Planning Guide

How to Size a Generator Set for Critical Loads

A reviewed guide to load definition, starting behavior, operating duty, site conditions and resilience inputs for critical-load generator selection.

CAMDA Technical Team Updated July 2026 12 min read Read guide

Key Takeaways

Build a verified load list with starting data
Define duty and redundancy before applying reserve
Include environment, installation and future load in the application review

Reliable generator sizing starts with the electrical behavior of the supported system. Running demand alone does not show motor starting, UPS recharge, transformer energization, load steps, redundancy or the required recovery sequence.

Technical planning workflow

Six-Step Project Guide

6 reviewed steps

Step 1

Define the Critical Load Profile

Build a verified load inventory with running demand, starting method, sequence and load priority.

Illustrative load inventory — confirm every value for the project
Load / input	Running kW	Starting factor	Sequence	Priority
Fire pump	45	3.0	1	Critical
HVAC chiller	75	1.8	2	High
Lighting and controls	35	1.2	3	High
UPS recharge	10	1.0	4	Review
Process motor	20	2.5	5	Critical

Example planning values only. Final load priority and sequence are maintained in the reviewed CMS article record.

Step 2

Calculate Power Requirements

Combine running demand, starting allowance, site derating and the agreed design margin.

Required capacity Running demand + starting allowance + design margin Verify transient response, site derating and duty before model release

Illustrative demand build-up Relative planning level; not generator performance data

Define the Critical Load Profile	Calculate Power Requirements	Apply Load Factors and Design Margins	Select the Generator Set	Validate Performance	Plan for the Long Term
38% 38% Build a verified load inventory with running demand, starting method, sequence and load priority.	46% 46% Combine running demand, starting allowance, site derating and the agreed design margin.	55% 55% Use diversity only where it is justified and record future-capacity assumptions.	63% 63% Review rating, alternator capability, controls, enclosure and balance-of-plant as one system.	72% 72% Verify transient response, starting sequence, voltage and frequency recovery, and site settings.	84% 84% Confirm service access, fuel autonomy, documentation and the path for future expansion.

Step 3

Apply Load Factors and Design Margins

Use diversity only where it is justified and record future-capacity assumptions.

Diversity factor

Largest acceptable load step and starting sequence

Future expansion

Unit count, redundancy, reserve and future-capacity allowance

Step 4

Select the Generator Set

Review rating, alternator capability, controls, enclosure and balance-of-plant as one system.

Selection point	Standby review	Prime review
Operating definition	Emergency duty	Average duty
Starting capability	Largest starting event	Load acceptance sequence
Runtime basis	Short-duration outage	Sustained operating profile
Site conditions	Apply project derating	Apply application rating basis

CAMDA engineer reviewing generator application information

Application engineering review

Compare the candidate set against the largest load step, motor-starting sequence, site derating, redundancy and future-capacity plan.

Step 5

Validate Performance

Verify transient response, starting sequence, voltage and frequency recovery, and site settings.

1 Reconcile the load list against the current single-line diagram
2 Check transient performance for the agreed sequence, not an assumed simultaneous start
3 Apply the applicable rating and site derating basis
4 Test the failure, recovery and maintenance operating scenarios

Sample validation profile Documented checkpoints across the agreed operating sequence

Reconcile the load list against the current single-line diagram	Check transient performance for the agreed sequence, not an assumed simultaneous start	Apply the applicable rating and site derating basis	Test the failure, recovery and maintenance operating scenarios
Reviewed input 72%	Model result 80%	Site setting 88%	Witness record 96%

Step 6

Plan for the Long Term

Confirm service access, fuel autonomy, documentation and the path for future expansion.

Maintenance access

Reconcile the load list against the current single-line diagram

Fuel autonomy

Check transient performance for the agreed sequence, not an assumed simultaneous start

Future capacity

Apply the applicable rating and site derating basis

Final selection checklist

Reconcile the load list against the current single-line diagram
Check transient performance for the agreed sequence, not an assumed simultaneous start
Apply the applicable rating and site derating basis
Test the failure, recovery and maintenance operating scenarios
Largest acceptable load step and starting sequence
Unit count, redundancy, reserve and future-capacity allowance

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How to Size a Generator Set for Critical Loads

Key Takeaways

Six-Step Project Guide

Define the Critical Load Profile

Calculate Power Requirements