Introduction #

Calculating factory electrical load is fundamental to designing safe, efficient, and code-compliant industrial electrical systems. Whether you're planning a new facility, expanding an existing one, or troubleshooting electrical issues, accurate load calculations ensure proper equipment sizing and prevent costly mistakes.

What is Factory Load? #

Factory load refers to the total electrical power demand of all equipment and systems in an industrial facility. It's measured in:

  • kW (kilowatts): Real power consumption
  • kVA (kilovolt-amperes): Apparent power requirement
  • Amperes: Current draw

Why Load Calculation Matters #

Accurate load calculations are essential for:

  1. Equipment Sizing: Transformers, generators, circuit breakers
  2. Safety: Preventing overloads and fires
  3. Code Compliance: Meeting NEC and local requirements
  4. Cost Optimization: Right-sizing equipment
  5. Future Planning: Accommodating expansion

Step-by-Step Load Calculation Process #

Step 1: Inventory All Electrical Loads #

Create a comprehensive list of all electrical equipment:

Production Equipment:
- Machine A: 15 kW
- Machine B: 20 kW
- Machine C: 10 kW
- Conveyor system: 5 kW

Lighting:
- Production area: 8 kW
- Office area: 2 kW

HVAC:
- Air conditioning: 25 kW
- Ventilation: 5 kW

Other:
- Office equipment: 3 kW
- Compressed air: 12 kW

Step 2: Calculate Total Connected Load #

Sum all individual loads:

Total Connected Load = 15 + 20 + 10 + 5 + 8 + 2 + 25 + 5 + 3 + 12
Total Connected Load = 110 kW

Step 3: Apply Diversity Factors #

Not all equipment operates simultaneously. Apply diversity factors:

Load Type Diversity Factor Reason
Production equipment 0.70-0.80 Machines cycle on/off
Lighting 0.90-1.00 Most lights on during work hours
HVAC 0.60-0.80 Varies with season and occupancy
Office equipment 0.50-0.70 Not all equipment used simultaneously
Welding 0.30-0.50 Intermittent operation

Example:

Production: 50 kW × 0.75 = 37.5 kW
Lighting: 10 kW × 0.95 = 9.5 kW
HVAC: 30 kW × 0.70 = 21 kW
Other: 20 kW × 0.60 = 12 kW

Diversified Load = 37.5 + 9.5 + 21 + 12 = 80 kW

Step 4: Determine Power Factor #

Calculate weighted average power factor:

Motors: 37.5 kW at 0.85 PF
Lighting: 9.5 kW at 1.0 PF
HVAC: 21 kW at 0.90 PF
Other: 12 kW at 0.80 PF

Weighted PF ≈ 0.87

Step 5: Calculate Apparent Power (kVA) #

kVA = kW ÷ Power Factor
kVA = 80 ÷ 0.87
kVA = 92 kVA

Step 6: Calculate Current #

For 3-phase systems at 480V:

Current = (kVA × 1000) ÷ (Voltage × √3)
Current = (92 × 1000) ÷ (480 × 1.732)
Current = 92,000 ÷ 831
Current = 110.7 Amperes

Step 7: Add Safety Margin #

Add 25% margin for safety and future growth:

Current with margin = 110.7 × 1.25 = 138.4 Amperes

Step 8: Select Circuit Breaker #

Round up to next standard breaker size:

Standard Breaker Sizes:
15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200, 225, 250, 300, 350, 400, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000

Selected: 150 Ampere breaker

Load Calculation Formulas #

Basic Formula #

Total Load (kW) = Σ (Individual Loads)

With Diversity #

Diversified Load (kW) = Σ (Load × Diversity Factor)

Apparent Power #

kVA = kW ÷ Power Factor

Current (3-Phase) #

Current (A) = (kVA × 1000) ÷ (Voltage × √3)

Current (Single-Phase) #

Current (A) = (kW × 1000) ÷ (Voltage × Power Factor)

Real-World Example #

Complete Factory Calculation #

Given:

  • Factory with 50 devices
  • Average 5 kW per device
  • Power factor: 0.85
  • Diversity factor: 0.75
  • Voltage: 480V, 3-phase
  • Safety margin: 25%

Calculation:

  1. Total Connected Load:
50 devices × 5 kW = 250 kW
  1. Diversified Load:
250 kW × 0.75 = 187.5 kW
  1. Apparent Power:
187.5 kW ÷ 0.85 = 220.6 kVA
  1. Current:
(220.6 × 1000) ÷ (480 × 1.732) = 265.4 A
  1. With Safety Margin:
265.4 A × 1.25 = 331.8 A
  1. Selected Breaker: 400 Ampere

Motor Load Calculations #

Motor Starting Current #

Motors draw 5-7 times rated current during startup:

Starting Current = Full Load Current × 6

Multiple Motors #

For multiple motors, consider:

  • Largest motor: Use full starting current
  • Other motors: Use running current
  • Staggered starting: Reduces peak demand

Example #

Given:

  • Motor 1: 50 HP (largest)
  • Motor 2: 25 HP
  • Motor 3: 25 HP

Calculation:

Motor 1 FLA: 65 A
Motor 1 Starting: 65 × 6 = 390 A
Motor 2 FLA: 34 A
Motor 3 FLA: 34 A

Total Starting Current = 390 + 34 + 34 = 458 A

Common Mistakes to Avoid #

  1. Ignoring Diversity Factors: Assuming all equipment runs simultaneously
  2. Overlooking Power Factor: Not accounting for reactive power
  3. Insufficient Safety Margin: Not planning for future growth
  4. Incorrect Voltage: Using wrong voltage for calculations
  5. Missing Loads: Forgetting lighting, HVAC, or other systems

Best Practices #

  1. Document Everything: Keep detailed records of all loads
  2. Use Software Tools: Leverage calculation software for accuracy
  3. Consult Standards: Follow NEC and local codes
  4. Plan for Growth: Include 20-25% margin for expansion
  5. Regular Updates: Review and update calculations periodically

Use our Factory Load Calculator to quickly calculate factory electrical loads with all the factors mentioned above.

Conclusion #

Accurate factory load calculations are essential for safe, efficient, and code-compliant electrical systems. By following these steps and using the proper formulas, you can ensure your facility has the right electrical infrastructure to support current and future operations.