Power Calc Toolsv0.2 · beta

Transformer Sizing Calculator

Standard kVA selection from load and growth margin, primary and secondary full-load amps, available fault current at the secondary with an AIC / SCCR check, and NEC 450.3(B) overcurrent protection.

Assumptionsinfinite primary source·bolted 3Ø fault at secondary terminals·ANSI standard kVA ratings·450.3(B) for ≤ 1000 V
One-line
Live · updates with inputs
SOURCE480 V350 A270.6 A FLA225 kVA4.50 %Z · 3Ø800 A624.5 A FLASECONDARY208 VAFC 13.9 kA≤ 22 kA rating
Inputs
Common systems
Transformer
225kVA3Ø
67 % loaded today83 % with growth
Fault current ≤ 22 kAPass
Primary FLA270.6 A
Secondary FLA624.5 A
Fault current (4.50 %Z)13.9 kA
Primary only · 125 % (next size up, Note 1)350 A
Primary + secondary · primary ≤ 250 %600 A
Secondary OCPD · 125 % (next size up, Note 1)800 A

BasisFLA = kVA·1000 / (√3·V) · Isc = FLA / %Z, infinite bus, %Z × 0.9 · OCPD per NEC 2023 Table 450.3(B), 240.6(A)

Standard 3Ø ratings · kVA
15304575112.51502253005007501000150020002500300037505000
Related:Cable sizing →Power factor →Load calculator →
Disclaimer. This tool is provided for preliminary sizing and engineering reference only. Results depend on assumptions that may not match your installation. Always verify with a licensed professional engineer and the applicable national wiring standard (NEC, IEC, CEC, AS/NZS, etc.) and local Authority Having Jurisdiction before construction.

Frequently asked questions

How do you size a transformer?

Convert the load to kVA (kVA = kW / PF, or √3 × V × I / 1000 for three-phase), add a growth margin — 20 to 25 % is common — and pick the next standard kVA rating. Standard three-phase ratings are 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 and 2500 kVA; single-phase ratings are 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 and 500 kVA.

How is transformer full-load current calculated?

For three-phase, FLA = kVA × 1000 / (√3 × V_LL). For single-phase, FLA = kVA × 1000 / V. For example, a 225 kVA transformer at 480 V has a primary FLA of 270.6 A and at 208 V a secondary FLA of 624.5 A.

How is the available fault current at the secondary calculated?

Assuming an infinite (zero-impedance) utility source, the bolted three-phase fault current at the secondary terminals is Isc = FLA_secondary / (%Z / 100). This is the worst case for the transformer itself — the real value is lower once source and cable impedance are included. The calculator can also reduce %Z by 10 % to cover the nameplate impedance tolerance, which gives a conservative (higher) fault current for checking equipment interrupting ratings.

What does the AIC / SCCR check mean?

NEC 110.9 requires overcurrent devices to have an interrupting rating at least equal to the available fault current at their line terminals, and 110.10 requires equipment short-circuit current ratings (SCCR) to be coordinated with it. The calculator compares the fault current at the transformer secondary with the rating you enter for the secondary main or panelboard. 110.24 also requires the available fault current to be field-marked on service equipment in other than dwelling units.

How is transformer overcurrent protection sized under NEC 450.3(B)?

For transformers of 1000 V and less, Table 450.3(B) allows either primary-only protection at 125 % of primary FLA (167 % if the current is below 9 A, 300 % below 2 A), or primary protection up to 250 % combined with secondary protection at 125 % of secondary FLA. Where 125 % does not match a standard rating, Note 1 permits the next higher standard size. Secondary conductors also need protection under 240.21(C).