Transformer Guide: Sizing, KVA Ratings & Selection

How Transformers Work

A transformer transfers electrical energy between circuits through electromagnetic induction. The voltage ratio equals the turns ratio:

V_p / V_s = N_p / N_s

Where p = primary, s = secondary, N = number of turns. A transformer with 480 primary turns and 12 secondary turns has a 40:1 ratio — 480V in becomes 12V out.

Key relationships:

• Power conservation: V_p × I_p = V_s × I_s (minus losses)
• Current ratio: I_p / I_s = N_s / N_p (inversely proportional)
• KVA: V × I / 1000 (single-phase), V × I × √3 / 1000 (three-phase)

Common Transformer Sizes

Step-Up vs Step-Down

NEC Transformer Requirements

• Primary overcurrent protection: NEC 450.3 — sized at 125% of primary current for transformers with primary-only protection
• Secondary overcurrent protection: Required for transformers over 9A secondary at 150V or less (NEC 450.3(B))
• Working clearance: NEC 110.26 — 36" minimum clearance in front of transformer
• Ventilation: Transformers generate heat. Dry-type transformers in enclosed spaces need ventilation or fans
• Grounding: NEC 250.30 — separately derived systems must be grounded per code

Worked Example

Scenario: Size a transformer for a small commercial building with 200A at 208Y/120V, 3-phase.

Step 1: KVA = 208 × 200 × √3 / 1000 = 208 × 200 × 1.732 / 1000 = 72.1 KVA

Step 2: For continuous loads (80% rule): 72.1 / 0.80 = 90.1 KVA

Step 3: Select next standard size: 112.5 KVA transformer

Step 4: Primary current at 480V: 112,500 / (480 × 1.732) = 135A

Common Transformer Mistakes

• Not derating for harmonics: Non-linear loads (computers, LED drivers, VFDs) create triplen harmonics that add to the neutral. Derate KVA by 50–70% for K-factor rated transformers.
• Ignoring inrush current: Transformers draw 6–12× rated current for 0.1 seconds at energization. Primary fuses must be time-delay (slow-blow) type.
• Wrong voltage connection: Verify primary voltage matches supply. A 480V transformer on 240V supply delivers half the secondary voltage.

Standards Reference

• NEC Article 450 — Transformers
• NEC 250.30 — Grounding Separately Derived Systems
• NEMA TP-1 — Transformer Efficiency
• UL 5085 — Low Voltage Transformers

Frequently Asked Questions

How do I size a transformer?

Calculate KVA = V × I / 1000 (single-phase) or KVA = V × I × √3 / 1000 (three-phase). For continuous loads, divide by 0.80 (80% rule). Select the next standard KVA size. Example: 120V × 100A = 12 KVA. For continuous use: 12 / 0.80 = 15 KVA.

What KVA transformer do I need for a house?

A typical US home has a 200A, 120/240V service. KVA = 240 × 200 / 1000 = 48 KVA. The utility transformer serving 4–6 homes is typically 25–50 KVA. Individual home transformers are not typically sized by the homeowner — the utility handles this.

Can a transformer be used backwards?

Yes, but with caution. A step-down transformer (480→120) used backwards becomes a step-up transformer (120→480). The secondary voltage may be 5–10% lower due to impedance losses designed into the primary. Also verify the insulation rating supports the higher voltage.

What is K-factor in transformers?

K-factor rates a transformer's ability to handle harmonic currents from non-linear loads. K-1 is standard (linear loads). K-4, K-13, K-20 are progressively more tolerant of harmonics. Use K-13 for modern commercial buildings with computers and LED lighting.

How hot do transformers get?

Dry-type transformers are rated by temperature class: 150°C (Class AA), 180°C (Class A), 220°C (Class B). A 75 KVA transformer at full load reaches 150–180°C internally. Ambient + temperature rise = total temperature. Proper ventilation keeps temperatures within rated limits.