Introduction #

Factory load calculation and transformer sizing are closely related but often misunderstood. Many engineers calculate load correctly but then select the wrong transformer size, leading to inefficiency, wasted capital, or insufficient capacity. This guide explains the relationship between load calculations and transformer sizing, shows how to convert load to transformer requirements, and provides practical selection criteria.

The Fundamental Relationship #

Basic Conversion #

Factory load is typically calculated in kW (real power).
Transformer size is rated in kVA (apparent power).

The relationship:

kVA = kW ÷ Power Factor

Transformer Size ≥ Calculated kVA (with margin)

Why This Matters #

Example:

Factory Load: 300 kW
Power Factor: 0.85
Required kVA: 300 ÷ 0.85 = 353 kVA

If you select a 300 kVA transformer (matching kW), it's undersized by 15%.

Step-by-Step: From Load to Transformer Size #

Step 1: Calculate Demand Load #

Start with your factory load calculation:

Connected Load: 500 kW
Diversity Factor: 0.70
Demand Load: 500 × 0.70 = 350 kW

Step 2: Determine Power Factor #

Calculate weighted power factor for your equipment mix:

Production Motors: 200 kW at 0.85 PF = 235.3 kVA
Lighting: 50 kW at 0.95 PF = 52.6 kVA
HVAC: 60 kW at 0.88 PF = 68.2 kVA
Welding: 40 kW at 0.50 PF = 80.0 kVA

Total: 350 kW, 436.1 kVA
Weighted PF: 350 ÷ 436.1 = 0.80

Step 3: Calculate Required kVA #

Required kVA = Demand Load ÷ Power Factor
Required kVA = 350 ÷ 0.80 = 437.5 kVA

Step 4: Add Safety Margin #

Safety Margin: 20%
Design kVA = 437.5 × 1.20 = 525 kVA

Step 5: Select Standard Transformer Size #

Standard transformer sizes (kVA):

  • 150, 200, 250, 300, 400, 500, 600, 750, 1000, 1250, 1500...

Selected: 600 kVA (next standard above 525 kVA)

Common Mistakes in Transformer Sizing #

Mistake 1: Matching kW to kVA #

Error:

Load: 300 kW
Transformer: 300 kVA

Problem:

  • Assumes PF = 1.0 (rarely true)
  • Transformer undersized
  • Overheating, reduced life

Correct:

Load: 300 kW
PF: 0.85
kVA: 300 ÷ 0.85 = 353 kVA
Transformer: 400 kVA

Mistake 2: Ignoring Power Factor #

Error: Using load kW directly as transformer kVA

Problem:

  • Underestimates kVA requirement
  • Transformer operates overloaded
  • Premature failure

Correct: Always divide by power factor

Mistake 3: Not Adding Safety Margin #

Error: Selecting transformer exactly at calculated kVA

Problem:

  • No room for load variations
  • No capacity for growth
  • Operating at 100% (inefficient)

Correct: Add 15-25% margin

Mistake 4: Oversizing Excessively #

Error: Selecting transformer 2× larger "to be safe"

Problem:

  • Wasted capital
  • Poor efficiency at low load
  • Higher no-load losses
  • Increased energy costs

Correct: Use appropriate margin (15-25%)

Transformer Loading Guidelines #

Optimal Loading Range #

Best Efficiency: 75-85% of rated capacity

Example:

Transformer: 500 kVA
Optimal Load: 375-425 kVA

Why:

  • High enough for good efficiency
  • Low enough to handle peaks
  • Room for growth

Loading Considerations #

Load Percentage Efficiency Notes
< 50% Poor High no-load losses
50-75% Good Acceptable efficiency
75-85% Excellent Optimal range
85-100% Good Near maximum
> 100% Poor Overload, reduced life

Real-World Example #

Facility:

  • Current load: 350 kVA
  • Peak load: 420 kVA
  • Growth planned: +80 kVA in 2 years

Analysis:

Current Peak: 420 kVA
Future Peak: 420 + 80 = 500 kVA
With 20% margin: 500 × 1.20 = 600 kVA

Selected: 600 kVA transformer

Loading:

  • Current: 350 ÷ 600 = 58% (acceptable)
  • Peak: 420 ÷ 600 = 70% (good)
  • Future: 500 ÷ 600 = 83% (optimal)

Special Considerations #

1. Motor Starting #

Large motors can cause voltage drops during starting:

Example:

Transformer: 500 kVA
Running Load: 400 kVA (80%)
Motor Starting: +200 kVA (momentary)
Peak During Start: 600 kVA (120% of rating)

Solution:

  • Oversize transformer for starting
  • Use soft starters or VFDs
  • Stagger motor starts
  • Consider starting kVA in sizing

2. Harmonic Loads #

Non-linear loads (VFDs, electronic equipment) create harmonics that increase apparent power:

Example:

Fundamental: 400 kVA
Harmonics: +50 kVA (12.5%)
Total: 450 kVA

Solution:

  • Add 10-15% for harmonics
  • Use K-rated transformers
  • Install harmonic filters

3. Future Expansion #

Plan for growth:

Example:

Current Load: 300 kVA
Planned Growth: 25% in 3 years
Future Load: 300 × 1.25 = 375 kVA
With Margin: 375 × 1.20 = 450 kVA
Selected: 500 kVA

4. Redundancy Requirements #

Critical facilities may need redundancy:

Example:

Required Capacity: 500 kVA
Redundancy: N+1 (two transformers)
Each Transformer: 500 kVA (can handle full load)
Or: 2 × 400 kVA (each handles 62.5% normally)

Complete Example: Load to Transformer Selection #

Facility Data #

Equipment Inventory:

  • Production machines: 50 × 10 kW = 500 kW
  • Welding: 5 × 25 kW = 125 kW
  • Lighting: 30 kW
  • HVAC: 50 kW
  • Compressed air: 40 kW
  • Office: 20 kW
  • Total Connected: 765 kW

Step 1: Apply Diversity Factors #

Category Connected (kW) Diversity Demand (kW)
Production 500 0.75 375
Welding 125 0.40 50
Lighting 30 0.95 28.5
HVAC 50 0.80 40
Compressed air 40 0.85 34
Office 20 0.65 13
Total 765 540.5

Step 2: Calculate Weighted Power Factor #

Category Demand (kW) PF kVA
Production 375 0.85 441.2
Welding 50 0.50 100.0
Lighting 28.5 0.95 30.0
HVAC 40 0.88 45.5
Compressed air 34 0.85 40.0
Office 13 0.75 17.3
Total 540.5 0.75 673.8

Weighted PF: 540.5 ÷ 673.8 = 0.80

Step 3: Add Safety Margin #

Demand kVA: 673.8 kVA
Safety Margin: 25% (growth planned)
Design kVA: 673.8 × 1.25 = 842.3 kVA

Step 4: Select Transformer #

Standard sizes near 842 kVA:

  • 750 kVA (89% loading - acceptable)
  • 1000 kVA (67% loading - good, room for growth)

Selected: 1000 kVA (provides room for future expansion)

Step 5: Verify Loading #

Current Load: 673.8 kVA
Transformer: 1000 kVA
Loading: 67.4% (good efficiency, room for growth)

Transformer Sizing vs. Load Calculation: Key Differences #

Load Calculation Focus #

  • Purpose: Determine actual power requirements
  • Output: kW and kVA demand
  • Factors: Diversity, power factor, load types
  • Result: What the facility actually needs

Transformer Sizing Focus #

  • Purpose: Select appropriate transformer capacity
  • Output: Transformer kVA rating
  • Factors: Standard sizes, loading efficiency, growth
  • Result: What transformer to install

The Connection #

Load calculation tells you what you need.
Transformer sizing tells you what to buy.

They're connected by:

  • Power factor (kW → kVA)
  • Safety margins
  • Standard equipment sizes
  • Efficiency considerations

Engineering Best Practices #

1. Always Calculate kVA from kW #

Never assume kW = kVA. Always:

kVA = kW ÷ Power Factor

2. Use Weighted Power Factor #

Don't use a single PF for all equipment:

  • Calculate PF for each category
  • Sum kVA values
  • Calculate weighted average

3. Add Appropriate Margin #

Consider:

  • Load variations (15-20%)
  • Future growth (5-10%)
  • Starting currents (if significant)
  • Total: 20-30% typical

4. Select Standard Sizes #

Use standard transformer ratings:

  • Better availability
  • Lower cost
  • Easier replacement

5. Verify Loading #

Check that selected transformer:

  • Handles current load (50-85% ideal)
  • Accommodates peaks (< 100%)
  • Allows for growth
  • Operates efficiently

Integration with Tools #

Factory Load Calculator #

Our Factory Load Calculator calculates:

  • Demand load (kW)
  • Apparent power (kVA)
  • Current requirements
  • Breaker sizing

Use the kVA result for transformer sizing, then:

  • Add safety margin
  • Select standard size
  • Verify loading

Calculate your load: Factory Load Calculator

Transformer Size Calculator #

Our Transformer Size Calculator helps:

  • Size transformers based on load
  • Account for power factor
  • Consider future expansion
  • Select appropriate rating

Size your transformer: Transformer Size Calculator

Conclusion #

Factory load calculation and transformer sizing are directly related through power factor and safety margins. Load calculation determines kW and kVA requirements, while transformer sizing selects the appropriate standard equipment. The key is converting kW to kVA using weighted power factor, adding appropriate safety margins, and selecting standard transformer sizes that provide optimal loading (75-85%) with room for growth. Always verify that selected transformers handle current loads efficiently while accommodating peaks and future expansion.