Updated
Load testing
Generator Load Testing Guide
A generator load test runs the set against a known electrical load in steps, typically 50% of nameplate kW for 30 minutes and then 75% for an hour in the US NFPA 110 annual test, while we record voltage, frequency, temperature and oil pressure. The manufacturer’s ratings for your set decide the exact steps.
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- A known load, in steps
- Readings recorded at each step
- Steps set by your rating
- Onsite across Gauteng
A run is not a test
What is generator load testing?
A load test is a run with a deliberate electrical load while we watch the set. We record voltage, frequency, coolant temperature, oil pressure and the engine note as each load step comes on. A set can start and idle perfectly and still fail when the building connects.
A load bank is a controlled load. We use one when switching your building on would be unsafe, unclear or impossible to repeat. Raising the load in steps shows whether the set holds voltage and speed at each one. NFPA 110, the US standard quoted by EC&M, uses 50% of nameplate kW for 30 minutes and then 75% for an hour for diesel sets that cannot reach 30% load in their monthly tests.
Commercial and industrial sites usually book a load test to prove the set can carry a defined load, not only that it starts. If your set already trips when load comes on, our trips-under-load guide covers that fault. We check the obvious causes first and then use the load bank to show the rest.
What we watch
Why test a generator under load?
What is a load bank?
A device that draws a set amount of power from the generator, so the test does not depend on whatever happens to be switched on in your building.
No-load versus loaded testing
A no-load run proves the engine starts and makes voltage. A loaded test asks it to hold that voltage and speed while it produces current. Each answers a different question.
Voltage and frequency under load
Both can look right at idle and sag once current flows. We take the readings during each load step, not only before the test.
Cooling, engine and fuel
Temperature, engine note and fuel delivery show their limits under load. A cooling fault or a fuel restriction can stay hidden on a five-minute idle.
Load steps
Raising the load in stages shows exactly where voltage, speed or temperature leaves the expected band. We set the steps from your set’s rating and the reason for the test.
When a load bank test is useful
After a major repair, when capacity is in doubt, when the building load is too small for a fair test, or when a diesel spends most of its life at light load.
Worked example: load steps for a 100 kVA set
A 100 kVA set at the typical 0.8 power factor is rated 80 kW. These steps follow the NFPA 110 (2013) figures quoted by EC&M, a US standard used here as a typical published reference. Your manufacturer’s ratings and your site’s requirements set the steps for your set.
| Step | Load | Duration | What we record |
|---|---|---|---|
| Monthly exercise | At least 30% of standby nameplate: 24 kW | The monthly run | Voltage, frequency, coolant temperature, oil pressure |
| Annual test, step 1 | 50% of nameplate kW: 40 kW | 30 minutes, continuous | The same readings, logged through the step |
| Annual test, step 2 | 75% of nameplate kW: 60 kW | 60 minutes, continuous | The same readings, plus any alarm or shutdown |
Source: EC&M: Testing generators with a portable load bank (quoting NFPA 110, 2013 edition). The annual steps apply under NFPA 110 to diesel sets that do not reach 30% load in their monthly exercise. Our cost guide lists load bank testing at R2,800 – R5,500. To book, see our load bank testing service.
Related Pages
The test, the trip and the size
Common Questions
Questions about generator load testing
What is generator load testing?
Running the generator against a known electrical load, in steps, while we record voltage, frequency and engine behaviour. A typical published reference, the US NFPA 110 annual test, uses 50% of nameplate kW for 30 minutes and then 75% for an hour.
What is a load bank?
Equipment that applies a known load, so the test does not rely on your building’s circuits.
Is a load test the same as starting the generator?
No. A start with no load does not show whether the set can carry current, hold speed or stay cool.
Why can a generator run fine with no load and fail under load?
Fuel delivery, cooling, alternator capacity and engine power are only fully tested when current flows. An idle run hides those limits.
How long does a load bank test take?
Under the NFPA 110 annual steps, the loaded part runs for at least 90 minutes: 30 minutes at 50% and 60 minutes at 75%. Setting up and connecting the load bank adds to that.
How much does a load bank test cost?
Our cost guide lists load bank testing at R2,800 – R5,500. We confirm your price in a written quote before the test.
Which standard do you test to?
We agree the steps and the record with you before the test, based on your generator’s rating and the reason for the test. If your site follows a standard such as NFPA 110, tell us and we plan the steps around it.
The set already trips when we switch the building on. Do we start with a load bank?
Tell us about the trip first. We check breakers, fuel and cooling, then use a controlled load to show the fault if it is still there.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Tell us what the test must prove
Capacity, a fault that only shows under load, or a check after a repair. The kVA from your data plate helps us bring the right load bank.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.