Transformer kVA Sizing and FLA
Total connected kVA + ANSI/IEEE step-series recommendation + primary / secondary FLA.
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
Details, formula, and sources
Total connected kVA = Σ load_i; required kVA = connected × (1 + reserve%). Recommended size from the ANSI/IEEE C57 standard step series (15 / 30 / 45 / 75 / 112.5 / 150 / 225 / 300 / 500 / 750 / 1000). FLA = kVA × 1000 / (V × √phases).
ANSI/IEEE C57 standard kVA step series by name; NEC 2023 (NFPA 70). Article 450 (Transformers).
Free read-only access at nfpa.org/freeaccess. ANSI/IEEE C57 licensed; step-series values free in carrier engineering literature.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: loads, 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