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

Real-World Case Study: Manufacturing Facility Expansion #

Project Background: A 15,000 m² manufacturing facility in Ohio needed to add 200kW of new production equipment. The existing electrical system had a 500kVA transformer serving the facility. Management needed to determine if the transformer could handle the additional load or if a costly upgrade was required.

Initial Assessment:

  • Existing connected load: 350kW (nameplate ratings)
  • Existing transformer: 500kVA, 480V, installed 8 years ago
  • Proposed new equipment: 200kW additional load
  • Total if added: 550kW connected load

Initial Concern: 550kW connected load on a 500kVA transformer seemed impossible. However, proper load analysis revealed a different story.

Detailed Load Analysis:

Step 1: Existing Load Diversity Analysis

  • Production equipment (200kW): Applied 0.75 diversity factor = 150kW
  • HVAC systems (80kW): Applied 0.70 diversity factor = 56kW
  • Lighting (40kW): Applied 0.95 diversity factor = 38kW
  • Office equipment (30kW): Applied 0.60 diversity factor = 18kW
  • Actual existing demand: 150 + 56 + 38 + 18 = 262kW

Step 2: Power Factor Analysis

  • Weighted average PF: 0.87
  • Existing kVA demand: 262kW ÷ 0.87 = 301kVA
  • Available transformer capacity: 500kVA - 301kVA = 199kVA available

Step 3: New Equipment Analysis

  • New equipment: 200kW connected
  • Diversity factor (new equipment): 0.80 (not all machines run simultaneously)
  • New demand: 200kW × 0.80 = 160kW
  • New kVA requirement: 160kW ÷ 0.87 = 184kVA

Step 4: Load Scheduling Optimization

  • Analyzed production schedules
  • Identified that peak existing load occurs 2-4 PM
  • New equipment could be scheduled for morning shift (8 AM - 12 PM)
  • Peak simultaneous load: 301kVA (existing peak) + 92kVA (50% of new load) = 393kVA

Solution:

  1. No transformer upgrade needed - 393kVA is well within 500kVA capacity (78% loading)
  2. Implemented load scheduling - Staggered operation to prevent simultaneous peaks
  3. Added load monitoring - Installed power meters to track actual demand
  4. Future-proofing: Identified that up to 250kW additional load could be added with proper scheduling

Results:

  • Cost savings: Avoided $50,000 transformer upgrade
  • Zero downtime: No electrical system modifications required
  • Optimized operations: Load scheduling improved overall efficiency
  • 12-month verification: Actual peak demand never exceeded 420kVA

Key Takeaway: This case study demonstrates the critical importance of applying proper diversity factors and load analysis. Simply adding nameplate ratings (350kW + 200kW = 550kW) would have led to an unnecessary $50,000 expense. Proper load calculation, considering diversity factors and load scheduling, revealed that the existing system had adequate capacity.

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

Standards & References #

All load calculation methods and safety factors in this guide are based on recognized international engineering standards:

IEEE Standards #

  • IEEE 141-1993 - Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book) - Provides diversity factors and load calculation methods
  • IEEE 241-1990 - Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book)

IEC Standards #

  • IEC 60364 - Low-voltage electrical installations - Part 4-41: Protection for safety, Part 5-52: Selection and erection of electrical equipment
  • IEC 60909 - Short-circuit currents in three-phase a.c. systems - Used for fault current calculations

NEC (National Electrical Code) #

  • NFPA 70 (NEC) - National Electrical Code
    • Article 220 - Branch-Circuit, Feeder, and Service Calculations
    • Article 430 - Motors, Motor Circuits, and Controllers
    • Article 450 - Transformers and Transformer Vaults

Industry Resources #

Engineer's Practical Insight #

From 12+ years of factory design experience: The biggest mistake I see is using connected load (sum of all nameplate ratings) instead of demand load (actual simultaneous usage). In a typical manufacturing facility, actual demand is only 60-75% of connected load due to diversity factors. I've seen projects where this mistake led to $100,000+ in unnecessary transformer and switchgear upgrades.

Practical diversity factors I use: For production equipment, I typically use 0.70-0.75 (machines cycle on/off). For HVAC, 0.60-0.70 depending on climate and building automation. For welding equipment, 0.30-0.40 because welding is highly intermittent. These aren't just numbers from a book—they're based on actual load monitoring data from dozens of facilities.

Critical field observation: Load scheduling can make or break a project. In one automotive plant expansion, we avoided a $50,000 transformer upgrade by simply scheduling the new equipment to run during off-peak hours. The existing 500kVA transformer had 180kVA available capacity—enough for the new 200kW load when properly scheduled. Always analyze load patterns, not just total connected load.

Safety margin reality check: Code minimum is 125% for continuous loads, but for factory applications with motor starting currents and future expansion, I recommend 20-25% margin. However, don't go overboard—oversizing by 50%+ wastes capital and reduces efficiency at light loads. The sweet spot is 20-30% above calculated demand load.

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.

About the Author: Michael Rodriguez, P.E. is a senior power systems engineer with 12+ years of experience in factory electrical design and facility expansion projects. He has designed electrical systems for automotive manufacturing plants, food processing facilities, and textile mills. Specializes in load analysis, transformer sizing, and electrical distribution system optimization. All content in this guide has been reviewed and validated by licensed engineers.