Before you read on — this isn't a DIY electrical job
This guide explains what load bank testing is, why it matters, and how it fits into a maintenance routine. Connecting a load bank to a generator involves high-current cabling and genuine arc flash risk if done incorrectly. Formal load bank testing should be carried out by a licensed electrician or a qualified generator technician, consistent with general electrical safety obligations under AS/NZS 3000 and AS/NZS 3010 — this guide is not a substitute for that expertise or for the generator manufacturer's own instructions.
Quick answer: A load bank is a device that applies a controlled electrical load to a generator so it can be tested under conditions that mimic real demand, rather than idling or running lightly loaded. Standby diesel generators are especially prone to a problem called wet stacking when they're run for long periods under little or no load — unburned fuel and carbon build up in the exhaust and cylinders because the engine never gets hot enough to burn cleanly. Load bank testing (or simply running the generator under a genuine loaded exercise cycle) deliberately loads the engine to burn that off and confirm the generator can actually deliver its rated output when it matters. There's no single Australian standard mandating a fixed testing interval the way the US's NFPA 110 does — but the underlying engineering problem, and the case for testing regularly, is real and well documented.
In 30 seconds
- A load bank simulates real electrical demand so a generator can be tested and exercised without waiting for an actual outage or peak-demand event.
- Wet stacking is the core reason standby gensets need this: running under light or no load for extended periods leaves unburned fuel and carbon in the exhaust, turbo and cylinders — a genuine, well-documented mechanical problem, not a sales scare tactic.
- Sources disagree on the exact minimum load threshold — different manufacturers and technical sources put it anywhere from 30% to 75% of rated output — so treat any single figure as a rough guide, not gospel, and check your own engine's documentation.
- There's no Australian equivalent to the US's NFPA 110 mandating a fixed load bank testing interval. AS/NZS 3010 governs the electrical safety of the installation (earthing, changeover, backfeed protection) — it doesn't set a testing schedule.
- Real-world practice varies enormously — from disciplined weekly loaded exercise runs plus an annual full load bank test, through to generators that get started but never properly loaded for years at a time.
- Some standby generators now ship with a factory-fitted automatic load bank built in specifically to manage this problem when real-world load is light.
- This is licensed-electrician or qualified-technician work once you're talking about connecting an actual load bank — the arc flash and high-current cabling risks are real.
What a load bank actually is
A load bank is a purpose-built device that draws a controlled amount of electrical power from a generator (or another power source) and converts it to heat, so the generator can be tested under conditions that resemble genuine demand rather than sitting idle or running a token light circuit. Instead of waiting for a real building load — or a real outage — to prove a generator can deliver its rated output, a load bank lets a technician apply that load deliberately, on a schedule, and measure how the generator responds.
There are three broad types, and the difference matters for what each is actually testing:
- Resistive load banks simulate simple loads like lighting and heating at unity power factor. A resistive bank can push a generator up to 100% of its nameplate kW rating and is well suited to burning off carbon and exercising the engine's fuel, cooling and exhaust systems — but it only reaches around 80% of the generator's nameplate kVA rating, because it doesn't exercise the reactive (electromagnetic) side of the alternator's output.
- Reactive load banks — inductive (lagging power factor) or capacitive (leading power factor) — simulate the kind of load created by motors, transformers and other electromagnetic equipment. These are typically run at around 0.8 power factor and are normally paired with a resistive bank rather than used alone, since real-world reactive loads (compressors, pumps, motor starts) are exactly the kind of load that catches out a generator that's only ever been tested with resistive heaters.
- Combined resistive/reactive load banks put both in one unit, letting a technician dial in a realistic power factor and load the generator to its full nameplate kVA rating in a single test — the most complete and realistic test available, and what's generally recommended for a proper annual test or a commissioning/acceptance test on a new installation.
If you're still working out what size generator or engine rating you actually need before any of this becomes relevant, our generator sizing guide and standby vs prime vs continuous engine ratings guide cover that groundwork.
Why standby generators specifically are at risk: wet stacking
Wet stacking is unburned diesel fuel and carbon residue building up in the exhaust manifold, turbocharger, cylinders and even the engine oil, because the engine isn't running hot enough or hard enough to burn fuel cleanly. The name comes from the oily, wet residue that shows up around the exhaust stack. It's a genuine, well-documented mechanical problem — not a beat-up to sell testing services — and the effects reported consistently across engine manufacturers and generator technical sources include fouled injectors, glazed cylinder walls, reduced turbo efficiency, fuel diluting the lubricating oil (which accelerates wear elsewhere in the engine), excessive smoke, and reduced power output. Left unaddressed for long enough, it can genuinely shorten engine life — and it's worth knowing that wet-stacking damage is often specifically excluded from engine manufacturer warranties.
This is precisely the risk profile of a typical standby generator: a unit that sits for weeks or months, gets started briefly to "make sure it still runs," or carries only a light circuit during a short outage, and never gets hot enough or loaded enough to burn off the unburned fuel. It's the opposite of a continuous-duty industrial generator that runs at or near its rated load routinely as a matter of course. Our standby vs prime vs continuous ratings guide goes into why standby-rated engines are specifically built and sized around short, occasional, high-output running rather than sustained load — which is exactly the usage pattern that makes wet stacking a real risk if that occasional running is also under-loaded.
There's no single agreed-upon minimum load percentage below which wet stacking is guaranteed to occur — and it's worth being upfront about that rather than quoting a false-precision number. Different manufacturers and technical sources put the risk threshold anywhere from around 30% of rated output up to 50–75%, with general agreement that the sweet spot for genuinely healthy running sits around 70–80% of rated capacity. Treat any single percentage you see quoted as a rough guide specific to that source, and check your own generator or engine manufacturer's documentation for their actual figure rather than assuming one number applies universally.
Is there an Australian standard that requires load bank testing?
Not in the way many people assume. AS/NZS 3010 (Electrical installations — Generating sets) is a real, current Australian/NZ standard, but its scope is electrical installation safety — changeover switching so a generator and mains supply can never be connected simultaneously, neutral and earthing arrangements, backfeed protection, and labelling. It governs how a generator is safely connected into an electrical installation, not how the generator's engine performance is verified over time. It does not set out a load bank testing procedure or a testing interval.
The United States has a genuinely different picture: NFPA 110, a specific standard for emergency and standby power systems, mandates concrete testing intervals for the systems it covers. Australia does not have a directly equivalent mandatory standard specifying load bank testing intervals for standby generators. That's a genuine gap worth being upfront about, not glossing over — the case for regular load testing here rests on sound engineering practice and manufacturer guidance, not on a specific clause in an Australian standard requiring it.
How often should you actually do it?
Formal guidance that does exist (mostly originating from the US, where the standards are more prescriptive) suggests a rhythm along these lines: a full load bank test roughly annually, alongside more frequent loaded exercise runs — commonly monthly or even weekly for critical standby systems — at a meaningful load, not just a no-load start-up check. Generator and engine manufacturers themselves tend to be less specific in public documentation, often deferring to "contact us for your model's recommendation" rather than publishing a universal number.
Real-world practice, based on how people who actually own and run standby gensets describe doing it, is considerably more varied than the formal guidance implies. At the disciplined end, some operators run a genuine weekly loaded test — transferring real load onto the generator for 15–20 minutes followed by a cooldown period — specifically because they've learned that a no-load weekly "start it up and let it idle" check doesn't get the engine hot enough to do any good, and can arguably make wet stacking worse rather than better by never letting the thermostat open. At the other end, plenty of smaller commercial and rural operators run their standby genset only when an actual outage happens, with no formal loaded exercise schedule at all — and some owners of smaller (8–10kW class) backup diesels report going years without seeing wet-stacking symptoms under fairly light, intermittent real-world use, which suggests the risk is somewhat scale- and duty-cycle-dependent rather than a fixed rule that applies identically to every generator.
The practical takeaway: there's a real spread between "textbook annual load bank test plus monthly loaded exercise" and what many standby generator owners actually do, and no single number is correct for every situation. What's consistent across every source, formal or informal, is that periodic loaded running — not just a no-load start check — is what actually protects the engine, and a properly instrumented load bank test is the only way to confirm the generator can truly deliver its full rated output under controlled, repeatable conditions.
Load bank testing vs testing under real building load
A load bank isn't the only way to properly load-test a generator. Where a facility has an automatic transfer switch and a genuinely substantial real electrical load, transferring that real load onto the generator for a scheduled test period achieves much the same result as a load bank — the generator is doing real work, not idling. This is common practice at the larger end of standby power (data centres, hospitals, multi-generator sites), sometimes done monthly as a no-load exercise plus an annual full transfer-load test. For a smaller commercial site, farm or dairy, real building load may simply not be large enough on its own to load the generator to a meaningful percentage of its rating — which is exactly the scenario a dedicated resistive or combined load bank is designed to solve, and also the scenario where a generator's own changeover switch setup matters: our manual vs automatic changeover switch guide covers how automatic transfer switches handle scheduled exercise cycles and what to look for if you want that capability built in.
Safety considerations
Load bank testing carries genuine electrical hazards that go beyond routine generator operation. Never disconnect load bank cabling without first dropping the load — doing so risks serious arc flash injury. Cabling has to be correctly rated for the current involved; undersized or incorrectly connected cabling can overheat or start a fire. A proper cooldown period after a loaded test protects the turbocharger and helps prevent oil coking, and testing without adequate airflow or in extreme heat can skew results or trigger false shutdowns. For all of these reasons, load bank testing — beyond simply running a generator under whatever real load it already has — is not a DIY job. It should be carried out by a licensed electrician or a qualified generator technician, using the correct cabling, exclusion zones and shutdown procedures, consistent with the general electrical safety obligations set out in AS/NZS 3000 and AS/NZS 3010.
Built-in load banks: some standby generators now come with one
Because chronic underloading is such a well-known problem for standby diesel generators specifically, some manufacturers now build an automatic load bank directly into the generator itself. Welling & Crossley's 24kVA three-phase standby diesel generator (Kubota-engined, in canopy, with a 400L tank) is a genuine example — it ships with an automatically applied artificial load specifically described as reducing the likelihood of engine glazing when real-world load is light, and helping the engine maintain optimal performance without relying on the site's own load or a separately scheduled test. It's a practical illustration of how seriously the wet-stacking problem is taken at the engineering level, not just as an afterthought maintenance topic.
How to reduce wet-stacking risk without a professional load bank test
Formal load bank testing needs a licensed electrician or technician, but there's a genuine amount an owner can do themselves to reduce the risk between professional tests.
Where AIMS fits
Through our relationship with Welling & Crossley, we have access to standby diesel generators across a range of sizes, including models built with a factory-fitted automatic load bank specifically to manage light-load running. If you're specifying a standby generator for a farm, dairy, or commercial property and want to avoid ending up with a chronically underloaded unit, get in touch with your actual load requirements and expected duty cycle — sizing correctly from the outset is the single biggest lever you have over this whole problem. You can also browse our broader generators and power stations range, and if you're weighing up a farm-based PTO-driven option instead of a standalone standby diesel unit, our PTO generator guide covers that alternative.
Frequently asked questions
What is load bank testing?
Load bank testing is the process of connecting a device called a load bank to a generator to apply a controlled electrical load, so the generator can be tested and exercised under conditions that resemble real demand rather than idling or running lightly loaded.
What's the difference between a resistive and a reactive load bank?
A resistive load bank simulates simple loads like lighting and heating at unity power factor and can push a generator to 100% of its kW rating, but only around 80% of its kVA rating. A reactive load bank (inductive or capacitive) simulates the kind of load created by motors and transformers, typically at around 0.8 power factor, and is normally paired with a resistive bank rather than used alone. A combined resistive/reactive load bank can test to the generator's full rated kVA at a realistic power factor in one test.
What is wet stacking?
Wet stacking is unburned diesel fuel and carbon residue building up in a generator's exhaust, turbocharger and cylinders because the engine isn't running hot enough or hard enough to burn fuel cleanly. It's named for the oily residue that appears around the exhaust stack, and can lead to fouled injectors, glazed cylinder walls, reduced turbo efficiency and reduced power output if left unaddressed.
Why are standby generators more prone to wet stacking than other generators?
Standby generators typically sit unused for long periods and, when they do run, often carry only a light load for a short time. That's the opposite of a continuous-duty generator that runs at or near its rated load routinely, and it's exactly the running pattern that causes wet stacking.
What percentage of load does a generator need to avoid wet stacking?
There's no single agreed figure. Different manufacturers and technical sources put the risk threshold anywhere from around 30% of rated output up to 50–75%, with general agreement that healthy running sits around 70–80% of rated capacity. Check your own generator or engine manufacturer's documentation rather than relying on one universal number.
Does an Australian standard require load bank testing?
No. AS/NZS 3010 governs the electrical installation safety of generating sets (changeover switching, earthing, backfeed protection) but doesn't set a load bank testing procedure or interval. Unlike the United States, which has NFPA 110 mandating testing intervals for standby power systems, Australia doesn't have a directly equivalent mandatory standard for load bank testing intervals.
How often should a standby generator be load bank tested?
Formal guidance commonly suggests an annual full load bank test alongside more frequent loaded exercise runs. In practice, real operators do this very differently — from disciplined weekly loaded tests to essentially no formal load testing at all, relying on real outages instead. There's no single correct interval for every situation; manufacturer guidance for your specific engine is the best starting point.
Can I load bank test a generator myself?
Connecting and operating an actual load bank involves high-current cabling and genuine arc flash risk, and should be done by a licensed electrician or qualified generator technician. What an owner can reasonably do themselves is run genuinely loaded exercise cycles (not just no-load starts), add temporary resistive load if the real building load is too light, and watch for visible warning signs like exhaust smoke or oily residue.
Is load bank testing the same as just running the generator under my building's real load?
They achieve a similar result if the real load is substantial enough — the generator is doing real work rather than idling. Where the real load isn't large enough to meaningfully load the generator, a dedicated load bank is the practical solution, since it can apply a controlled load regardless of what the building actually draws at the time.
Do all standby generators need an external load bank test?
Not necessarily. Some standby generators, including certain Welling & Crossley models, are built with a factory-fitted automatic load bank designed to reduce the risk of light-load running without needing a separately scheduled external test — though a periodic professional test is still worth discussing with your supplier or technician.
What actually causes chronic underloading in a standby generator?
The most common practical cause is oversizing — buying a generator rated well above the property's actual load "just in case." An oversized unit may never be loaded to a healthy percentage of its rating even during a genuine outage, which is why correct sizing matters as much for engine health as for cost.
What warning signs suggest a generator has been affected by wet stacking?
Oily residue around the exhaust stack, excessive smoke, fouled injectors, reduced power output and increased exhaust backpressure are commonly reported signs. These are worth acting on rather than ignoring, since the underlying fuel dilution of the lubricating oil can accelerate wear elsewhere in the engine.
This guide is general information current as at August 2026. Load bank and load bank testing definitions are sourced from generator and load bank industry technical publications; the AS/NZS 3010 scope described here reflects independently verified standard-scope summaries, not the full clause text of the paid standard itself; real-world testing frequency guidance reflects a genuine range of manufacturer guidance and reported operator practice, not a single universal figure. This guide does not replace the generator manufacturer's own documentation, a proper risk assessment, or advice from a licensed electrician or qualified generator technician for your specific equipment and site.
