How to Calculate Factory Load: Complete Step-by-Step Guide
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:
- Equipment Sizing: Transformers, generators, circuit breakers
- Safety: Preventing overloads and fires
- Code Compliance: Meeting NEC and local requirements
- Cost Optimization: Right-sizing equipment
- 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:
-
Total Connected Load:
50 devices × 5 kW = 250 kW -
Diversified Load:
250 kW × 0.75 = 187.5 kW -
Apparent Power:
187.5 kW ÷ 0.85 = 220.6 kVA -
Current:
(220.6 × 1000) ÷ (480 × 1.732) = 265.4 A -
With Safety Margin:
265.4 A × 1.25 = 331.8 A -
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: 32 A
Motor 3 FLA: 32 A
Peak Current = 390 + 32 + 32 = 454 A
Lighting Load Calculations #
General Lighting #
NEC requirements for general lighting:
- Industrial: 2 VA per square foot minimum
- Warehouse: 0.5 VA per square foot minimum
- Office: 3.5 VA per square foot minimum
Example #
Given:
- Factory area: 10,000 sq ft
- Industrial lighting requirement
Calculation:
Lighting Load = 10,000 × 2 VA = 20,000 VA = 20 kVA
HVAC Load Calculations #
Cooling Load #
HVAC cooling load depends on:
- Building size and insulation
- Occupancy
- Equipment heat generation
- Climate
Heating Load #
Heating load typically 60-80% of cooling load in moderate climates.
Example #
Given:
- Factory: 20,000 sq ft
- Cooling: 1 ton per 400 sq ft
- Heating: 70% of cooling
Calculation:
Cooling: 20,000 ÷ 400 = 50 tons
50 tons × 3.5 kW/ton = 175 kW
Heating: 175 × 0.70 = 122.5 kW
Using Our Factory Load Calculator #
Our Factory Load Calculator simplifies the process:
- Enter number of devices
- Specify load per device
- Set power factor and diversity
- Get instant results:
- Total load (kW)
- Apparent power (kVA)
- Current (Amperes)
- Recommended breaker size
The calculator includes:
- Automatic diversity factor application
- Power factor correction
- Safety margin calculations
- 3-phase current calculations
Code Requirements #
NEC Article 220 #
NEC requirements for load calculations:
- General lighting: Minimum VA per square foot
- Receptacles: Minimum number and load
- Motors: Full load current + starting current
- Diversity: Apply appropriate factors
- Demand factors: For specific load types
Local Codes #
Always check local electrical codes for:
- Additional requirements
- Different diversity factors
- Special load types
Common Mistakes #
1. Ignoring Diversity #
Problem: Using total connected load
Result: Oversized equipment, wasted money
Solution: Apply appropriate diversity factors
2. Wrong Power Factor #
Problem: Assuming 1.0 power factor
Result: Undersized equipment
Solution: Use actual measured power factor
3. Forgetting Starting Currents #
Problem: Only considering running current
Result: Breakers trip on startup
Solution: Account for motor starting currents
4. No Safety Margin #
Problem: Sizing exactly to calculated load
Result: No room for growth or errors
Solution: Add 20-25% safety margin
Best Practices #
- Be thorough: List all electrical loads
- Use realistic diversity: Don't be overly conservative
- Measure power factor: Don't guess
- Account for growth: Include future expansion
- Document everything: Keep detailed records
- Verify with measurements: Compare calculations to actual usage
Load Monitoring #
Regular Monitoring #
Monitor actual load regularly:
- Weekly: Spot checks
- Monthly: Detailed analysis
- Seasonally: Account for variations
- After changes: Recalculate after modifications
Load Analysis #
Analyze load patterns:
- Peak demand: Highest usage periods
- Average demand: Typical usage
- Load factor: Average ÷ Peak
- Diversity factor: Actual ÷ Connected
Conclusion #
Accurate factory load calculation is essential for safe, efficient, and code-compliant electrical systems. By following the step-by-step process, using proper formulas, and accounting for all factors, you can size equipment correctly and ensure reliable operation.
Key takeaways:
- Inventory everything: List all electrical loads
- Apply diversity: Not all loads operate simultaneously
- Consider power factor: Use actual PF, not assumed
- Add margins: Include safety and growth margins
- Use tools: Calculators simplify complex calculations
For quick calculations, use our Factory Load Calculator to get instant results with detailed explanations and recommendations.