Sizing a backup generator for new construction is one of the most critical decisions you’ll make during the design phase. Get it right, and you have reliable power protection for decades. Get it wrong, and you face expensive replacement, system failure when you need it most, or wasted money on excessive capacity. This guide walks […]
How to Size a Generator for New Construction
Sizing a backup generator for new construction is one of the most critical decisions you’ll make during the design phase. Get it right, and you have reliable power protection for decades. Get it wrong, and you face expensive replacement, system failure when you need it most, or wasted money on excessive capacity.
This guide walks you through the complete generator sizing process using professional engineering methodology. Whether you’re a facility manager, project engineer, architect, or building owner, you’ll understand exactly how to determine the correct generator size for your new commercial building.
By the end of this guide, you’ll know:
How professional load calculations are performed
What the electrical codes require
Common sizing mistakes and how to avoid them
How to optimize costs without compromising reliability
Why Proper Generator Sizing Matters
The Consequences of Incorrect Sizing
Undersized Generators:
When a generator is too small for the connected load:
System failure under load – Generator cannot carry the building’s electrical demand
Overload trips – Protective devices shut down power entirely
Voltage sag – Equipment may malfunction or be damaged
The most critical failure: During an emergency when you need backup power most, an undersized generator fails to operate properly or shuts down completely.
Oversized Generators:
While less catastrophic than undersizing, excessive capacity creates problems:
Higher initial cost – Larger generators cost significantly more
Fuel inefficiency – Generators running at <30% capacity waste fuel
Wet stacking – Light loads cause unburned fuel accumulation in diesel engines
Increased maintenance – Larger systems cost more to maintain
Space requirements – Bigger generators need more room and larger pads
Unnecessary complexity – Parallel systems when single unit would suffice
The financial impact: Oversizing by 50% can add $30,000-$100,000+ to project costs depending on system size, with no operational benefit.
The Right-Sizing Goal
Professional generator sizing aims for:
100% of critical load capacity at rated output
20-30% future growth margin for equipment additions
Proper motor starting capacity without voltage sag
Efficient operating range (40-80% load typical)
Code compliance for all applicable standards
Optimized first cost without compromising reliability
Understanding Load Types and Calculations
Connected Load vs. Demand Load
Connected Load: The total nameplate rating of all electrical equipment that could potentially operate simultaneously.
Example:
HVAC: 200 kW
Lighting: 50 kW
Receptacles: 30 kW
Elevators: 40 kW
Process equipment: 80 kW
Total connected: 400 kW
Demand Load: The actual maximum electrical demand considering diversity (not everything runs simultaneously at full capacity).
Same example with diversity factors:
HVAC: 200 kW × 0.85 = 170 kW
Lighting: 50 kW × 0.75 = 37.5 kW
Receptacles: 30 kW × 0.50 = 15 kW
Elevators: 40 kW × 0.60 = 24 kW
Process equipment: 80 kW × 0.90 = 72 kW
Total demand: 318.5 kW
Critical distinction: Sizing to connected load (400 kW) wastes money. Sizing to demand load (318.5 kW) is appropriate, but you must use verified diversity factors, not guesses.
Critical vs. Non-Critical Loads
Critical Loads (Must operate during outage):
Life safety systems (emergency lighting, exit signs, fire alarm)
Essential equipment (servers, critical manufacturing, medical equipment)
HVAC for critical areas (server rooms, cleanrooms, operating rooms)
Security and communications systems
Fire pumps and sprinkler systems
Elevator recall (at least one elevator)
Non-Critical Loads (Can be shed during outage):
General office lighting (beyond egress)
Comfort HVAC for non-critical areas
Convenience receptacles
Non-essential equipment
Electric water heaters
Most kitchen equipment
Sizing approach: Generator only needs to supply critical loads. Proper load shedding can reduce required generator size by 30-50% compared to serving entire building.
The Professional Load Calculation Process
Step 1: Inventory All Critical Equipment
Create comprehensive list including:
For each piece of equipment, document:
Equipment description and location
Nameplate rating (kW or HP)
Voltage and phase (120V, 208V, 480V, 3-phase, etc.)
Power factor (if known, otherwise assume 0.8-0.85)
Sum all running loads for total steady-state demand.
Step 3: Calculate Motor Starting Loads
Why motor starting matters:
Electric motors draw 3-6 times their running current when starting (called “locked rotor” or “inrush” current). This surge lasts only 1-10 seconds but can:
Cause voltage sag affecting other equipment
Trip generator overload protection
Damage motor contactors and starters
Create nuisance problems for sensitive electronics
Starting current multipliers by motor type:
Across-the-Line Starting (Direct Online):
Small motors (<5 HP): 4-5× running current
Medium motors (5-50 HP): 5-6× running current
Large motors (>50 HP): 6-7× running current
Soft-Start or Reduced Voltage:
2-3× running current (reduced inrush)
Variable Frequency Drives (VFD):
1-1.5× running current (minimal inrush)
Critical calculation:
You must size generator to handle:
All running loads PLUS
Starting current of largest motor PLUS
25% margin for voltage regulation
Example calculation:
Building loads:
Steady-state running: 250 kW
Largest motor: 75 HP (56 kW running)
Starting method: Across-the-line
Starting multiplier: 6×
Starting kVA calculation:
Running kW × (1/PF) = Running kVA
56 kW × (1/0.85) = 65.9 kVA running
Starting kVA = Running kVA × Starting Multiplier
65.9 × 6 = 395 kVA starting requirement
Total generator requirement during motor start:
Steady loads: 250 kW = 294 kVA (at 0.85 PF)
Motor running: Included in steady loads
Motor starting: 395 kVA
TOTAL: 294 + 395 = 689 kVA
Convert to kW: 689 kVA × 0.8 = 551 kW peak demand
If same motor had VFD: Starting requirement would be ~66 kVA instead of 395 kVA, allowing much smaller generator (300-350 kW range).
Step 4: Apply Diversity Factors
Diversity factors account for:
Not all equipment operates simultaneously
Some loads cycle on/off
Partial loading of equipment
Time-of-day variations
National Electrical Code (NEC) provides diversity factors for certain load types, but these are minimum requirements. Professional engineering judgment required.
Typical diversity factors:
Lighting:
Office: 0.75-0.85
Warehouse: 0.80-0.90
Retail: 0.90-1.00 (most lights on)
Receptacle Loads:
Office: 0.40-0.60
Industrial: 0.50-0.70
Laboratory: 0.60-0.80
HVAC:
Cooling: 0.85-0.95 (most units run)
Heating: 0.70-0.85 (staging)
Ventilation: 0.90-1.00 (continuous)
Important: Conservative approach uses higher diversity factors (closer to 1.00) for critical facilities. Liberal approach uses lower factors but increases risk of undersizing.
Step 5: Add Future Growth Allowance
Industry standard: 20-30% capacity margin for future additions.
Why growth allowance matters:
Buildings add equipment over 20-30 year generator lifespan
Technology changes (more servers, electric vehicles, etc.)
For legally required emergency systems (life safety):
Generator must supply 100% of emergency load
No diversity factors allowed for emergency loads
Capacity verification required through engineering analysis
Must handle all emergency loads simultaneously
Article 701 – Legally Required Standby Systems:
For systems required by code but not immediate life safety:
Generator must supply 100% of legally required loads
Limited diversity may be acceptable with engineering justification
Load management systems may be used
Article 702 – Optional Standby Systems:
For non-required backup power:
No specific sizing requirements
Engineering discretion
Load management and prioritization acceptable
NFPA 110 – Emergency Power Systems
Applies to: Healthcare, high-rise buildings, facilities with emergency power requirements
Key sizing requirements:
Capacity Rating:
Generator must be rated for 100% of emergency plus legally required standby loads
No overloading allowed even temporarily
Must operate continuously at rated load
Voltage Regulation:
±10% of nominal voltage under all loading conditions
Starting large motors must not cause excessive voltage sag
Frequency regulation ±5% under all conditions
Ambient Conditions:
Generator rated capacity at site altitude and temperature
Derating required for high altitude (3.5% per 1,000 feet above sea level)
Derating required for high temperatures (typically derated above 77°F)
Michigan considerations:
Altitude derating usually minimal (below 1,500 feet elevation)
Temperature rating must consider summer highs (90-95°F)
Cold weather packages required for reliable winter starting
IEEE Standards
IEEE 446 (Orange Book) – Emergency and Standby Power Systems:
Recommended practices include:
Detailed load calculation methodology
Motor starting analysis
System one-line diagrams
Coordination studies
Testing and commissioning procedures
Professional engineers reference IEEE 446 for best practices beyond code minimums.
Common Generator Sizing Mistakes
Mistake #1: Using Rules of Thumb
Wrong approach: “We have a 50,000 square foot building, so we need 500 kW (10 watts/square foot).”
Why it’s wrong:
Building types vary dramatically in load density
Office: 5-8 W/sq ft
Warehouse: 2-4 W/sq ft
Data center: 50-200+ W/sq ft
Manufacturing: 10-50+ W/sq ft
Rule of thumb error margin: 50-200% in either direction
Correct approach: Actual load calculation based on installed equipment.
Mistake #2: Ignoring Motor Starting
Wrong assumption: “We have 300 kW of connected load, so a 350 kW generator is fine.”
Why it fails: If you have large motors with across-the-line starting, you may need 500-600 kW generator to handle starting surge without voltage sag or shutdown.
Correct approach: Calculate starting kVA for all motors, size generator accordingly, or specify soft-starters/VFDs to reduce starting load.
Mistake #3: No Growth Allowance
Short-sighted thinking: “We’ll size it exactly for current needs and save money.”
Five years later:
Building adds 10 computer servers
HVAC system upgraded
Security system expanded
EV charging stations added
Generator now undersized
Replacement cost: $150,000+ vs. $20,000 to add capacity during initial installation
Correct approach: Plan for 20-30% growth from day one.
Scenario: Building is at 5,000 feet elevation in Colorado. Engineer sizes 500 kW generator based on load calculations.
Reality: Generator derated 17.5% at altitude (3.5% per 1,000 ft)
Actual capacity: 500 × 0.825 = 412.5 kW
Now undersized for 500 kW load
Correct approach: Account for site conditions. Order 600 kW generator to achieve 500 kW capacity at altitude.
Michigan advantage: Low altitude minimizes this concern, but summer temperatures still require consideration.
Mistake #5: Single-Phase vs. Three-Phase Confusion
Common error:
Building has 480V 3-phase electrical service
Total load: 300 kW across all three phases
Engineer specifies 300 kW generator
Generator is single-phase or load is unbalanced
Problem: If load is unevenly distributed across phases, one phase may be overloaded even if total is within capacity.
Example:
Phase A: 150 kW
Phase B: 100 kW
Phase C: 50 kW
Total: 300 kW, but Phase A exceeds single-phase capacity
Correct approach: Balance loads across phases or size generator for maximum single-phase load.
Mistake #6: Fuel Type Selection Without Analysis
Diesel vs. Natural Gas decision made on:
Personal preference
“We’ve always used diesel”
First cost only
Better analysis considers:
Diesel advantages:
Independent fuel supply (on-site storage)
Higher energy density
Better performance at low temperatures
Lower installation cost (no gas service required)
Diesel disadvantages:
Fuel degradation (requires testing/polishing)
On-site fuel storage regulations
Higher maintenance cost
Emissions considerations
Natural Gas advantages:
No fuel storage required
No fuel degradation issues
Lower emissions
Lower maintenance costs
Unlimited runtime (assuming gas supply)
Natural Gas disadvantages:
Depends on utility gas supply (may fail during disaster)
Gas service upgrade may be expensive
Lower power density (larger engine for same output)
Cold weather performance challenges
Correct approach: Analyze both options with total lifecycle costs, reliability requirements, and site constraints.
🏗️ SIZING A GENERATOR FOR YOUR NEW BUILDING?
Get professional load calculations from Michigan’s generator experts. We’ll analyze your electrical drawings, calculate exact requirements, and recommend the right-sized system.
“How did you verify the engineer’s load calculations?”
“What specific generator model and manufacturer are you proposing?”
“Is the generator sized for Michigan weather conditions?”
“What is the generator’s actual capacity at our site elevation and temperature?”
“How are you handling the largest motor starting requirement?”
“What testing will be performed before system turnover?”
“What training will you provide to our facilities staff?”
“What is your emergency service response time and coverage?”
Red Flags During Contractor Selection
Warning signs:
No load calculation review or verification
“Our standard size for buildings like this”
Significant size variance between bidders (indicates someone is wrong)
No discussion of motor starting requirements
Vague specifications (“Generac or equal”)
No commissioning or testing plan
No training or documentation offered
Quality contractors:
Review and verify load calculations
Ask detailed questions about building operations
Provide detailed equipment specifications
Include comprehensive testing plan
Offer training and documentation
Discuss maintenance requirements and options
Conclusion: The Value of Proper Generator Sizing
Generator sizing is engineering, not guesswork. The investment in professional load calculations and proper sizing delivers:
Reliability Benefits: ✓ System operates properly when needed ✓ Adequate capacity for all critical loads ✓ Proper voltage regulation during motor starting ✓ Compliance with all applicable codes ✓ Reduced risk of emergency power failure
Financial Benefits: ✓ Optimized first cost (no oversizing waste) ✓ Efficient fuel consumption ✓ Reduced maintenance costs ✓ Proper loading prevents wet stacking ✓ Growth capacity without replacement
Long-Term Benefits: ✓ 20-30 year service life expected ✓ Flexibility for building changes ✓ Reliable performance throughout lifespan ✓ Maintained warranty coverage ✓ Asset value protection
The cost difference between properly sized and incorrectly sized generators:
Oversized: Waste $30,000-$100,000+ on initial cost plus ongoing inefficiency
Undersized: Replace entire system ($150,000-$500,000+) or face failure during emergencies
Properly sized: Optimized investment with decades of reliable service
Professional engineering pays for itself many times over.
Next Steps: Generator Sizing for Your Project
Free Generator Sizing Consultation
Wolverine Power Systems offers complimentary generator sizing consultations for new construction projects in Michigan:
✓ Generator sizing requires professional load calculations, not rules of thumb ✓ Motor starting loads often determine generator size, not running loads ✓ Diversity factors reduce required capacity but must be verified ✓ 20-30% growth allowance prevents future undersizing ✓ NEC, NFPA 110, and IEEE standards govern sizing requirements ✓ Undersizing causes failure; oversizing wastes money ✓ Site conditions (altitude, temperature) affect capacity ✓ Load management and soft-starters can reduce required generator size ✓ Professional engineering review is essential during design ✓ Proper commissioning verifies sizing accuracy
Generator sizing is one of the most important decisions in new construction. Get it right with professional engineering and experienced contractors.
About the Author
Wolverine Power Systems has served Michigan’s commercial construction market since 1997, providing generator sales, engineering support, installation, and commissioning for new construction generator projects. Our experienced generator team works with architects, electrical engineers, and contractors to ensure proper generator sizing and code-compliant installations.
