Load faults
Generator Trips or Shuts Down Under Load
A generator that runs fine on its own but trips, slows or stops when you connect equipment is usually overloaded, struggling with motor starting current, or short of fuel at high demand. Collapsing voltage or frequency, a faulty breaker, the alternator, overheating or a controller protection can also cause it. We test the set under load to find out which.
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- Tested under real load
- Speed, voltage and heat recorded
- Load bank testing when capacity is in doubt
- Onsite across Gauteng
When the building connects
Why does a generator run fine until load is applied?
The fault needs current, fuel flow or heat that an unloaded run never asks for. A motor, a lift, a cold room or the whole board can demand more at the moment of connection than the set can start and hold.
We record what was connected when it tripped, whether the engine note fell, whether the voltage sagged, and whether a breaker opened or the controller shut the engine down. Each of those endings points to a different cause.
If you need to know whether the generator is big enough, or whether it can still make its rated output, a load bank test answers that under controlled conditions. Our generator size guide explains why the kVA on the plate is not the whole answer.
Measured, not guessed
Starting current, not just running load
A motor started direct-on-line typically draws 6 to 7 times its full-load current until it reaches speed (Schneider Electric). We list the pumps, compressors and air conditioners on your generator supply and check which ones start at the same moment, because their combined starting current can trip a set that carries the running load comfortably.
Fault guide
Generator trips or shuts down under load: what it can mean and what to do
A set that runs fine off load but trips, slows or stops when equipment connects is usually overloaded, struggling with motor starting current, or short of fuel at high demand. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| The breaker trips when big appliances start | Overload or high motor starting current | Load and sizing check |
| Trips even at light load | A faulty breaker, a short circuit or a wiring fault | Control panel repairs |
| The earth leakage unit trips only on generator power | The generator earthing, the changeover arrangement or the house wiring | Changeover switch repairs |
| The engine bogs down, then the set trips | Fuel delivery or governor response | Fuel system repairs |
| Trips after a long run with a temperature alarm | A cooling restriction or low coolant | Cooling system repairs |
01
What you may notice
- The output breaker trips when the building or a large appliance connects
- The engine slows or stalls when load comes on
- The controller shows an overload, under-frequency or under-voltage alarm
- It runs for a while on load and then shuts down
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- Total connected load larger than the set can carry
- Motor starting current from pumps, compressors or air conditioners
- Fuel delivery that cannot keep up at higher load
- Voltage or frequency collapsing under load
- A faulty or incorrectly rated breaker
- Overheating that only appears when the set works hard
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- What was switched on just before it tripped?
- Which alarm shows on the controller after the trip?
- Does it trip immediately, or after running for a while?
- Is any equipment new since the generator was installed?
04
Stop using the generator and call if you notice
- A breaker that trips again as soon as you reset it
- A burning smell or smoke from the panel or alternator
- High temperature alarms or steam
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Connected load, starting loads and the set rating
- How quickly voltage and frequency recover when load comes on
- Fuel delivery and engine power under load
- Breaker rating, condition and trip settings
- A load bank test where the set needs proving under controlled load
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- What equipment is on the generator supply
- The alarm shown after the trip
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Typical repair costs for this fault
Published ranges, last reviewed October 2026. The callout is R400 – R700 in business hours and R700 – R1,400 after hours, and it is waived if you accept the quote. Your written quote after diagnosis is the price that applies.
- Voltage fluctuation diagnosis and repair
- R1,500 – R5,000
- Control panel repair or reprogramming
- R3,500 – R14,000
- Load bank testing
- R2,800 – R5,500
Step 9: book a diagnosis visit.
Related Pages
Load, size, speed and heat
- Load Bank Testing A known load instead of a guess
- Generator Sizes kVA against the connected load
- Low Frequency Speed falls when load comes on
- Surging or Hunting Speed that will not settle
- Low Voltage Voltage sags under load
- Overheating Temperature shutdown on a run
- Generator Fault Finding Separating the symptom first
Common Questions
Questions about generators that trip under load
Why does my generator trip only when I connect the load?
Because an unloaded run never asks for the current, fuel or cooling that the load needs. The weakness shows up when that demand arrives.
Does a trip mean the generator is too small?
Sometimes. It can also be fuel, engine speed, a breaker, voltage or heat. We rule those out before we conclude the set is undersized.
The engine note drops when the lights come on. What is that?
Usually overload, or a fuel or governor limit. We read the frequency while it happens instead of adjusting anything by ear.
A breaker opens but the engine keeps running. Is that an engine fault?
Not necessarily. An electrical trip with the engine still running is a different fault from a shutdown that stops the engine, and we test it differently.
Can one large motor cause the trip?
Yes. A motor can draw several times its running current while it starts, so what starts at the same moment matters.
Will a load bank test show this?
It shows whether your set can carry a defined load. We book it when capacity is the question, not when a breaker or fuel fault is already obvious.
It also gets hot and then stops. Is that the same problem?
It may be linked. Heat after a loaded run brings in the overheating checks, so mention both and we will watch the temperature as well as the trip.
What should I list when I book?
Tell us what was switched on, whether the engine stopped or a breaker opened, and the kVA on the data plate.
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 was connected when it tripped
The kVA, the fuel type, and whether the engine stopped or a breaker opened are the three facts that help us most.
- 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.