Transformer Sizing Calculator
Enter the load and voltages to get the standard kVA size, primary and secondary full-load amps and protection ratings.
Step-by-step
- Enter valid values to begin.
Standard kVA sizes follow common dry-type and pad-mount ratings (1-phase: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167 kVA; 3-phase: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA). Primary and secondary protection follow NEC 450.3(B) for transformers 1000 V and below with primary-only or primary-plus-secondary protection rules summarised below.
Transformer Sizing: kVA, Full-Load Amps and Overcurrent Protection
Core Engineering Principles
A transformer is rated in kVA, not kW, because the copper and iron inside it heat according to current and voltage, regardless of the power factor of the load. That’s why you divide kW by power factor first. It also means a transformer feeding a poor power-factor load can be overloaded while the wattmeter reads fine. Add up the load in kVA, add margin for growth, and pick the next standard size up. I generally add 20–25% for growth on a building and I never size to exactly 100%.
Full-load amps follow directly from the kVA and voltage: I = kVA × 1000 / V for single-phase, and divide by 1.732 as well for three-phase. You need that current on both sides. The primary current sizes the primary protection and feeder, the secondary current sizes the secondary conductors and panel. Remember the primary current is smaller because the primary voltage is higher.
Required kVA = Load kVA × (1 + growth %) • OCPD (450.3(B)): primary 125% (167% if under 9 A), secondary 125%
NEC & Standard References
NEC 450.3(B) covers transformers rated 1000 V or less. With only primary protection, the device must be no more than 125% of the primary rated current when the current is 9 A or more, 167% if between 2 and 9 A, and 300% if under 2 A. You can use the next higher standard size from 240.6 if 125% doesn’t land on one. If you also protect the secondary at 125% (167% if under 9 A), the primary device may go up to 250%. 450.9 requires ventilation clearance, 240.21(C) governs secondary conductor tap rules, and 450.13 covers accessibility. Standard 3-phase sizes are 15, 30, 45, 75, 112.5, 150, 225, 300, 500 and 750 kVA.1. Required: 45 × 1.25 = 56.25 kVA, so the next standard size is 75 kVA.
2. Primary FLA = 75,000 / (1.732 × 480) = 90.2 A.
3. Secondary FLA = 75,000 / (1.732 × 208) = 208.2 A.
4. Primary OCPD: 90.2 × 1.25 = 112.8 A → 125 A (next standard).
5. Secondary: 208.2 × 1.25 = 260.3 A → 300 A (next standard). Check that the secondary conductor can be protected by it.
- kVA, not kW. If the loads are mostly motors at 0.8 PF, 45 kW is 56 kVA before you add growth.
- Inrush on energizing. A transformer can draw 8–12 times full-load current for a few cycles; time-delay fuses or breakers with an adjustable instantaneous setting help.
- Harmonics. Nonlinear loads need K-rated transformers or oversizing, and the neutral may need to be doubled.
- Clearances. Dry-type units need air space around them per the manufacturer; stuffing one in a closet is a common cause of overheating.
- Ground the secondary properly. A separately derived system needs its own grounding electrode conductor and bonding jumper per 250.30.