Transformer kVA sizing equation
Transformer sizing starts by converting the connected load to kVA, then applies demand, growth, loading margin, and ambient derating.
where:
- Sconnected
- Connected apparent load[kVA]
- P
- Connected real power[kW]
- PF
- Power factor
- hp
- Motor horsepower input
- η
- Motor efficiency as a decimal
- D
- Demand factor as a decimal
- G
- Future growth allowance as a decimal
- Ltarget
- Target transformer loading as a decimal
- Kambient
- Ambient temperature derating factor
- Sreq
- Required transformer rating before standard rounding[kVA]
- Sstd
- Selected standard transformer rating[kVA]
- Isec
- Secondary full-load current[A]
- Vsec
- Secondary voltage[V]
For single-phase transformers, the current equation removes √3. Final selection should also check impedance, inrush, harmonics, cooling class, and enclosure conditions.
Assumptions
- Ambient derate uses an approximate 1% capacity reduction per C above 30 C
- Target loading represents normal operating margin, commonly 75-80%
- Standard rating series is a planning reference
Important Warnings
- Final transformer selection must check inrush, impedance, harmonics, cooling class, enclosure, temperature rise, and applicable standards.
- Do not use this as a certified transformer specification.
FAQ
Why divide by target loading?
A transformer intended to run at 80% normal loading needs a nameplate rating larger than the design load.
Why does ambient temperature increase the size?
Hot ambient conditions reduce transformer cooling capacity, so the required nameplate kVA increases.