Transformer Sizing Calculator
Size a transformer two ways: from a known kVA, or from the secondary load you need to serve. Get the kVA and the primary and secondary full-load amps for single- or three-phase systems — the two currents that drive conductor and overcurrent-device sizing on each side.
Transformer
Sizing
Three-phase: A = VA ÷ (V × √3). Round up to the next standard transformer size and verify primary/secondary OCPD per NEC 450.3.
Transformer Sizing
Prepared with the NORDIX Transformer Sizing Calculator (nordixhq.com/tools/transformer-sizing-calculator)
| Transformer size | 75 kVA, three-phase |
| Sized from | entered 75 kVA |
| Primary | 480 V — 90.2 A FLA |
| Secondary | 208 V — 208.2 A FLA |
Method: A = VA ÷ (V × √3). Round up to the next standard transformer kVA and verify primary/secondary overcurrent protection per NEC 450.3. Nameplate impedance, temperature rise, and inrush are manufacturer data — confirm before ordering.
How this calculator works
A transformer’s full-load current on each winding is its apparent power divided by the winding voltage — with the √3 factor on three-phase systems because kVA is a three-phase total while amps flow in each line:
three-phase: A = VA ÷ (V × √3) single-phase: A = VA ÷ V
In “secondary load” mode the same identity runs in reverse — kVA = V × A × √3 ÷ 1000 — to find the apparent power a given secondary current demands, which you then round up to the next standard transformer size (three-phase dry-types step 15, 30, 45, 75, 112.5, 150, 225, 300, 500 kVA and up). The full-load amps are what NEC 450.3 protection percentages are applied against, so getting them right is step one of every transformer submittal.
Worked example
A 75 kVA three-phase dry-type transformer, 480 V delta primary to 208Y/120 V secondary — the calculator’s default inputs:
- Primary FLA: 75,000 ÷ (480 × 1.732) = 75,000 ÷ 831.4 = 90.2 A.
- Secondary FLA: 75,000 ÷ (208 × 1.732) = 75,000 ÷ 360.3 = 208.2 A.
- Protection check (NEC Table 450.3(B)): with primary-only protection, 125% × 90.2 = 112.8 A → next standard rating 125 A; with a secondary device too, the secondary maximum is 125% × 208.2 = 260.2 A → 300 A.
Reproduce it above (three-phase, 75 kVA, 480/208), then print the spec line sheet from the results panel.
Reference: standard sizes with maximum protection
The standard three-phase dry-type ladder at 480 V Δ → 208Y/120 V, with full-load amps and the maximum overcurrent devices worked from NEC Table 450.3(B) and the standard ratings of 240.6(A) — the 125% cells round up to the next standard size (Note 1), the 250% primary ceiling rounds down:
| kVA | Primary FLA | Max OCPD — pri. only (125%) | Secondary FLA | Max sec. OCPD (125%) |
|---|---|---|---|---|
| 15 | 18 A | 25 A | 41.6 A | 60 A |
| 30 | 36.1 A | 50 A | 83.3 A | 110 A |
| 45 | 54.1 A | 70 A | 124.9 A | 175 A |
| 75 | 90.2 A | 125 A | 208.2 A | 300 A |
| 112.5 | 135.3 A | 175 A | 312.3 A | 400 A |
| 150 | 180.4 A | 250 A | 416.4 A | 600 A |
| 225 | 270.6 A | 350 A | 624.5 A | 800 A |
| 300 | 360.8 A | 500 A | 832.7 A | 1200 A |
| 500 | 601.4 A | 800 A | 1,387.9 A | 2000 A |
| 750 | 902.1 A | 1200 A | 2,081.8 A | 3000 A |
| 1000 | 1,202.8 A | 1600 A | 2,775.7 A | 4000 A |
Frequently asked questions
How do I calculate transformer full-load amps?
Three-phase: amps = kVA × 1000 ÷ (volts × 1.732). Single-phase: amps = kVA × 1000 ÷ volts. A 75 kVA three-phase transformer at 480 V primary / 208 V secondary runs 90.2 A on the primary and 208.2 A on the secondary — which is why a 75 kVA unit pairs so naturally with a 200 A, 208 V panel. This calculator does both windings at once from either a known kVA or a secondary load in amps.
What size transformer do I need for a 200 A, 208 V three-phase panel?
kVA = volts × amps × 1.732 ÷ 1000 = 208 × 200 × 1.732 ÷ 1000 ≈ 72 kVA, so the next standard size up is 75 kVA. Size from the calculated load on the panel, not its bus rating, when you have a real load calc — a 200 A panel carrying 120 A of computed load only needs a transformer for the load plus growth.
What are the standard transformer kVA sizes?
Common dry-type distribution sizes run 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA for three-phase, and 15, 25, 37.5, 50, 75, and 100 kVA for single-phase. Always round the calculated kVA up to the next standard size — there is no partial credit between steps, and the odd-looking 112.5 and 37.5 are genuine catalog sizes, not typos.
How do I size the primary and secondary breakers for a transformer?
NEC 450.3(B) governs transformers 1000 V and under. With protection on both windings and currents of 9 A or more, the primary device may be up to 250% of primary FLA and the secondary up to 125% of secondary FLA (rounding up to the next standard 240.6(A) rating where 125% doesn't land on one); with primary-only protection the limit is 125%. Note 450.3 protects the transformer — the secondary conductors have their own rules in Article 240. Verify against the code edition adopted in your jurisdiction.
Should I oversize a transformer for future load?
Some margin is cheap insurance — 20–25% above the calculated load is a common planning allowance (a rule of thumb, not a code number), and it also helps with harmonic-rich electronic loads. Don't swing wildly oversized either: transformers burn core losses around the clock whether loaded or not, and a lightly loaded unit wastes energy for its whole service life. For motor-heavy or K-rated applications, confirm with the manufacturer's data.
Method: A = VA ÷ (V × √3) three-phase, A = VA ÷ V single-phase; load mode inverts the same identity. The protection table applies NEC Table 450.3(B) (transformers 1000 V and less, all currents ≥ 9 A) against the 240.6(A) standard ratings — the under-9 A rows (167%/300%) and over-1000 V transformers (Table 450.3(A)) follow different cells. This sizes the transformer, not the installation: nameplate impedance, temperature rise, and inrush are manufacturer data, and secondary conductors have their own rules. Verify against the governing NEC edition — the AHJ governs.
The full protection tables live on the transformer overcurrent protection chart (and the MV chart for over 1000 V). Pair with the Wire Ampacity Calculator for the winding conductors and the Service & Load Calculator for the demand the transformer must serve.