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Generator performance class G1 G2 G3 G4 describes electrical performance requirements, including voltage and frequency response under defined conditions. A class label alone does not establish duty rating, prove an arbitrary load step, or confirm compatibility with every connected device.
The purchasing risk lies in the missing conditions behind the letter. A bid can name G3 while leaving the applied load, recovery definition and test configuration unclear. This guide separates the standard’s test basis from your site’s operating sequence and turns both into reviewable acceptance requirements.
Key takeaways
- Specify duty rating separately from electrical performance class.
- Review voltage and frequency together, including recovery bands and times.
- Separate ISO steps from the project’s pickup and rejection sequence.
- Treat G4 as agreed requirements that must be written down.
- Accept evidence only for the configuration and conditions it covers.
What Generator Performance Class G1 G2 G3 G4 Actually Specifies
Class and duty rating answer different questions
Duty rating describes the operating duty associated with a stated output. Performance class addresses how the generating set maintains and restores electrical conditions within its specified test framework. Two offers with the same kVA can have different performance evidence. Neither the kVA number nor a class label substitutes for the other.
Start the comparison with separate entries for output, duty, voltage, frequency, site conditions and performance class. Our guide to ISO 8528 generator ratings explains the duty side. Keep that selection visible when reviewing transient performance so the supplier knows the operating basis behind the enquiry.
Illustrative scenario: Two tender responses state the same capacity. One includes a defined acceptance sequence and recovery criteria; the other says only G3. Ask the second bidder for the missing test basis. The shared capacity does not resolve that gap, and the difference alone does not establish which set meets the project.
Read the categories without creating an automatic ladder
The Cummins technical paper explains the class framework and associated electrical parameters. This comparison is qualitative, not a substitute for the normative tables.
| Class | General meaning | What the buyer still needs |
|---|---|---|
| G1 | Basic electrical performance requirements | Confirmation that loads tolerate the specified behavior |
| G2 | General-purpose electrical requirements | Applicable limits, load steps and equipment tolerances |
| G3 | More demanding electrical requirements | Evidence for defined class tests and the project sequence |
| G4 | Requirements agreed by purchaser and manufacturer | Written parameters, limits and test conditions |
Do not assign a class solely from a building label. Hospital, factory or data center does not identify the load sequence or every device’s tolerance. Check applicable project rules and equipment requirements separately.
Prepare the load schedule and collect equipment tolerance sheets before comparing quotations. Include them with a Shanhua generator enquiry and request a configuration-specific response. Any performance commitment needs confirmation in the written offer; this article is not a compliance declaration for a particular set.
Read Voltage and Frequency Response Together
Separate settled operation from a load-change event
Steady-state behavior describes settled operating conditions. Transient behavior concerns the disturbance after a change, such as adding or rejecting load. A specification should address both.
Equipment can trip during a disturbance even when the set later settles within the agreed band. Define the allowed excursion and duration together; a minimum voltage reading alone cannot describe the full event.
Voltage and frequency are related parts of the response, but they are not interchangeable readings. Engine and governing behavior matter to frequency. Alternator, excitation and regulation characteristics matter to voltage, alongside the effect of the connected load. Use the alternator specification guide for background rather than treating the controller label as a whole-set guarantee.
Record the excursion and recovery definition
Define the initial condition, instant of load change, largest excursion and recovery criterion. State whether a percentage is referenced to the rated value or the value immediately before the event. Otherwise similar percentages may describe different quantities.
A dip is a downward excursion; a rise is an upward excursion. Application and rejection require their own assessment. A favorable result in one direction does not prove the other. The report should identify the event behind each trace and the voltage and frequency channels recorded.
Recovery time is meaningful only when the destination band and timing rule are stated. Ask when timing starts, which band must be reached, and how sustained recovery is determined. Do not silently substitute the recovery band for the steady-state tolerance. Cummins explains these distinctions, but its historical limit table explicitly uses an older edition.
Read the trace as a sequence
A conceptual response curve starts at the pre-event value, moves to an excursion, then returns toward a defined band. It may cross that band before remaining within it. These are different points on the trace, so returned to normal is too vague for acceptance.
Compare voltage and frequency on a common time basis. Check whether the load remained applied and whether another event occurred before settling. Request underlying traces and event labels, not merely a screenshot stamped pass. The record should let a reviewer understand the sequence without guessing.
Specify ISO Load Steps Separately from Project Steps
Keep the standard profile explicit
Kohler’s transient-performance paper explains acceptance in relation to brake mean effective pressure, or BMEP. This engine loading parameter features in the discussed ISO step framework. Do not replace that framework with an invented universal tender percentage.
A class claim does not automatically mean full rated load can be accepted from no load in one step while meeting every desired limit. Acceptance and rejection are distinct events. A full-load rejection statement cannot prove full-load acceptance. Ask for the standard edition, applicable sequence and supporting report.
Describe what actually happens on site
Record what is already running, what starts next, what disconnects, and how long controls allow between events. State real power in kW and apparent power in kVA where relevant. Include power factor and load characteristics. A motor, UPS or nonlinear load should not be assessed as an unspecified resistive block.
Use the generator load schedule guide to assemble the sequence. Keep sustained demand, starting events and later recharge demand distinguishable. A connected-load total hides the order that drives the performance question.
Illustrative scenario: A pump starts while a UPS restores battery charge. Listing only combined steady demand leaves the transition undefined. The reviewer needs the pump’s starting method, UPS input behavior and control timing. This hypothetical scheduling example does not describe a customer test or assert that either load will cause failure.
Give each event a row containing initial load, added or removed load, load type, starting method, duration and interval before the next event. Attach equipment tolerances. Mark assumptions clearly so the supplier can confirm them or return a deviation.
Caterpillar’s transient specifications paper highlights why custom steps need careful specification. Keep one schedule for the standard test basis and another for project events. Request evidence addressing each.
If a report supports only the standard profile, mark project-profile confirmation as a separate open item. This avoids treating one test schedule as implicit approval of another.
If the sequence changes after selection, reopen the performance review. A delayed restart or changed control setting can alter the event. The original report remains evidence for its original conditions, not automatic confirmation of the revised sequence.
Write Agreed G4 and Equipment Acceptance Limits
Replace an unexplained label with an agreement
The useful question after G4 is: agreed to what? G4 requires explicit requirements between purchaser and manufacturer. It is not a universal row automatically stricter than every G3 parameter. The agreement must establish both required behavior and the method of demonstrating it.
Write parameters individually: voltage and frequency variation, transient excursions, recovery bands, recovery times and associated load conditions. Identify the standard and project additions. Where a project value differs from the standard basis, identify that difference instead of concealing it in a general compliance statement.
Translate equipment tolerances into requirements
Equipment limits may involve magnitude, duration, frequency range or restart behavior. Collect these limits from its supplier. Calling a device sensitive does not create a testable criterion. Selecting a more demanding class does not remove the need to understand the device.
For UPS loads, review transfer behavior, input controls, recharge and generator interaction with the relevant parties. The diesel generator and UPS compatibility guide covers that interface. This guide explains performance acceptance, not certification of every UPS combination.
Motor starting method and demand belong in the schedule. Allowed voltage behavior must come from equipment and project requirements. Avoid assigning a universal class to all motors or assuming steady kW describes starting.
When documents disagree or omit a limit, record the open item and ask the responsible supplier or designer to resolve it. Do not fill a blank with a familiar legacy percentage. An identified missing criterion is more useful than a precise number with no traceable authority.
Build a Reviewable Performance Clause
A one-line requirement leaves conditions implicit. Use this educational worksheet as a starting point, then have the responsible parties complete and approve it. It is not a finished specification or a guarantee that a particular generator can meet the resulting requirements.
Complete the identification and test basis
| Entry | What to record | Purpose |
|---|---|---|
| Standard and edition | Designation, edition and additions | Prevent mixed tables and definitions |
| Complete-set configuration | Engine, alternator, control, settings | Connect evidence to the offer |
| Duty and output | Duty, voltage, frequency, rated output | Establish the operating basis |
| Site conditions | Ambient, altitude, installation assumptions | Make conditions reviewable |
| ISO profile | Applicable acceptance and rejection sequence | Identify the standard claim |
| Project profile | Initial load, events, intervals, load type | Describe actual demand |
| Electrical limits | Excursions, references, bands, times | Define acceptance |
| Evidence and deviations | Records, exclusions, approval route | Expose exceptions |
The ISO catalogue identifies ISO 8528-5:2025, published in May 2025. Older papers remain useful explanations, but their tables must not be silently relabeled as current limits. State the edition adopted by the contract and consult its complete requirements.
Make the clause usable in bid comparison
Require the bidder to identify the proposed configuration and test basis, respond to every project event and list deviations explicitly. Decide who reviews deviations and what evidence closes them before treating the offer as accepted.
Separate to be confirmed from complies. If a bidder proposes another sequence, check whether it matches site controls and operating needs. Carry conditions involving settings or accessories into the equipment schedule and test documents. Otherwise the accepted requirement may become detached from delivered equipment.
The guaranteed technical particulars guide helps organize agreed values. Keep terminology consistent across enquiry, returned schedule, order and test plan. A reviewer should be able to trace a requirement without interpreting different shorthand at each stage.
Send the completed worksheet, load sequence and tolerance documents with your enquiry. Request written confirmation or an exception for each requirement. Give each worksheet row a document reference and revision.
When a supplier revises an offer, compare those references so a changed assumption is visible before the order is placed. Do not infer acceptance from an unanswered field, generic brochure or class label.
Check the Test Evidence and Resolve Exceptions
Match the report to the offered set
Compare report identification with the proposed engine, alternator, control and settings. Review duty, output, voltage, frequency and conditions. For evidence from a related configuration, request an explanation of differences and relevance. Similar model names do not close the question.
Illustrative scenario: A bid includes a report for a different alternator arrangement. The purchaser records the mismatch and requests applicable evidence or a technical explanation. This does not establish failure; it shows why report applicability needs review separately from the pass result.
Understand load-bank scope
Avtron’s testing article explains load-bank and power-factor considerations. Resistive and resistive-reactive tests impose different electrical conditions. Record the test equipment and the loads it represents.
Include the applied kW, kVA and power factor in the event record. A test description that names only the bank capacity leaves the actual operating point unclear.
A resistive-bank pass does not prove every motor or nonlinear-load interaction. A limited report may still support a defined requirement. Identify that requirement and the remaining questions. Those may need further analysis or an agreed test representing the relevant characteristics.
Separate prediction, factory evidence and installed confirmation
A calculation or simulation predicts behavior under assumptions. A factory report records a particular test. Installed-system evidence addresses the actual arrangement within its test scope. Label these evidence types clearly because they answer different questions.
Use the generator acceptance testing procedure for the wider FAT and SAT process. Retain event traces, references, conditions and criteria for class assessment. Demonstrating two transients does not establish that every required class parameter was checked.
Record exceptions with the responsible party, evidence needed and closure decision. When settings change or retesting is required, preserve the earlier result and identify the changes. Explain resolution in the final record instead of replacing history with a new pass label.
Questions to Settle Before Ordering
Is G3 always necessary instead of G2?
No universal application rule follows from class names alone. Compare requirements with equipment tolerances and project obligations. A building category does not provide all load data. The designer should establish requirements and the supplier should demonstrate the offered configuration against them.
Does G3 mean full-load pickup in one step?
Do not assume so. Check standard and project profiles separately. Request evidence for the actual initial load, applied load and recovery criteria. A rejection result does not answer a different acceptance test.
Is G4 automatically better than G3?
G4 means requirements are agreed. Read the agreement parameter by parameter. Without limits, conditions and demonstration method, the label is insufficient for comparing bids or judging compliance.
Which standard edition should the order name?
Name the edition adopted for the contract and applicable project requirements. The catalogue is not the full standard. Check historical technical papers against the contract before copying numerical tables into an enquiry.
What if frequency falls during an actual event?
Compare the event with the approved schedule, configuration and settings before assigning a cause. Our frequency drop troubleshooting guide covers that task. A class label alone does not show whether a field event matches previously assessed conditions.
Before ordering, confirm duty, configuration, standard profile, project sequence, equipment limits and evidence route. Include supporting documents with your Shanhua enquiry and request a written response to unresolved items. Aim for a traceable agreement with a clear basis for judging the delivered set.
