What is transformer KVA and why does the size matter?

Transformer KVA (kilovolt-amperes) is the apparent power rating — the maximum load the transformer can continuously supply. Undersizing causes overheating and premature failure; oversizing wastes money. The correct transformer size equals the total connected load kVA multiplied by a safety margin (typically 125%).

What does NEC 450 say about transformer sizing?

NEC Article 450 covers transformer installation requirements, including overcurrent protection per NEC 450.3. For 600V and below, the primary OCPD must not exceed 125% of the primary full-load current when primary current is 9 A or greater. Transformer sizing itself follows load calculations under NEC Article 220.

How do I use this transformer sizing calculator?

Select single-phase or three-phase, enter secondary voltage, then input either load current (A) or load power (kW or kVA). Set a safety margin and optionally enter primary voltage to get primary FLA and OCPD sizing. The calculator outputs the calculated kVA, minimum required kVA with margin, and the correct standard transformer size to specify.

What is the difference between single-phase and three-phase transformer sizing?

Single-phase transformer kVA = (V × I) / 1000. Three-phase transformer kVA = (1.732 × V × I) / 1000, where V is line-to-line voltage. Three-phase transformers have different standard sizes (3, 6, 9, 15, 30, 45, 75, 112.5 kVA...) compared to single-phase (0.5, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25 kVA...).

What are the standard transformer KVA ratings?

Standard single-phase transformer sizes include: 0.5, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA. Standard three-phase sizes include: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 kVA. Always size up to the next standard rating above the required kVA.

How do I calculate transformer full load amps (FLA)?

Single-phase FLA = (kVA × 1000) / V. Three-phase FLA = (kVA × 1000) / (1.732 × V). Calculate FLA for both primary and secondary windings using their respective voltages. Primary FLA is used to size the primary OCPD per NEC 450.3.

Frequently asked questions about transformer sizing

Common transformer sizing questions: Apply a 125% safety margin (or 25% above calculated load) as a minimum. For non-linear loads (VFDs, computers, UPS systems), apply additional K-factor derating — typically size up by 20–30% extra. NEC 450.3(B) sets primary OCPD at 125% FLA for conductors rated ≥9 A. IEC 60076 governs transformer standards internationally.

⚡ Electrical Calculator

Transformer Sizing Calculator

NEC 450 · IEC 60076 · KVA Sizing · Full Load Amps · OCPD Guidance

⚙️ System Configuration

System Type

Secondary Voltage (V)

📋 Load Input

Load Current (A)

Power Factor (optional, for kW display)

📐 Sizing Options

Safety Margin (%)

Primary Voltage (V) — for FLA & OCPD

Primary voltage is optional — used to calculate primary full-load amps and NEC 450.3 OCPD sizing.

Please fill in all required fields.

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Calculated Load
(kVA)

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With Margin
(kVA)

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Standard Size
to Specify (kVA)

📊 Load Breakdown

Calculated load —

Safety margin (+25%) —

Minimum required —

Next standard transformer size ↓ —

Headroom above minimum —

Standard Sizes — Nearest Options

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Single-phase: kVA = (V × I) / 1,000 | Three-phase: kVA = (√3 × V × I) / 1,000
From kW: kVA = kW / PF | FLA = (kVA × 1,000) / V (1Φ) or (kVA × 1,000) / (√3 × V) (3Φ)
NEC 450.3(B): Primary OCPD ≤ 125% × primary FLA (for FLA ≥ 9 A). Standard sizes per NEC 240.6(A).
For non-linear loads (VFDs, UPS, computers), apply additional K-factor derating — consult transformer manufacturer.
Results are indicative. Always verify with a licensed electrical engineer before specifying equipment.

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Transformer Sizing Calculator

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