Nameplate power describes steady operation, not the source demand during starting. Pumps, fans, chillers, compressors and production equipment create short transients that can cause voltage dip, contactor dropout, controller reset or breaker trips when sizing is wrong.
First, Confirm Whether This Article Matches Your Project
Owners, procurement teams, facility managers and electrical leads buying a first set, expanding, replacing an old unit or adapting to a changed load profile.
It separates symptoms, decision criteria, site inputs and engineering boundaries for generator sizing.
A sizing basis that records operating duty, critical loads, starting sequence, environmental derating and future-expansion limits.
Generator size should be calculated from the load schedule, starting methods and sequence—not by adding nameplate ratings and applying a fixed margin.
Check 01
What Information Must the Load Schedule Include?
Equipment names and kW alone cannot define starting transients or reactive demand. Electrical and operating data are also required.
Record rated power, voltage, phases and power factor, plus DOL, star-delta, soft-starter or VFD starting. Document starting time, frequency, allowable voltage dip and normal, standby or time-based operating status. Without written evidence, do not assume the load schedule is complete.
- Rated Power, Voltage, Phases and Power Factor
- DOL, Star-Delta, Soft-Starter or VFD Starting
- Starting Time, Frequency and Allowable Voltage Dip
- Normal, Standby and Time-Based Operating Status
Check 02
How Much Does Sequenced Starting Change the Result?
Simultaneous recovery of large motors stacks their transients. Proper delays and priorities can often reduce required generator size without sacrificing operations.
Define which loads start first after an outage and prevent chillers, pumps and elevators from reconnecting together. Reserve stable power for controls and safety, and document that each step stabilizes before the next. Without written evidence, do not assume the sequence is workable.
- Define Which Loads Start First After an Outage
- Prevent Chillers, Pumps and Elevators from Reconnecting Together
- Reserve Stable Power for Controls and Safety Loads
- Require Each Load Step to Stabilize Before the Next
Check 03
Why Starting Method Changes Generator Size
Soft starters and VFDs can reduce mechanical and electrical transients, but harmonics, bypass, control logic and generator compatibility still require review.
Confirm starting-current multiple and duration, plus VFD input characteristics and harmonics. Document soft-starter bypass and protection settings, and provide starter data to the sizing engineer. Without written evidence, do not assume starting demand is known.
- Confirm Starting-Current Multiple and Duration
- Review VFD Input Characteristics and Harmonics
- Confirm Soft-Starter Bypass and Protection Settings
- Provide Starter Data to the Sizing Engineer
Check 04
How to Validate the Sizing Result on Site
Calculations must be validated through the loaded sequence, starting voltage and frequency recovery time, especially for critical large motors.
Record voltage and frequency before and during starting, and observe engine speed and smoke. Document controller alarms, breaker status, worst-case load combination and recovery sequence. Without written evidence, do not assume the calculation is proven.
- Record Voltage and Frequency Before and During Starting
- Observe Engine Speed and Black-Smoke Response
- Check Controller Alarms and Breaker Status
- Validate the Worst-Case Load Combination and Recovery Sequence
Decision record
Put Each Decision in the Project Record, Not Just a Verbal Confirmation
Selection should not begin with brand and price. Turn loads, duty and site constraints into reviewable project conditions, then compare supplier proposals on the same basis.
| Decision Point | Prepare on Site | Confirm in Design or Acceptance |
|---|---|---|
| What Information Must the Load Schedule Include? | Rated Power, Voltage, Phases and Power FactorDOL, Star-Delta, Soft-Starter or VFD Starting | Starting Time, Frequency and Allowable Voltage DipNormal, Standby and Time-Based Operating Status |
| How Much Does Sequenced Starting Change the Result? | Define Which Loads Start First After an OutagePrevent Chillers, Pumps and Elevators from Reconnecting Together | Reserve Stable Power for Controls and Safety LoadsRequire Each Load Step to Stabilize Before the Next |
| Why Starting Method Changes Generator Size | Confirm Starting-Current Multiple and DurationReview VFD Input Characteristics and Harmonics | Confirm Soft-Starter Bypass and Protection SettingsProvide Starter Data to the Sizing Engineer |
| How to Validate the Sizing Result on Site | Record Voltage and Frequency Before and During StartingObserve Engine Speed and Black-Smoke Response | Check Controller Alarms and Breaker StatusValidate the Worst-Case Load Combination and Recovery Sequence |
Common Mistakes
These Shortcuts Leave the Risk Until After the Outage
Simply Adding Equipment Nameplate Ratings
Nameplate ratings do not show motor starting demand, coincident operation or recovery order. Capacity may look sufficient on paper yet fail to start the site loads.
Replacing Engineering Calculations with a Fixed Percentage
Reserve must separately address starting demand, environmental derating, future expansion and redundancy. One blanket percentage cannot define those boundaries.
Comparing Only the Generator-Set Price
A low purchase price is not a low total cost when fuel, distribution, installation, spares, maintenance and outage losses are excluded.
Handover Check
At Minimum, the Project Should Retain These Deliverables
End customers do not need to perform every calculation, but they should receive clear documents and know what data supports each conclusion.
Load Schedule and Starting Matrix
Record each load’s power, voltage, power factor, starting method, coincidence factor and recovery sequence.
Capacity and Rating-Basis Statement
State prime or standby duty, kW and kVA basis, environmental correction and capacity justification.
Expansion and Redundancy Limits
State whether future loads, paralleling conditions, maintenance windows and N+1 requirements have been allowed for.
Like-for-Like Proposal Comparison
Compare equipment, fuel, installation, maintenance, spares and service scope in one table.