Introduction #

Factory load calculation mistakes cost thousands of dollars in wasted equipment, cause production downtime, and create safety hazards. After reviewing hundreds of factory electrical designs, we've identified the most common errors engineers make—and how to avoid them. This guide covers real mistakes from actual projects, explains why they happen, and provides solutions you can apply immediately.

Mistake 1: Using Connected Load Instead of Demand Load #

The Error #

Sizing equipment based on the sum of all nameplate ratings without applying diversity factors.

Example:

Equipment inventory:
- 50 machines × 10 kW = 500 kW connected
- Transformer selected: 500 kVA
- Breaker selected: 600 A

Why It Happens #

  • Simpler calculation (just addition)
  • "Safe" approach (oversizing seems better)
  • Lack of understanding of diversity factors
  • Pressure to complete design quickly

The Problem #

Reality:

  • Actual demand load: 350 kW (with diversity)
  • Transformer operates at 70% load (inefficient)
  • Wasted capital: $8,000
  • Annual energy waste: $1,800/year

The Solution #

Always calculate demand load:

Connected Load: 500 kW
Diversity Factor: 0.70
Demand Load: 500 × 0.70 = 350 kW
Power Factor: 0.85
Apparent Power: 350 ÷ 0.85 = 412 kVA

Selected Transformer: 500 kVA (appropriate)

Use our Factory Load Calculator to automatically apply diversity factors.

Mistake 2: Ignoring Power Factor in kVA Calculations #

The Error #

Assuming kW = kVA, or using incorrect power factor values.

Example:

Calculated load: 300 kW
Assumed: 300 kW = 300 kVA
Selected transformer: 300 kVA

Why It Happens #

  • Misunderstanding of power factor concept
  • Using unity PF for simplicity
  • Not calculating weighted power factor
  • Assuming all equipment has same PF

The Problem #

Reality:

  • Actual power factor: 0.80 (mixed loads)
  • Required kVA: 300 ÷ 0.80 = 375 kVA
  • Transformer undersized by 25%
  • Breaker trips during peak loads
  • Production downtime: $12,000

The Solution #

Always calculate kVA correctly:

Real Power: 300 kW
Power Factor: 0.80 (weighted)
Apparent Power: 300 ÷ 0.80 = 375 kVA

Selected Transformer: 400 kVA (next standard)

Calculate weighted power factor for mixed loads. See our guide: Typical Power Factor Values for Industrial Equipment.

Mistake 3: Overestimating Diversity Factors #

The Error #

Using overly optimistic diversity factors to reduce calculated load.

Example:

Connected Load: 400 kW
Diversity Factor Used: 0.50 (too optimistic)
Demand Load: 400 × 0.50 = 200 kW

Why It Happens #

  • Trying to reduce equipment costs
  • Lack of actual measurement data
  • Using factors from different industry
  • Not understanding equipment operating patterns

The Problem #

Reality:

  • Actual diversity factor: 0.75
  • Actual demand: 400 × 0.75 = 300 kW
  • System undersized by 50%
  • Constant breaker trips
  • Equipment damage from voltage drops

The Solution #

Use realistic diversity factors:

  • Measure actual demand when possible
  • Use industry standards as starting point
  • Document assumptions
  • Add safety margin

See: Diversity Factor Explained in Factory Load Calculation

Mistake 4: Not Accounting for Motor Starting Currents #

The Error #

Sizing breakers based only on running current, ignoring starting inrush.

Example:

Motor: 50 HP, 65 A running current
Breaker selected: 70 A (based on running current only)

Why It Happens #

  • Focus on steady-state operation
  • Unfamiliar with motor starting characteristics
  • Assuming breakers handle inrush automatically
  • Not considering multiple motors starting

The Problem #

Reality:

  • Starting current: 65 A × 6 = 390 A
  • Breaker trips on every start
  • Production delays
  • Motor damage from repeated starts

The Solution #

Account for starting currents:

Running Current: 65 A
Starting Current: 65 × 6 = 390 A
Breaker Type: Time-delay (handles inrush)
Breaker Size: 100 A (with time delay)
Or: Soft starter to reduce inrush

For main breakers, ensure coordination with motor protection.

Mistake 5: Insufficient Safety Margin #

The Error #

Sizing exactly at calculated load without adequate margin.

Example:

Calculated Demand: 250 kW
Design Load: 250 kW (no margin)
Transformer: 250 kVA (exact match)

Why It Happens #

  • Trying to minimize equipment costs
  • Overconfidence in calculations
  • Pressure to reduce capital expenditure
  • Not considering load variations

The Problem #

Reality:

  • Peak loads: 270 kW (8% over)
  • Breaker trips during peak production
  • Voltage drops affect equipment
  • No room for future growth

The Solution #

Always add appropriate safety margin:

Demand Load: 250 kW
Safety Margin: 20%
Design Load: 250 × 1.20 = 300 kW
Design kVA: 300 ÷ 0.85 = 353 kVA

Selected Transformer: 400 kVA (with margin)

See: Safety Margin in Factory Load: How Much Is Enough?

Mistake 6: Mixing Single-Phase and Three-Phase Loads Incorrectly #

The Error #

Incorrectly combining single-phase and three-phase loads in calculations.

Example:

Three-phase load: 200 kW
Single-phase load: 50 kW
Total calculated: 250 kW (incorrect addition)

Why It Happens #

  • Not understanding phase balance
  • Treating all loads the same
  • Incorrect conversion methods
  • Lack of three-phase system knowledge

The Problem #

Reality:

  • Single-phase loads create phase imbalance
  • Neutral current not accounted for
  • Incorrect breaker sizing
  • Potential equipment damage

The Solution #

Handle separately:

Three-phase load: 200 kW (balanced)
Single-phase load: 50 kW per phase
Total per phase: 200/3 + 50 = 116.7 kW
Total three-phase equivalent: 116.7 × 3 = 350 kW

Or distribute single-phase loads across phases for balance.

Mistake 7: Not Considering Future Expansion #

The Error #

Sizing for current load only, with no allowance for growth.

Example:

Current Load: 300 kW
Transformer: 315 kVA (sized for current only)
No expansion capacity

Why It Happens #

  • Focus on immediate needs
  • Budget constraints
  • Uncertainty about future plans
  • "We'll add capacity later" thinking

The Problem #

Reality:

  • Expansion needed in 18 months
  • New equipment: +80 kW
  • Total required: 380 kW
  • Transformer overloaded
  • Need to replace transformer: $15,000

The Solution #

Plan for growth:

Current Load: 300 kW
Planned Growth: 20% in 2 years
Design Load: 300 × 1.20 = 360 kW
Design kVA: 360 ÷ 0.85 = 424 kVA

Selected Transformer: 500 kVA (room for growth)

See: Factory Load Planning for Future Expansion

Mistake 8: Incorrect Breaker Sizing #

The Error #

Sizing breakers too small (trips) or too large (poor protection).

Example:

Calculated Current: 180 A
Breaker Selected: 200 A (too close, no margin)

Why It Happens #

  • Exact matching to calculated current
  • Not considering starting currents
  • Ignoring code requirements
  • Not accounting for ambient temperature

The Problem #

Reality:

  • Starting currents cause trips
  • Load variations exceed rating
  • Breaker operates near limit
  • Premature failure

The Solution #

Size with appropriate margin:

Calculated Current: 180 A
Starting Current Consideration: +25%
Design Current: 180 × 1.25 = 225 A
Breaker Selected: 250 A (next standard, with margin)

Ensure breaker coordinates with upstream protection.

Mistake 9: Using Generic Power Factor for All Equipment #

The Error #

Assuming all equipment has the same power factor (often 0.85 or 1.0).

Example:

All equipment: PF = 0.85
Welding equipment: Also 0.85 (incorrect)

Why It Happens #

  • Simplification for calculation
  • Lack of equipment-specific data
  • Not calculating weighted average
  • Assuming "typical" values apply

The Problem #

Reality:

  • Welding: PF = 0.40 (very poor)
  • Weighted PF: 0.72 (not 0.85)
  • kVA underestimated by 18%
  • Transformer undersized

The Solution #

Calculate weighted power factor:

Equipment A: 200 kW at 0.85 PF = 235.3 kVA
Equipment B: 50 kW at 0.40 PF = 125.0 kVA
Total: 250 kW, 360.3 kVA
Weighted PF: 250 ÷ 360.3 = 0.694

See: Typical Power Factor Values for Industrial Equipment

Mistake 10: Not Validating Calculations #

The Error #

Not checking results for reasonableness or comparing to similar facilities.

Example:

Calculated: 50 kW for 10,000 m² facility
No validation
Proceeded with design

Why It Happens #

  • Time pressure
  • Overconfidence
  • Lack of reference data
  • No review process

The Problem #

Reality:

  • Typical load: 150-200 kW for this size
  • Calculation error: Off by 75%
  • System severely undersized
  • Complete redesign required

The Solution #

Always validate:

  • Compare to similar facilities
  • Check against industry benchmarks
  • Review with experienced engineer
  • Use multiple calculation methods
  • Measure after installation

How to Avoid These Mistakes #

1. Use Proper Methodology #

Follow systematic approach:

  1. Complete equipment inventory
  2. Apply diversity factors
  3. Calculate weighted power factor
  4. Add safety margin
  5. Select standard equipment sizes

2. Document Assumptions #

Record:

  • Diversity factors used
  • Power factor values
  • Safety margin selected
  • Future expansion plans
  • Special considerations

3. Validate Results #

Check:

  • Reasonableness (compare to benchmarks)
  • Equipment sizing (not too close to limits)
  • Code compliance
  • Coordination (breakers, transformers)

4. Review with Tools #

Use calculators and tools:

5. Learn from Experience #

After installation:

  • Measure actual loads
  • Compare to calculated
  • Adjust methods for future
  • Build reference database

Integration with Factory Load Calculator #

Our Factory Load Calculator helps avoid these common mistakes by:

  • Automatically applying diversity factors
  • Calculating weighted power factor
  • Adding appropriate safety margins
  • Providing equipment sizing recommendations

Try it now: Factory Load Calculator

Conclusion #

Factory load calculation mistakes are costly and dangerous. The most common errors stem from oversimplification, incorrect assumptions, and lack of validation. By understanding these mistakes, applying proper methodology, and using appropriate tools, you can avoid costly errors and design efficient, reliable electrical systems. Always document assumptions, validate results, and learn from actual measurements after installation.