In short. A generator is overloaded when it is asked for more kW than the engine can make, more current than the alternator can carry on any one phase, or a bigger starting surge than the set can absorb. Each shows different symptoms: frequency drop and black smoke for engine overload, heat and breaker trips for alternator overload, voltage collapse and stalling for starting overload. A clamp meter and ten minutes of readings tell you which one you have, and the cure is usually load rearrangement, not a bigger set.
Who this page is for: facility managers, electricians and apartment associations whose set trips, smokes, overheats or “struggles” when certain loads come on — and anyone deciding whether they really need a larger generator.
Key facts
- At 415 V three-phase, rated current is about 1.39 A per kVA: 62.5 kVA ≈ 87 A, 125 kVA ≈ 174 A, 250 kVA ≈ 348 A, 500 kVA ≈ 696 A.
- Most sets are rated at 0.8 power factor, so the engine’s limit in kW is 80% of the kVA figure: a 125 kVA set is a 100 kW engine.
- Overload is judged per phase. A set can be at 85% on average and still overloaded on one phase.
- A prime-rated set is commonly allowed about 10% overload for one hour in every twelve hours. A standby-rated set has no overload allowance at all. See prime power and standby power.
- A direct-on-line motor draws about 6 to 7 times its running current while it starts. Starting surges overload more small sets than running load does.
- Frequency falling below about 49 Hz under load on a set that normally holds 50 Hz is the engine telling you it has run out of power.
Three different overloads
“Overload” covers three separate limits, and confusing them leads to wrong fixes.
Engine (kW) overload
The engine converts fuel into shaft power. When the electrical load in kW exceeds what the engine can deliver at rated speed, the governor opens the fuel rack fully and speed still falls. Frequency drops, the exhaust turns dark, exhaust and coolant temperatures climb, and on electronic engines the controller may derate or log a fault. Engine overload is about real power: heaters, motors doing work, compressors, pumps.
Alternator (current) overload
The alternator’s limit is heat in its windings, set by current — and current depends on both kW and power factor. A load at 0.7 power factor draws about 14% more current than the same kW at 0.8. Lightly loaded engines can therefore sit behind badly overloaded alternators. Symptoms are hot windings, a smell of hot varnish, breaker or controller overcurrent trips, and, if it continues, insulation failure. Phase unbalance concentrates the heat on one winding.
Starting (transient) overload
When a large motor, lift, compressor or air-conditioning plant starts, it briefly demands several times its running kVA. If that surge exceeds what the set can supply, voltage collapses, contactors drop out, lights dim, UPS units transfer to battery and the motor may fail to accelerate. The set may be well within its rating the rest of the time. See motor starting and voltage dip.
Signs of overload, and what each means
| Sign | Which limit | What is happening | First action |
|---|---|---|---|
| Frequency falls below about 49 Hz under load | Engine (kW) | Governor at full fuel, speed still dropping | Measure kW; shed heaters and non-essential motors |
| Black smoke when a load comes on, clearing after | Engine, transient | Fuel exceeds available air during the load step | Stagger large loads; check air filter and turbo |
| Continuous dark exhaust at high load | Engine (kW) | Running beyond the clean-combustion limit | Measure kW against rating; check air filter restriction |
| Coolant temperature rises only at high load | Engine (kW), cooling margin | Heat rejection exceeds radiator capacity, worse in summer | Check radiator, airflow and ambient derating |
| Breaker or controller overcurrent trip | Alternator (current) | Current above setting on one or more phases | Read current on each phase; check unbalance |
| Alternator very hot, varnish smell | Alternator (current) | Winding temperature too high | Reduce load; check power factor and ventilation |
| Voltage collapses, contactors chatter at motor start | Starting (transient) | Motor inrush beyond set’s transient capability | Change starting method or sequence; see motor starting |
| Set stalls when the lift or borewell pump starts | Starting (transient) | Starting kVA several times the set rating | Soft starter, star-delta or VFD; sequence loads |
What overload does to the machine
Engine. Sustained kW overload raises cylinder pressures and exhaust temperatures. Turbochargers run hotter and faster; piston crowns, rings and valves suffer; lubricating oil degrades faster; and in summer the cooling system loses its margin first. On CPCB IV+ engines, aftertreatment components see higher temperatures and the controller may derate to protect them. Repeated overload shortens overhaul intervals noticeably.
Alternator. Insulation life roughly halves for every 10 °C rise above its rated temperature — the long-established rule of thumb for electrical insulation. An alternator that runs 20 °C hot on one phase may lose three-quarters of its winding life. Overload also stresses the automatic voltage regulator (AVR) and the exciter, which work harder to hold voltage. See alternator temperature rise and insulation.
Switchgear and cables. Cables sized for the set’s rated current overheat under overload, especially in trenches and on hot rooftops. Terminals loosen and discolour. Changeover contacts pit and weld.
How to check load properly
- Know the rating. Read the nameplate kVA, kW and voltage. Work out rated current: kVA × 1,000 ÷ (1.732 × voltage). At 415 V this is kVA × 1.39.
- Read current on each phase with a clamp meter at the set’s output terminals or panel, at the busiest time of day. Note R, Y and B separately, plus neutral.
- Read voltage and frequency at the same moment. Many controllers display kW, kVA and power factor directly; note them.
- Calculate per-phase loading (phase current ÷ rated current), average loading, phase unbalance, and kW if the controller does not show it: kW = 1.732 × V × I(average) × PF ÷ 1,000.
- Watch a start. Start the largest motor while watching voltage and frequency. A dip below about 80% of rated voltage, or slow recovery, shows a starting problem.
- Repeat in summer. Engine and cooling margins shrink as the air gets hotter; a set that copes in December can struggle in May. See ambient temperature derating.
Worked example (illustrative)
Situation. An office building runs on a 62.5 kVA, 415 V set during outages. The output breaker trips about once a week in the afternoon, and the alternator feels very hot. The building manager wants to buy an 82.5 kVA set.
Step 1 — rated current. 62.5 × 1,000 ÷ (1.732 × 415) = 62,500 ÷ 718.8 = 86.9 A per phase.
Step 2 — readings at 3 pm. R = 92 A, Y = 81 A, B = 70 A. Power factor 0.72. Frequency 49.9 Hz.
Step 3 — per-phase loading. R: 92 ÷ 86.9 = 106%. Y: 81 ÷ 86.9 = 93%. B: 70 ÷ 86.9 = 81%. Average current (92 + 81 + 70) ÷ 3 = 81 A, which is 93% of rated. The R phase is overloaded even though the average is within rating.
Step 4 — unbalance. Largest deviation from average ÷ average = (92 − 81) ÷ 81 = 13.6%. Most alternator makers want unbalance well below this for full-load running.
Step 5 — engine load. kW = 1.732 × 415 × 81 × 0.72 ÷ 1,000 = 41.9 kW. The engine is rated 62.5 × 0.8 = 50 kW, so it is at 84%. Frequency at 49.9 Hz confirms the engine is comfortable.
Diagnosis. This is an alternator current problem caused by phase unbalance and low power factor, not an engine overload. A larger set would hide it for a while at a much higher cost.
Fix. The building’s single-phase air-conditioners and water heaters were redistributed so that about 10 A moved from R to B. New readings: R = 82 A, Y = 81 A, B = 80 A, all at 92–94% of rated current. The power factor was traced to old fluorescent-fitting ballasts and lightly loaded motors; correction was planned on the mains side with an automatic panel that disconnects capacitors on generator supply. The trips stopped.
Second example: a starting overload (illustrative)
A farmhouse has a 30 kVA set and a 15 kW borewell motor started direct-on-line. Running kVA of the motor at about 90% efficiency and 0.85 power factor: 15 ÷ (0.90 × 0.85) = 19.6 kVA — comfortably within 30 kVA. But direct-on-line starting draws about six times that: roughly 118 kVA for a few seconds. The set’s voltage collapses and the motor stalls. A star-delta starter cuts the surge to about a third (about 39 kVA), and a soft starter or VFD can bring it lower still. The set was fine; the starting method was not.
Clamp-meter load-check worksheet
| Item | Value | How to work it out |
|---|---|---|
| Set rating (kVA) | Nameplate | |
| Engine rating (kW) | kVA × 0.8, or nameplate | |
| Voltage (V, line to line) | Panel meter or multimeter | |
| Rated current (A) | kVA × 1,000 ÷ (1.732 × V); at 415 V, kVA × 1.39 | |
| R-phase current (A) / % of rated | Reading ÷ rated current × 100 | |
| Y-phase current (A) / % of rated | Reading ÷ rated current × 100 | |
| B-phase current (A) / % of rated | Reading ÷ rated current × 100 | |
| Neutral current (A) | Clamp on neutral | |
| Average current (A) | (R + Y + B) ÷ 3 | |
| Unbalance (%) | Largest deviation from average ÷ average × 100 | |
| Power factor | Controller or power analyser | |
| Load (kW) / % of engine rating | 1.732 × V × average current × PF ÷ 1,000 | |
| Frequency at this load (Hz) | Below about 49 Hz suggests engine overload | |
| Voltage dip at largest motor start (%) | (Rated V − lowest V) ÷ rated V × 100 | |
| Ambient temperature (°C) and time | Summer afternoon readings are the critical ones |
Use the full-load current chart to check rated current for other sizes and voltages.
Exceptions and cautions
- Do not add power-factor capacitors on the generator side without engineering advice. At light load, capacitors can push the set into leading power factor, which the AVR cannot control; voltage rises and the set may trip or damage equipment.
- UPS, VFD and LED loads distort current. A true-rms clamp meter is essential; an average-reading meter can under-read by 20% or more on these loads. See UPS and non-linear loads.
- Clamp readings are taken on live conductors. Only a qualified electrician should take them, with correct PPE and an insulated meter rated for the location.
- Altitude and heat derating reduce what the set can deliver. A nameplate rating applies at the maker’s reference conditions, not at 46 °C on a tin-roofed plinth.
- If the set is genuinely too small after balancing and starting fixes, size properly with generator sizing rather than buying “one size up”.
Fixing overload without buying a bigger set
- Balance the phases. Move single-phase loads until all three phases are within about 10% of each other.
- Sequence large loads. Stagger lift, pump and compressor starts by 10 to 30 seconds after transfer, using timers in the changeover or load panels.
- Change motor starting. Star-delta, soft starters or VFDs on the largest motors cut starting kVA dramatically.
- Shed non-essential loads on generator. Water heaters, some air-conditioning, decorative lighting and EV chargers can be held off generator supply by contactor interlocks.
- Restore the engine’s own capacity. A choked air filter, dirty radiator, worn injectors or a tired turbocharger reduce available power. A service often brings back 5 to 10% of lost performance.
- Only then size up — or parallel a second set, which also gives redundancy. See N+1 redundancy.
Common mistakes
- Judging load from the average current and missing an overloaded phase.
- Assuming every trip is an overload, when it may be a mis-set controller, a loose current-transformer connection or an earth fault.
- Buying a larger set to cure a starting problem, then running it lightly loaded for years and causing wet stacking.
- Raising breaker settings to stop trips. This removes protection without removing the cause.
- Measuring in the cool morning and assuming the set will cope on summer afternoons.
What to send us
Send the set’s kVA, make and running hours; a photograph of the nameplate; your three phase currents, neutral current, voltage, frequency and power factor at the busiest time; a list of the largest motors with their kW and starting method; and when the trips or symptoms happen. A short video of the controller screen while the largest motor starts is very useful. Use the service and AMC enquiry, or WhatsApp the details to 98852 01608. If you do need a larger or additional set, our generator sales enquiry handles sizing, and you can use “Get price” once the size is agreed.
Questions engineers ask
How do I know if my generator is overloaded?
Read the current on each phase with a clamp meter and compare it with rated current, which is about 1.39 A per kVA at 415 V. Also watch frequency: if it falls below about 49 Hz under load, the engine is out of power. Black smoke, high coolant temperature, overcurrent trips and voltage collapse at motor starts are the other signs.
Can a generator run at 100% load?
A prime-rated set can carry its full prime rating for part of the day, and usually 10% more for one hour in twelve, but its average load over 24 hours is normally kept to about 70% of prime (some makers allow 80%). Constant full load calls for a continuous rating. A standby-rated set should not be run at full load continuously and has no overload allowance.
Why does my set trip when the average load is only 80%?
Almost always because one phase is much more heavily loaded than the others, or because a large motor start briefly pushes current far above rated. Check each phase separately and watch a motor start on the controller.
What happens if a generator is overloaded for a long time?
The engine runs hotter, smokes and wears faster; the alternator windings overheat and their insulation life falls sharply, roughly halving for every 10 °C above rating. Cables, terminals and changeover contacts also overheat. Repeated overload shortens the time to overhaul.
Does low power factor overload a generator?
It overloads the alternator rather than the engine. At lower power factor the same kW draws more current. At 0.7 power factor, current is about 14% higher than the same kW at 0.8, so the windings heat up even though the engine is comfortable.
Is a bigger generator the answer?
Often not. Phase balancing, staggered starts and better motor starters fix most overload complaints. A set that is too large for the normal load runs lightly loaded and suffers wet stacking. Size up only after the load has been measured and these fixes tried.
Can I raise the breaker setting to stop the trips?
No. Settings protect the alternator and cables. Raising them leaves the cause in place and removes the protection that stops a fire or winding failure.
Related pages
- Full-load current chart
- Generator sizing
- Motor starting on generators
- Motor starting generator size calculator
- Will your breakers trip on generator supply?
- Oversized and under-loaded generators
- Overheating and high coolant temperature
- Load bank testing
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.