A transformer kVA calculator for sizing and full-load current
This transformer kVA calculator does the three jobs that come up when you choose or check a distribution transformer: it sizes a transformer from your load, it turns volts and amps into kVA, and it turns a kVA rating into full-load amps on the primary and secondary. It works for single-phase and three-phase supplies and always shows its working.
For sizing, enter the connected load in kW (with its power factor) or kVA, your demand or diversity factor, an allowance for future growth and the maximum loading you are willing to run at. The calculator then picks the next size up from a list of standard ratings: Indian ratings from IS 1180 (Part 1):2014 for 11–33 kV distribution transformers, US ratings from the Department of Energy efficiency rules in 10 CFR 431.196, or your own list.
The demand factor, growth and loading limit are design judgements, not fixed rules, so they are inputs you control. Treat the answer as a planning estimate and confirm the final choice with the utility, the supplier and a qualified electrical engineer.
How to use the Transformer kVA Sizing Calculator
- Choose Three-phase or Single-phase and what to calculate: size from load, kVA from volts and amps, or amps from kVA.
- Pick the standard ratings list for your country, or choose Custom and type the sizes your supplier offers.
- For sizing, enter the load, its unit and power factor, your demand factor, growth percentage and maximum loading.
- Enter the secondary voltage (433 V, 415 V, 480 V, 240 V…) and, for primary current, the HV voltage such as 11 kV.
- Read the recommended rating, its loading and both full-load currents, then copy the working.
Transformer kVA Sizing Calculator features
Next standard size
Rounds the required kVA up to a real rating rather than an odd number no one makes.
Both windings
Primary and secondary full-load amps for the chosen rating, and a table for every rating in the list.
Three jobs in one
Transformer sizing, kVA from V and I, and transformer kVA to amps.
Your assumptions, visible
Demand factor, growth and loading limit are labelled inputs and appear in the working.
Indian and US ratings
IS 1180 and 10 CFR 431.196 lists, read 24 October 2026, plus a custom list.
Transformer kVA formulas
Apparent power is kVA = V × I ÷ 1,000 for a single-phase transformer and kVA = √3 × VL × I ÷ 1,000 for three-phase, where VL is the line-to-line voltage. Turned around, the full-load current is I = kVA × 1,000 ÷ V (single-phase) or kVA × 1,000 ÷ (√3 × VL).
For sizing the calculator works out: required kVA = (load kW ÷ power factor) × demand factor × (1 + growth) ÷ maximum loading. Example: 150 kW of load at a power factor of 0.85 is 176.5 kVA; with a demand factor of 0.8 it becomes 141.2 kVA, and 20% growth gives 169.4 kVA. The next IS 1180 rating is 200 kVA, whose full-load current is 266.7 A at 433 V and 10.5 A at 11 kV. On the US list the same load needs 225 kVA.
| Rating | 3-phase at 433 V | 3-phase at 480 V | 1-phase at 240 V |
|---|---|---|---|
| 25 kVA | 33.3 A | 30.1 A | 104.2 A |
| 100 kVA | 133.3 A | 120.3 A | 416.7 A |
| 250 kVA | 333.3 A | 300.7 A | 1,041.7 A |
| 1,000 kVA | 1,333.4 A | 1,202.8 A | — |
Standard transformer ratings used here
- India, three-phase (IS 1180 Part 1:2014, Amendment 4): 16, 25, 63, 100, 160, 200, 250, 315, 400, 500, 630, 1,000, 1,250, 1,600, 2,000 and 2,500 kVA, with a 433 V secondary. The standard also lists 6.3, 10, 20 and 40 kVA as non-preferred.
- India, single-phase: 5, 10, 16 and 25 kVA preferred; 50, 75 and 100 kVA non-preferred.
- US (10 CFR 431.196): single-phase 10 to 833 kVA (10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500, 667, 833) and three-phase 15 to 2,500 kVA, with 3,750 and 5,000 kVA in the table for units made from April 2029.
- Large motors, welding sets, non-linear loads and high ambient temperatures may justify a bigger unit or a K-rated transformer.
Transformer kVA Sizing Calculator FAQ
How do I calculate transformer kVA?
Multiply volts by amps and divide by 1,000; for three-phase multiply by √3 as well. 415 V × 200 A × 1.732 ÷ 1,000 = 143.8 kVA, so the next standard Indian rating is 160 kVA.
How do I size a transformer for my load?
Convert the load to kVA (kW ÷ power factor), apply a demand factor for loads that do not run together, add growth, divide by the loading you are comfortable with, then round up to a standard rating. This transformer sizing calculator shows each step.
How do I use a transformer kVA to amps calculator for 3-phase?
Amps = kVA × 1,000 ÷ (1.732 × line voltage). A 200 kVA, 11 kV/433 V transformer carries 266.7 A on the 433 V side and 10.5 A on the 11 kV side at full load.
What demand factor should I use?
It depends on the installation and local rules. Use measured demand where you have it; otherwise follow your electrical code, utility guidelines or a designer’s judgement. The calculator leaves it as an input for that reason.
Should a transformer run at 100% load?
It is rated to deliver its kVA continuously under standard conditions, but many designers plan for 70–80% to leave room for growth and reduce losses and temperature. Set the maximum loading to suit.
What is the difference between kVA and kW for a transformer?
Transformers are rated in kVA because heating depends on current, whatever the power factor. kW = kVA × power factor, so a 100 kVA transformer delivers about 85 kW at 0.85 PF.