Written by Tan, Electrical Engineer @ Transformer4U · Updated May 2026
A few months ago, a contractor called asking for “a standard-sized transformer for an office building.” When I asked which kVA, he said, “I don’t know — just the normal one.” That conversation lasted 40 minutes because “normal” depends on the building’s electrical load, the number of floors, and whether you’re feeding single-phase or three-phase equipment.
This guide gives you the complete transformer size chart so you can skip that 40-minute call. Every standard size from 1 kVA to 5,000 kVA is listed below — with full load amps, typical applications, and a step-by-step transformer selection chart to match the right size to your project.
Quick Answer: Standard transformer sizes follow ANSI/IEEE C57.12.00 preferred ratings. Common single-phase sizes: 5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA. Common three-phase sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000, 1,500, 2,000, 2,500 kVA. Jump to the complete size chart or selection guide.
What Are Standard Transformer Sizes?
Standard transformer sizes are predefined kVA ratings established by industry standards (ANSI/IEEE C57.12.00 and NEMA) that manufacturers commonly produce. These standard sizing levels exist for practical reasons:
- Availability: Standard sizes are stocked by distributors, cutting lead times from 30+ weeks to days
- Cost: Standard sizing is 15–40% cheaper than custom-manufactured transformers in most power systems
- Interchangeability: Replacement is straightforward — same kVA, same footprint, same connections
- Code compliance: Breaker sizing, wire sizing, and NEC 450.3 overcurrent protection tables are all built around standard kVA sizes
From the factory floor: We keep our most popular three-phase sizes (75, 112.5, 150, 225, 300, and 500 kVA) in stock at all times. Any time a customer requests a non-standard size like 180 kVA or 400 kVA, lead time jumps from 1–2 weeks to 16–20 weeks, and cost increases 25–35%. My advice: always design around standard sizes whenever possible.
Standard Transformer Size Chart: All kVA Ratings
Single-Phase Standard Transformer Sizes
| kVA | FLA @ 120V | FLA @ 240V | FLA @ 480V | Typical Application |
|---|---|---|---|---|
| 1 | 8.3 | 4.2 | 2.1 | Control panel, signal circuit |
| 1.5 | 12.5 | 6.3 | 3.1 | Control circuit, doorbell system |
| 2 | 16.7 | 8.3 | 4.2 | Control transformer, PLC supply |
| 3 | 25.0 | 12.5 | 6.3 | Lighting panel, machine control |
| 5 | 41.7 | 20.8 | 10.4 | Small HVAC, single outlet panel |
| 7.5 | 62.5 | 31.3 | 15.6 | Workshop, small motor load |
| 10 | 83.3 | 41.7 | 20.8 | Residential service entrance |
| 15 | 125.0 | 62.5 | 31.3 | Small commercial, pole-mount |
| 25 | 208.3 | 104.2 | 52.1 | Multi-family residential |
| 37.5 | 312.5 | 156.3 | 78.1 | Small retail, community well pump |
| 50 | 416.7 | 208.3 | 104.2 | Medium commercial, strip mall |
| 75 | 625.0 | 312.5 | 156.3 | Large commercial, small factory |
| 100 | 833.3 | 416.7 | 208.3 | Industrial subpanel |
| 167 | 1,391.7 | 695.8 | 347.9 | Large pad-mount distribution |
| 200 | 1,666.7 | 833.3 | 416.7 | Medium industrial substation |
| 250 | 2,083.3 | 1,041.7 | 520.8 | Utility distribution feeder |
| 333 | 2,775.0 | 1,387.5 | 693.8 | Utility distribution |
| 500 | 4,166.7 | 2,083.3 | 1,041.7 | Large distribution feeder |
Bold = most commonly ordered sizes. FLA = Full Load Amps (FLA = kVA × 1,000 ÷ V). Source: ANSI/IEEE C57.12.00 preferred ratings.
Three-Phase Standard Transformer Sizes
| kVA | FLA @ 208V | FLA @ 480V | Voltage Class | Typical Application |
|---|---|---|---|---|
| 3 | 8.3 | 3.6 | Low Voltage | Control panel, small 3φ motor |
| 6 | 16.7 | 7.2 | Low Voltage | Small lighting panel |
| 9 | 25.0 | 10.8 | Low Voltage | Machine tool control |
| 15 | 41.6 | 18.0 | Low Voltage | Small commercial HVAC |
| 30 | 83.3 | 36.1 | Low Voltage | Retail store, restaurant |
| 45 | 124.9 | 54.1 | Low Voltage | Small office building |
| 75 | 208.2 | 90.2 | Low Voltage | Medium commercial, schools |
| 112.5 | 312.3 | 135.3 | Low Voltage | Small manufacturing, warehouse |
| 150 | 416.4 | 180.4 | Low Voltage | Medium industrial plant |
| 225 | 624.6 | 270.6 | Low Voltage | Hospital, campus building |
| 300 | 832.8 | 360.8 | Low Voltage | Large commercial complex |
| 500 | 1,388.0 | 601.4 | Low Voltage | Data center, heavy manufacturing |
| 750 | 2,082.0 | 902.1 | Low Voltage | Large industrial campus |
| 1,000 | — | 1,202.8 | Medium Voltage | Utility substation, mine |
| 1,500 | — | 1,804.2 | Medium Voltage | Petrochemical, steel mill |
| 2,000 | — | 2,405.6 | Medium Voltage | Major industrial facility |
| 2,500 | — | 3,007.0 | Medium Voltage | Large substation |
| 3,000 | — | 3,608.4 | Medium Voltage | Utility power distribution |
| 3,750 | — | 4,510.6 | Medium Voltage | Utility distribution |
| 5,000 | — | 6,014.1 | Medium Voltage | Large utility substation |
Bold = most commonly ordered sizes. FLA = kVA × 1,000 ÷ (V × √3). Gray rows = medium voltage transformers (primary voltage 2.4–34.5 kV). Sources: ANSI/IEEE C57.12.00; Schneider Electric FAQ FA91532.
Standard Sizes by Transformer Type
Not all transformer types come in every size. Here’s what’s typically available for each type:
| Transformer Type | Typical kVA Sizes | Primary & Secondary Voltage | Best For |
|---|---|---|---|
| Dry-Type (Ventilated) | 1 – 2,500 kVA | 480V–600V / 208V–240V | Indoor commercial & industrial; hospitals, data centers, schools |
| Dry-Type (Cast Resin) | 25 – 12,500 kVA | Up to 36 kV / 480V–600V | Medium voltage indoor; fire-risk areas, high humidity environments |
| Pad-Mount (Oil-Filled) | 25 – 2,500 kVA | 2.4–34.5 kV / 120–600V | Outdoor ground-level, residential subdivisions, commercial parks |
| Pole-Mount (Oil-Filled) | 5 – 167 kVA (1φ); 15 – 500 kVA (3φ) | 2.4–34.5 kV / 120–240V | Overhead utility distribution, rural service |
| Oil-Immersed (Large Power) | 1,000 kVA – 500+ MVA | 34.5–765 kV / 2.4–34.5 kV | Power substations, generation plants, transmission grid |
Key insight for indoor installations: If your project is inside a building (especially near occupied spaces), dry-type transformers are almost always required. Oil-filled units need fire-rated vaults per NEC 450.26, adding $10,000–$50,000 to installation costs. We’ve seen projects switch from oil to dry-type mid-construction when the vault cost was discovered — always specify transformer type early in the design.
Transformer Selection Chart: How to Choose the Right Size
Follow these four steps to select the correct standard transformer size for your project:
Step 1: Determine Your Phase Configuration
Is your electrical load single-phase or three-phase? Check your main electrical panel or service entrance. Residential is almost always single-phase. Commercial and industrial phase systems over 15 kVA are typically three-phase.
Step 2: Calculate Your Minimum kVA
Single-phase: kVA = V × I ÷ 1,000
Three-phase: kVA = V × I × 1.732 ÷ 1,000
From kW: kVA = kW ÷ Power Factor (typical PF = 0.8–0.9)
Don’t know your load voltage or amps? Check the equipment nameplate or electrical schematic. For an entire building, add up all branch circuit loads or check the main breaker size. Use our transformer kVA calculator for quick results.
Step 3: Add Margin and Round Up
Never select the exact calculated kVA. Add 20–25% for:
- Motor starting surge (6–8× full load for a few seconds)
- Future load growth (plan for 5–10 years ahead)
- Altitude/temperature derating (above 1,000m or 30°C)
Example: A 480V three-phase system draws 150A. kVA = 480 × 150 × 1.732 ÷ 1,000 = 124.7 kVA. Adding 25% margin = 155.9 kVA. The next standard size up is 225 kVA (not 150 kVA — you’d have zero margin at 150).
Step 4: Choose the Transformer Type
| Installation Location | Recommended Type | Why |
|---|---|---|
| Indoor (near people) | Dry-type ventilated | No oil = no fire hazard, no vault needed |
| Indoor (fire-risk zone) | Dry-type cast resin | Self-extinguishing, humidity resistant |
| Outdoor (ground level) | Pad-mount | Tamper-resistant cabinet, weatherproof |
| Outdoor (overhead) | Pole-mount | Space-saving for utility distribution |
| Substation / utility grid | Oil-immersed power | Most efficient at high voltage / high power |
Common Transformer Sizes by Application
Here’s what we ship most frequently for each building type — based on actual order data from our factory:
| Application | Typical Transformer Size | Type | Notes |
|---|---|---|---|
| Single-family home | 15–25 kVA (1φ) | Pole-mount | One transformer often serves 4–8 homes |
| Small retail store | 30–45 kVA (3φ) | Dry-type | Lighting + HVAC + POS systems |
| Office building (per floor) | 75–150 kVA (3φ) | Dry-type | Lighting + IT + HVAC per floor |
| Restaurant | 45–75 kVA (3φ) | Dry-type | Commercial kitchen = high load |
| Hospital / medical center | 225–500 kVA (3φ) | Dry-type | Critical load + redundancy required |
| School / university | 150–300 kVA (3φ) | Dry-type/Pad-mount | Per building on campus |
| Data center | 500–2,500 kVA (3φ) | Dry-type (K-rated) | K-13 or K-20 for harmonic loads |
| Manufacturing plant | 300–1,000 kVA (3φ) | Dry-type/Oil-filled | Motor loads may need 125% start factor |
| Residential subdivision (20–50 homes) | 75–167 kVA (1φ) pad-mount | Pad-mount | Ground-level, tamper-resistant |
A lesson from the field: We once supplied a 75 kVA dry-type unit for a new restaurant. Six months later, the owner added a walk-in freezer and a second deep fryer bank. The transformer ran at 95% load during peak dinner hours — you could hear the 120Hz hum two rooms away, and the winding temperature hit 145°C. We replaced it with a 112.5 kVA unit. The moral: always ask the owner about their expansion plans, not just today’s menu.
FAQ
What are standard transformer sizes?
Standard single-phase sizes per ANSI/IEEE C57.12.00: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, and 500 kVA. Standard three-phase sizes: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3750, and 5000 kVA.
How do I select the right transformer size?
Step 1: Calculate your kVA using V × I / 1,000 (single-phase) or V × I × 1.732 / 1,000 (three-phase). Step 2: Add a 20–25% safety margin. Step 3: Round up to the next standard kVA size. Step 4: Choose the type (dry-type, pad-mount, or oil-filled) based on installation location.
What is the most common transformer size for commercial buildings?
The most common sizes are 75, 112.5, 150, 225, and 300 kVA three-phase dry-type units. For small retail: 45 or 75 kVA. For office buildings and hospitals: 225–500 kVA.
What is the difference between dry-type and oil-filled transformer sizes?
Dry-type transformers are available from 1 kVA to about 2,500 kVA (low voltage) and up to 10 MVA (medium voltage cast resin). Oil-filled units range from 10 kVA to over 500 MVA. Oil-filled are more compact per kVA but require fire-resistant vaults indoors.
What are standard pad-mount transformer sizes?
Single-phase pad-mount: 25, 37.5, 50, 75, 100, and 167 kVA. Three-phase pad-mount: 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, and 2500 kVA. These are oil-filled units for outdoor ground-level installation.
How do I calculate the kVA for a transformer?
Single-phase: kVA = Volts × Amps / 1,000. Three-phase: kVA = Volts × Amps × 1.732 / 1,000. If you know your load in kW, divide by the power factor: kVA = kW / PF. Always select the next standard size above your calculated kVA.
What size transformer do I need for a 200-amp service?
For a 200A single-phase 240V service: kVA = 240 × 200 / 1,000 = 48 kVA → select a 50 kVA transformer. For a 200A three-phase 480V service: kVA = 480 × 200 × 1.732 / 1,000 = 166.3 kVA → select a 225 kVA transformer (includes growth margin).
Need Help Finding the Right Transformer Size?
Picking the wrong standard transformer size costs more than just the price difference — it means return shipping, reinstallation labor, and project delays. Our engineering team sizes every order before manufacturing to catch issues early.
Contact Transformer4U for a free sizing consultation, or browse our product range: