Generator Sizing Chart

Use this worksheet after the calculator result to record base load, starting load, load sequencing, voltage dip notes, application mode, derating, recommended generator size, fuel estimate, output current, ATS rating, and conductor review.

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Quick reference table

A generator sizing chart is a calculator-led planning screen, not an order-ready generator selection. It organizes connected load, starting load, load sequencing, voltage dip, power factor, standby or prime mode, altitude and temperature derating, fuel, ATS rating, manufacturer data, utility requirements when applicable, adopted NEC checks, and AHJ requirements before equipment selection.

Generator sizing worksheet

Generator sizing worksheet
Checkpoint	Record from calculator	Review before selection
Load basis	Base kW, kVA, PF, load unit	Confirm load schedule, diversity, and essential load list
Starting load	Peak starting load and method	Coordinate motor starting, load sequencing, and voltage dip
Application mode	Standby, prime, or continuous	Confirm duty rating, runtime expectations, and code classification
Derating	Altitude and temperature factors	Check manufacturer data for site conditions
Output equipment	Generator current, ATS, conductor ampacity	Verify adopted NEC, listing, and equipment ratings
Fuel screen	Fuel rate and total fuel required	Confirm tank, ventilation, runtime, and logistics

Generator decision handoff

Generator decision handoff
Design issue	Document on the chart	Why it changes the answer
Motor starting	Largest motor, starter method, step sequence	Starting kVA and voltage dip can control size
Nonlinear loads	UPS, VFD, rectifier, medical or IT load notes	Waveform and alternator heating may change manufacturer sizing
Transfer equipment	ATS rating, poles, bypass, service or non-service use	Transfer switch selection is not the same as kW sizing
Fuel and runtime	Fuel type, consumption, tank size, refill plan	Runtime can govern project acceptance even when kW is adequate
Site conditions	Altitude, ambient temperature, enclosure, ventilation	Derating can lower usable generator output

Formula basis

Required generator kW = larger of base load and starting load, adjusted for growth, safety margin, application factor, and environmental derating.

Base load is the converted kW value from the selected load unit.
Starting load is the motor or transient load screen entered in the calculator.
Derating reflects application mode, altitude, ambient temperature, and manufacturer assumptions.
Recommended size is the next planning generator size above the calculated requirement.

Worked examples

Standby generator recordRecord connected kW, starting kW, PF, safety margin, load growth, derating, recommended size, output current, ATS rating, and fuel estimate before requesting manufacturer review.

Motor-starting dominated loadA facility with modest running kW but one large motor should keep starting method, sequence, acceptable voltage dip, and generator response notes beside the calculator result.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

The worksheet assumes calculator loads are planning values and not a substitute for a complete load schedule or manufacturer sizing program.
Generator selection can change with motor starting, nonlinear loads, voltage dip limits, fuel system, emissions, transfer equipment, and project code classification.
Standby, prime, continuous, emergency, optional standby, and legally required standby use cases can carry different equipment and review paths.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Use this chart as a planning worksheet; verify adopted NEC requirements, NFPA emergency or standby power requirements where applicable, manufacturer data, utility interconnection, equipment ratings, fuel system requirements, and AHJ review before generator selection.

How to use this chart

1Record load and modeCopy total load, load unit, power factor, application mode, starting load, and starting method from the calculator.

2Document deratingRecord altitude, temperature, safety factor, load growth, application factor, and manufacturer derating assumptions so the recommended size is traceable.

3Prepare equipment reviewUse the worksheet to list generator output current, conductor ampacity, ATS rating, fuel system, ventilation, and manufacturer checks.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Capture connected loadWrite base connected load, converted kW, required kVA, PF, load schedule assumptions, and essential or optional load grouping.
Capture starting conditionDocument starting load, starting multiplier or method, load sequencing, acceptable voltage dip, and nonlinear-load notes.
Capture final review itemsList manufacturer sizing, code classification, transfer equipment, fuel storage, ventilation, emissions, utility interconnection, and AHJ review items.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Sizing only from running kW while ignoring motor starting, load sequencing, voltage dip, and nonlinear load behavior.
Treating standby, prime, and continuous ratings as interchangeable without checking manufacturer duty rating and code classification.
Forgetting ATS rating, fuel system, ventilation, grounding, output conductor, and utility interconnection checks after the generator size looks adequate.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Can the recommended size be ordered directly?

No. Treat it as a planning size. Manufacturer sizing, voltage dip review, load sequencing, fuel system, transfer equipment, and code classification still need review.

Why record starting load separately?

Motor and transient loads can control generator sizing even when the steady connected load appears smaller.