Quick answer: Three-phase: kVA = (√3 × V × I) / 1,000. Example: 200 kW, PF 0.85 → 235 kVA → select 300 kVA (standard). FLA at 480V: I = 300,000 ÷ (1.732 × 480) = 360.8 A. Short-circuit at %Z = 5%: I_sc = 360.8 ÷ 0.05 = 7,216 A. Use this transformer sizing calculator workflow with the Transformer Calculator for instant sizing.
Quick Answer
How do I size a transformer?
| System | Formula |
|---|---|
| Single-Phase | kVA = (V × I) / 1000 |
| Three-Phase | kVA = (√3 × V × I) / 1000 |
| From Load | kVA = kW / Power Factor |
→ Use the Transformer Calculator for instant sizing.
Standard Transformer Sizes
Single-Phase Transformers
| kVA | 120V FLA | 240V FLA | 480V FLA |
|---|---|---|---|
| 0.5 | 4.2 | 2.1 | 1.0 |
| 1 | 8.3 | 4.2 | 2.1 |
| 2 | 16.7 | 8.3 | 4.2 |
| 3 | 25.0 | 12.5 | 6.3 |
| 5 | 41.7 | 20.8 | 10.4 |
| 7.5 | 62.5 | 31.3 | 15.6 |
| 10 | 83.3 | 41.7 | 20.8 |
| 15 | 125 | 62.5 | 31.3 |
| 25 | 208 | 104 | 52 |
| 37.5 | 313 | 156 | 78 |
| 50 | 417 | 208 | 104 |
| 75 | 625 | 313 | 156 |
| 100 | 833 | 417 | 208 |
Three-Phase Transformers
| kVA | 208V FLA | 480V FLA | 600V FLA |
|---|---|---|---|
| 15 | 41.7 | 18.0 | 14.4 |
| 30 | 83.3 | 36.1 | 28.9 |
| 45 | 125 | 54.1 | 43.3 |
| 75 | 208 | 90.2 | 72.2 |
| 112.5 | 312 | 135 | 108 |
| 150 | 416 | 180 | 144 |
| 225 | 625 | 271 | 217 |
| 300 | 833 | 361 | 289 |
| 500 | 1,388 | 601 | 481 |
| 750 | 2,082 | 902 | 722 |
| 1000 | 2,776 | 1,203 | 962 |
| 1500 | 4,164 | 1,804 | 1,443 |
| 2000 | 5,552 | 2,406 | 1,925 |
| 2500 | 6,940 | 3,007 | 2,406 |
Transformer Sizing Formulas
From Current (Amperes)
Single-Phase:
kVA = (V × I) / 1000
Three-Phase:
kVA = (√3 × V × I) / 1000
kVA = (1.732 × V × I) / 1000
From Power (kW)
kVA = kW / Power Factor
Example: 100 kW load, PF = 0.85
kVA = 100 / 0.85 = 117.6 kVA
Select: 150 kVA transformer
Current from kVA
Single-Phase:
I = (kVA × 1000) / V
Three-Phase:
I = (kVA × 1000) / (√3 × V)
I = (kVA × 1000) / (1.732 × V)
Sizing Guidelines
Load Factor Considerations
| Application | Typical Load Factor | Sizing Factor |
|---|---|---|
| Continuous (100%) | 1.0 | 1.0-1.25 |
| Intermittent (80%) | 0.8 | 0.9-1.0 |
| Cyclic (60%) | 0.6 | 0.7-0.8 |
Future Growth Allowance
| Situation | Recommended Margin |
|---|---|
| Minimal growth expected | 10-15% |
| Moderate growth | 20-25% |
| High growth potential | 30-50% |
Sizing Formula with Margin
kVA (selected) = kVA (calculated) × (1 + Growth Factor)
Example: 180 kVA calculated, 25% growth
kVA = 180 × 1.25 = 225 kVA
Common Voltage Configurations
Distribution Transformers
| Primary | Secondary | Configuration |
|---|---|---|
| 4160V | 480/277V | Delta-Wye |
| 13.8kV | 480/277V | Delta-Wye |
| 480V | 208/120V | Delta-Wye |
| 480V | 240/120V | Delta-Delta |
| 240V | 208/120V | Delta-Wye |
Secondary Voltage Systems
| System | Line-Line | Line-Neutral | Use |
|---|---|---|---|
| 208/120V | 208V | 120V | Commercial |
| 480/277V | 480V | 277V | Industrial |
| 600/347V | 600V | 347V | Industrial (Canada) |
| 240/120V | 240V | 120V | Residential |
Impedance and Short Circuit
Transformer Impedance (%Z)
Typical impedance values:
| kVA Range | Typical %Z |
|---|---|
| 0-15 | 2-3% |
| 15-75 | 3-4% |
| 75-300 | 4-5% |
| 300-1000 | 5-6% |
| 1000-2500 | 5.75-6.5% |
Short Circuit Current Calculation
I_sc = I_FLA / (Z % / 100)
Or:
I_sc = I_FLA × (100 / Z%)
Example: 500 kVA transformer, 480V, Z = 5.75%
I_FLA = 500,000 / (1.732 × 480) = 601A
I_sc = 601 / 0.0575 = 10,452A
Note: Actual fault current depends on utility source impedance.
Voltage Regulation
Voltage Drop Calculation
V_drop (%) ≈ %Z × (I_load / I_rated) × cos(θ)
For lagging power factor:
V_drop = %R × cos(θ) + %X × sin(θ)
Where:
- %R = Resistance component of impedance
- %X = Reactance component of impedance
- θ = Power factor angle
Tap Settings
Most transformers have ±2.5% or ±5% taps:
| Tap Position | Primary Voltage Adjustment |
|---|---|
| +5% | Primary reduced 5% (boosts secondary) |
| +2.5% | Primary reduced 2.5% |
| Nominal | Standard ratio |
| -2.5% | Primary increased 2.5% |
| -5% | Primary increased 5% (reduces secondary) |
Usage: Adjust taps to maintain secondary voltage under load.
NEC Transformer Requirements
Overcurrent Protection (NEC 450.3)
NEC 450.3(B) applies to transformers rated 1,000V or less on both sides. This is a simplified summary — the full NEC 450.3 table has additional conditions; always verify against the adopted NEC edition and your AHJ.
Transformers ≤ 1,000V (Both Primary and Secondary) — NEC 450.3(B):
| Protection Configuration | Rated Primary Current | Max Primary OCP | Max Secondary OCP |
|---|---|---|---|
| Primary only | ≥ 9A | 125%* | Not required |
| Primary only | < 9A | 167%* | Not required |
| Primary + Secondary | Primary ≥ 9A, Secondary ≥ 9A | 250%* | 125%* |
| Primary + Secondary | Primary ≥ 9A, Secondary < 9A | 250%* | 167%* |
*If the calculated percentage is not a standard fuse or breaker size, the next higher standard size is permitted.
For transformers with primary > 1,000V: See NEC 450.3(A) — protection levels of 150% to 600% depending on whether the transformer has primary-only or primary-plus-secondary protection and rated current levels.
Conductor Sizing (NEC 450.3)
- Primary conductors: Based on primary current
- Secondary conductors: Based on secondary current
Grounding (NEC 250.30)
For separately derived systems:
- System bonding jumper required
- Grounding electrode conductor required
- Equipment grounding required
Worked Examples
Example 1: Single-Phase Sizing
Given: 50A load at 240V, PF = 0.9
Solution:
kVA = (V × I) / 1000
kVA = (240 × 50) / 1000
kVA = 12 kVA
Add 25% margin: 12 × 1.25 = 15 kVA
Select: 15 kVA single-phase transformer
Example 2: Three-Phase Sizing from kW
Given: 200 kW load, 480V, PF = 0.85
Solution:
kVA = kW / PF
kVA = 200 / 0.85
kVA = 235.3 kVA
Add 15% margin: 235.3 × 1.15 = 270.6 kVA
Select: 300 kVA three-phase transformer
Example 3: Full Load Current
Given: 150 kVA, 480V three-phase
Solution:
I = (kVA × 1000) / (√3 × V)
I = (150 × 1000) / (1.732 × 480)
I = 150,000 / 831.4
I = 180.4 A
Example 4: Short Circuit Calculation
Given: 1000 kVA, 480V, Z = 5.75%
Solution:
I_FLA = 1,000,000 / (1.732 × 480) = 1,203A
I_sc = 1,203 / 0.0575 = 20,922A
Available short circuit: ~21,000A
Select equipment with AIC ≥ 22,000A
Transformer Losses and DOE Efficiency Standards
Loss Types
No-Load Losses (Core / Iron): Constant regardless of load; present 24/7 whenever the transformer is energized. Caused by eddy currents and hysteresis in the core. Typically 0.5–1.5% of rated kVA for modern dry-type units.
Load Losses (Copper / I²R): Proportional to the square of load current. At half load, copper losses are 25% of full-load copper losses.
P_copper = P_copper_rated × (I / I_rated)²
Efficiency Calculation
Efficiency = kVA_out × PF / (kVA_out × PF + P_core + P_copper)
Typical modern dry-type distribution transformer efficiency: 97–99% at full load; highest at 50–75% load (where core losses ≈ copper losses).
DOE Minimum Efficiency Requirements (10 CFR 431 Subpart K)
Effective January 1, 2016, all new general-purpose low-voltage dry-type distribution transformers (1–333 kVA, single- and three-phase, 600V class) sold in the US must meet minimum efficiencies at 35% of rated load:
| kVA Rating | Configuration | Min. Efficiency (at 35% load) |
|---|---|---|
| 15 kVA | Single-phase | 97.0% |
| 25 kVA | Single-phase | 97.3% |
| 37.5 kVA | Single-phase | 97.5% |
| 75 kVA | Single-phase | 97.9% |
| 15 kVA | Three-phase | 97.0% |
| 30 kVA | Three-phase | 97.5% |
| 75 kVA | Three-phase | 98.0% |
| 150 kVA | Three-phase | 98.4% |
| 300 kVA | Three-phase | 98.6% |
| 500 kVA | Three-phase | 98.7% |
Per DOE 10 CFR 431, Table 4 (low-voltage dry-type). NEMA Premium Efficiency models typically exceed these minimums. For liquid-immersed transformers ≥ 10 kVA, separate DOE efficiency rules apply per 10 CFR 431, Table 2. Always confirm with the current CFR and manufacturer data.
Loading for Maximum Efficiency
Maximum efficiency occurs when core losses equal copper losses. This typically occurs at 50–75% of full-load rating. Design practice often targets transformer loading at 60–80% of rating to balance efficiency, growth capacity, and thermal life.
Common Mistakes to Avoid
| Mistake | Why It's Wrong | Correct Approach |
|---|---|---|
| Using kW instead of kVA | kW ignores reactive | Size by kVA |
| No growth margin | Future overload | Add 15-25% |
| Wrong voltage | Mismatched system | Verify primary/secondary |
| Ignoring impedance | Short circuit issues | Check AIC rating |
Related Calculators
| Calculator | Use When... |
|---|---|
| Transformer Calculator | kVA sizing |
| 3-Phase Power Calculator | Load calculations |
| Short Circuit Calculator | Fault current |
| Wire Size Calculator | Feeder sizing |
Summary
Sizing Formulas:
- Single-Phase: kVA = V × I / 1000
- Three-Phase: kVA = √3 × V × I / 1000
- From kW: kVA = kW / PF
Standard Sizes (kVA):
- Single-Phase: 0.5, 1, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100
- Three-Phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500
Always:
- Size by kVA, not kW
- Add growth margin (15-25%)
- Check fault current vs. equipment rating
- Verify primary and secondary protection per NEC
FAQ
What's the difference between kVA and kW for transformers?
Transformers are rated in kVA (apparent power) because they handle both real (kW) and reactive (kVAR) power. kVA = kW / Power Factor. Always size by kVA to ensure adequate capacity.
How much should I oversize a transformer?
For general commercial: 15-25% margin. For industrial with growth potential: 25-50%. Consider load factor, diversity, and future expansion.
What is transformer impedance?
Impedance (%Z) represents voltage drop and limits short circuit current. Higher Z = more voltage drop but lower fault current. Typical values: 3-6% for distribution transformers.
How do I protect a transformer per NEC?
NEC 450.3 requires primary overcurrent protection. For transformers >9A secondary, use 125% of rated current. May use up to 250% if conditions met. Secondary protection also required in most cases.
What voltage taps are for?
Taps adjust the turns ratio to compensate for high or low supply voltage. Typically ±2.5% or ±5%. Adjust de-energized to maintain proper secondary voltage under load.