How to Read This Transformer Sizing Table

When specifying a dry-type transformer for commercial or light industrial applications, you need three exact numbers: the kVA rating, the full-load amps (FLA) on both windings, and the maximum Overcurrent Protective Device (OCPD) sizes. The table below provides these values for standard 3-Phase, 480V Delta Primary to 208Y/120V Wye Secondary dry-type transformers—the most common voltage configuration in North American commercial buildings.

Column Breakdown & Standards Applied:

  • kVA Rating: The standard manufacturer size based on IEEE C57.12.01.
  • Primary / Secondary Amps: Calculated using the 3-phase power formula: I = (kVA × 1000) / (V × √3).
  • Primary & Secondary OCPD (Breaker Size): Sized per NEC Article 450.3(B). For transformer currents over 9A, the NEC permits sizing the OCPD at 125% of the full-load current. If the 125% calculation does not land on a standard breaker size, NEC 240.6(A) permits rounding up to the next standard size (e.g., 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A).
Bookmark Quick-Jumps: The most frequently installed sizes in commercial tenant fit-outs are 50 kVA (small office/retail), 75 kVA (standard commercial floor), and 112.5 kVA (heavy commercial/multi-tenant). These rows are highlighted in the table below.

Complete Transformer Sizing & NEC Breaker Table

kVA Rating Primary Voltage Primary Amps (FLA) Pri. OCPD (Max Breaker) Secondary Voltage Secondary Amps (FLA) Sec. OCPD (Max Breaker)
15 kVA 480V 3Φ 18.0 A 25 A 208Y/120V 3Φ 41.6 A 60 A
30 kVA 480V 3Φ 36.1 A 50 A 208Y/120V 3Φ 83.2 A 110 A
45 kVA 480V 3Φ 54.1 A 70 A 208Y/120V 3Φ 124.9 A 175 A
50 kVA ★ 480V 3Φ 60.1 A 80 A 208Y/120V 3Φ 138.8 A 175 A
75 kVA ★ 480V 3Φ 90.2 A 125 A 208Y/120V 3Φ 208.2 A 300 A
112.5 kVA ★ 480V 3Φ 135.3 A 175 A 208Y/120V 3Φ 312.3 A 400 A
150 kVA 480V 3Φ 180.4 A 225 A 208Y/120V 3Φ 416.4 A 600 A
225 kVA 480V 3Φ 270.6 A 350 A 208Y/120V 3Φ 624.6 A 800 A
300 kVA 480V 3Φ 360.8 A 450 A 208Y/120V 3Φ 832.7 A 1000 A

Note: For 1-Phase 240V to 120/240V applications, use the formula I = (kVA × 1000) / V. A 25 kVA 1-phase transformer yields 104A primary and 208A secondary.

Derating Rules: When the Base Table Fails

The table above assumes standard installation conditions defined by IEEE C57 and manufacturer guidelines: an ambient temperature not exceeding 40°C (104°F) and an altitude below 1,000 meters (3,300 feet). If your installation deviates, the base kVA rating must be derated.

Ambient Temperature Derating

Standard 150°C rise dry-type transformers are designed for a maximum 40°C ambient environment. If you are installing the transformer in a hot mechanical room, a rooftop enclosure, or an unventilated space where ambient temperatures exceed 40°C, you must derate the transformer capacity by 0.4% for every 1°C above 40°C.

  • Example: A 75 kVA transformer installed in a room that reaches 50°C ambient. The temperature is 10°C over the limit. Derating = 10 × 0.4% = 4%. The transformer's true capacity is now 72 kVA. If your calculated load is 74 kVA, you must step up to a 112.5 kVA unit.

Altitude Derating

Thinner air at high elevations reduces the convective cooling efficiency of the transformer windings. For installations above 1,000 meters (3,300 feet), derate the kVA rating by 0.3% for every 100 meters (330 feet) above 1,000m.

What This Table Cannot Tell You (Critical Edge Cases)

  1. Harmonic Loads (K-Factor): This table assumes linear loads. Modern commercial buildings are packed with non-linear loads (VFDs, LED drivers, server power supplies). If your non-linear load exceeds 50% of the total, a standard transformer will overheat due to eddy current losses. You must specify a K-13 rated (or K-20 for data centers) transformer. The kVA and breaker sizing remain the same, but the internal winding construction is reinforced.
  2. Magnetizing Inrush Current: When a transformer is first energized, it draws an inrush current that can be 10 to 12 times the primary FLA for a few cycles. If you use a standard thermal-magnetic breaker sized exactly to the table, it may nuisance-trip on startup. You must select breakers with high magnetic trip thresholds (e.g., 10x to 15x In) or use time-delay fuses on the primary side to ride through the inrush.

Decision Path: Sizing Your Next Transformer

Use this decision tree to move from a raw load calculation to a concrete, orderable part number. Do not size a transformer based purely on the sum of breaker handles; size it based on the actual calculated continuous and non-continuous load.

Step Action / Calculation Example Scenario
1. Calculate Total Load Sum the actual continuous and non-continuous load currents (in Amps) on the secondary side. Calculated load is 120A continuous at 208V 3-Phase.
2. Convert to kVA Use formula: (Amps × Volts × 1.732) / 1000 (120 × 208 × 1.732) / 1000 = 43.2 kVA
3. Apply Continuous Margin Multiply by 1.25 (125%) to comply with NEC continuous load rules and prevent operating at 100% thermal capacity. 43.2 kVA × 1.25 = 54 kVA
4. Select Standard Size Round UP to the next standard IEEE C57 kVA rating from the table above. Next standard size above 54 kVA is 75 kVA.
5. Determine K-Factor Is the load >50% non-linear (electronics, VFDs, LED)? If Yes = K-13. If No = Standard (K-1). Office space with heavy PC/LED load. Yes, requires K-13.
The Final Concrete Pick: For the 120A continuous 208V load scenario above, do not order a generic 75 kVA unit. Order a 75 kVA, 480V Delta to 208Y/120V Wye, K-13 rated dry-type transformer (e.g., Eaton V13T75K or Square D EE75T3H). Protect the primary with a 125A breaker (with high magnetic inrush tolerance) and the secondary with a 300A main breaker panel.