About Transformer Sizing Calculator
The transformer sizing calculator determines how large a distribution transformer must be to serve a given electrical load. It converts the load to apparent power in kVA, applies a growth or spare-capacity factor, and then selects the next standard ANSI/IEEE C57.12 rating at or above the required capacity so the transformer is neither overloaded nor grossly oversized.
Enter the load as a current, a real power in kW with its power factor, or directly as kVA, and pick single-phase or three-phase. The tool reports the connected load in kVA, the required capacity after the growth factor, the recommended standard size, the primary and secondary full-load currents, and the resulting percent loading and spare capacity of the chosen transformer.
How It Works
- Choose single-phase or three-phase and how you will specify the load (current, kW, or kVA).
- For current, the tool computes apparent power: three-phase kVA = sqrt(3) * V_LL * I / 1000, single-phase kVA = V * I / 1000. For kW input, kVA = kW / PF.
- A growth/spare factor (for example 1.25 for 25% spare) multiplies the connected load to give the required capacity.
- The next standard rating at or above the required kVA is selected, and full-load currents are computed at that rating: I = kVA*1000 / (sqrt(3)*V) for three-phase and kVA*1000 / V for single-phase.
Worked Example
A three-phase 480 V load draws 100 A. The apparent power is kVA = sqrt(3) * 480 * 100 / 1000 = 83.1 kVA. Applying a 25% growth factor gives a required capacity of 83.1 * 1.25 = 103.9 kVA, so the next standard rating is 112.5 kVA. The secondary full-load current at 112.5 kVA is 112500 / (sqrt(3) * 480) = 135.3 A. The connected load uses 83.1 / 112.5 = 73.9% of the transformer, leaving about 26% spare capacity.
Formulas
- Apparent power from current
3ph: kVA = sqrt(3) * V_LL * I / 1000 | 1ph: kVA = V * I / 1000- Apparent power from real power
kVA = kW / PF- Required capacity
required kVA = load kVA * growth factor- Full-load current at the selected rating
3ph: I = kVA*1000 / (sqrt(3)*V) | 1ph: I = kVA*1000 / V- Percent loading
% loading = load kVA / standard kVA * 100
Standards & References
- ANSI/IEEE C57.12.00 — general requirements for liquid-immersed transformers
- IEEE C57.12.01 — dry-type transformers (standard kVA ratings)
- IEC 60076 — power transformers
- NEC (NFPA 70) Article 450 — transformer installation and protection
Frequently Asked Questions
Why is the apparent power in kVA and not kW?
A transformer is limited by current and voltage, i.e. by apparent power (kVA), regardless of power factor. Sizing on kW would undersize the transformer whenever the load power factor is below unity, so transformers are always rated and sized in kVA.
What growth or spare factor should I use?
A common practice is 1.25, leaving the transformer loaded to about 80% of its rating, which provides headroom for load growth and avoids continuous full-load operation. Use a larger factor for loads expected to grow quickly and a smaller one for fixed, well-defined loads.
Where does the sqrt(3) come from in three-phase sizing?
For a balanced three-phase system the total apparent power relates to the line-to-line voltage and line current by S = sqrt(3) * V_LL * I. The sqrt(3) factor accounts for the 120-degree phase displacement between the three phases when line-to-line voltage is used.
Does this calculator size overcurrent protection?
No. It selects the transformer kVA and reports full-load currents. Primary and secondary overcurrent protection must be sized separately per NEC Article 450 (or the applicable code), using the full-load currents reported here as a starting point.