratings; the smallest sizes are dry-type catalog ratings).

How do I calculate transformer full-load amps?

FLA = kVA × 1000 ÷ V for single-phase and kVA × 1000 ÷ (1.732 × V) for three-phase, using the line-to-line voltage of the winding. A 75 kVA three-phase transformer is 90.2 A at 480 V and 208.2 A at 208 V.

Is 125% sizing required for transformers?

The 125% factor comes from NEC rules for continuous loads on feeders and overcurrent devices, and many designers apply it to transformer kVA too. It is not a universal transformer code requirement, so the tool makes it optional. Final sizing follows the NEC Article 220 load calculation, the engineer of record and the manufacturer.

What overcurrent protection does NEC 450.3(B) allow?

For transformers 1000 V or less with primary-only protection: 125% of primary FLA when it is 9 A or more (next standard size up permitted), 167% from 2 to 9 A, 300% below 2 A. With secondary protection, the primary may be up to 250% and the secondary up to 125% (167% below 9 A). Conductor ampacity and the adopted code edition still govern.

Transformer Sizing Calculator

Single-phase kVA = V × I ÷ 1000; three-phase kVA = √3 × V × I ÷ 1000 (or kVA = kW ÷ PF). Multiply by 1.25 for continuous loads and add a growth allowance, then pick the next standard size. Example: 100 A at 480 V three-phase = 83.1 kVA; with 25% growth → 103.9 kVA → 112.5 kVA, 135.3 A secondary.

Load & System Parameters

Circuit Configuration

Single-Phase Three-Phase (Balanced)

Load given as

Load Current (A)

Secondary voltage (V) (single-phase)

Primary voltage (V) optional

Continuous load (3 h or more): size at 125% of the load

Growth allowance (%)

Spare capacity for future load (0–100%). Applied after the continuous-load factor.

Sizing Summary

Enter load parameters and click Calculate.

Transformer full-load amps: three-phase

FLA = kVA × 1000 ÷ (1.732 × V_LL).

kVA	208 V	240 V	480 V	600 V
15	41.6	36.1	18.0	14.4
30	83.3	72.2	36.1	28.9
45	124.9	108.3	54.1	43.3
75	208.2	180.4	90.2	72.2
112.5	312.3	270.6	135.3	108.3
150	416.4	360.8	180.4	144.3
225	624.5	541.3	270.6	216.5
300	832.7	721.7	360.8	288.7
500	1,387.9	1,202.8	601.4	481.1
750	2,081.8	1,804.2	902.1	721.7
1000	2,775.7	2,405.6	1,202.8	962.3
1500	4,163.6	3,608.4	1,804.2	1,443.4

Transformer full-load amps: single-phase

FLA = kVA × 1000 ÷ V.

kVA	120 V	240 V	277 V	480 V
5	41.7	20.8	18.1	10.4
10	83.3	41.7	36.1	20.8
15	125.0	62.5	54.2	31.3
25	208.3	104.2	90.3	52.1
37.5	312.5	156.3	135.4	78.1
50	416.7	208.3	180.5	104.2
75	625.0	312.5	270.8	156.3
100	833.3	416.7	361.0	208.3
167	1,391.7	695.8	602.9	347.9
250	2,083.3	1,041.7	902.5	520.8

Worked examples

Three-phase from amps. 100 A at 480 V: 1.732 × 480 × 100 ÷ 1000 = 83.1 kVA. With 25% growth: 103.9 kVA → 112.5 kVA standard. Secondary FLA = 112,500 ÷ (1.732 × 480) = 135.3 A, so the load is 73.9% of rating.

Continuous kW load, step-down. 60 kW at 0.85 PF, continuous, 0% growth, 480 V to 208Y/120 V: 60 ÷ 0.85 = 70.6 kVA × 1.25 = 88.2 kVA → 112.5 kVA. FLA is 312.3 A at 208 V and 135.3 A at 480 V. Primary-only protection: 125% × 135.3 = 169.1 A → 175 A maximum.

Single-phase. 80 A at 240 V = 19.2 kVA; with 25% growth 24.0 kVA → 25 kVA, FLA 104.2 A.

Standard ratings follow ANSI/IEEE C57.12.00 preferred kVA ratings and common dry-type catalogs; three-phase ratings above 1500 kVA are usually liquid-filled pad-mount or substation units. Not a substitute for an engineered load calculation.

Summary

This calculator converts a load given in amps, in kW with a power factor, or directly in kVA into the apparent power a transformer must supply, applies an optional 125% continuous-load factor and a growth allowance, and selects the next standard ANSI/NEMA kVA rating. It then reports secondary and primary full-load current, percent loading, spare capacity, and the maximum overcurrent device ratings allowed by NEC Table 450.3(B) for transformers rated 1000 V or less.

How it works

Choose single-phase or balanced three-phase, and how you know the load: amps, kW with power factor, or kVA.
Enter the secondary (load-side) voltage and, optionally, the primary voltage for primary full-load current.
Tick "continuous load" to size at 125%, and set a growth allowance (default 25%).
Load kVA = V × I ÷ 1000 (single-phase) or √3 × V × I ÷ 1000 (three-phase); from kW, kVA = kW ÷ PF.
Required kVA = load kVA × 1.25 (if continuous) × (1 + growth %). The next standard rating at or above it is selected, and FLA = kVA × 1000 ÷ V (÷ √3 for three-phase).

Use cases

Preliminary sizing of a dry-type distribution transformer for a panel or building addition.
Finding the kVA needed for a kW load at a given power factor.
Getting primary and secondary full-load amps for a 480–208Y/120 V step-down transformer.
Checking how loaded an existing transformer is and how much spare capacity remains.
Estimating maximum primary and secondary overcurrent device sizes under NEC 450.3(B).

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu