Transformer Sizing
Required kVA from load and voltages.
Example
You enter
- Load (kW) 90
- Power factor 0.9
- Primary voltage (V) 480
- Secondary voltage (V) 208
- Phase three
You get
- Required kVA 100
- Next standard kVA 112.5
Checked against published worked examples from: Alfa Transformer.
Details, formula, and sources
Formula or tablekVA = kW / power factor, rounded up to the next ANSI/IEEE C57 standard size; full-load current = kVA x 1000 / (√3 × V_LL) three-phase or kVA x 1000 / V single-phase, on each winding. Overcurrent protection (NEC 450.3) is not computed here.
Source and editionNEC 2023 (NFPA 70). Article 450 by name; the standard kVA steps per ANSI/IEEE C57. Overcurrent protection per Section 450.3, Table 450.3(B), is a separate step this tile does not compute.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesload_kW, power_factor, primary_V, secondary_V, phase, required_kVA, next_standard_kVA
- Voltage-tolerance band ANSI C84.1 ±5%ANSI C84.1 by name