Transformer kVA Sizing and FLA

Total connected kVA + ANSI/IEEE step-series recommendation + primary / secondary FLA.

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Example

You enter

Loads kVA 25; kVA 18; watts 7500, pf 0.85; kVA 15
Primary V 480
Secondary V 208
Phase three
Future-growth reserve % 25

You get

Connected kVA 66.82
Required (with reserve) 83.53 kVA
Recommended transformer 112.5

Checked against published worked examples from: NFPA / ANSI/IEEE C57.

Details, formula, and sources

Formula or tableTotal connected kVA = Σ load_i; required kVA = connected × (1 + reserve%). Recommended size from the ANSI/IEEE C57 standard step series (three-phase 15 / 30 / 45 / 75 / 112.5 / 150 / 225 / 300 / 500 / 750 / 1000; single-phase 10 / 15 / 25 / 37.5 / 50 / 75 / 100 / 167 / 250 / 333 / 500). FLA = kVA × 1000 / (V × √phases).

Source and editionANSI/IEEE C57 standard kVA step series by name; NEC 2023 (NFPA 70). Article 450 (Transformers).

Free accessFree read-only access at nfpa.org/freeaccess. ANSI/IEEE C57 licensed; step-series values free in carrier engineering literature.

What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

API field namesloads, primary_V, secondary_V, phase, growth_reserve_pct, connected_kVA, required_kVA, recommended_kVA

Default future-growth reserve 25% unless user suppliesengineering practice for general-purpose service transformers
Three-phase factor √3 ≈ 1.732physical fact