In short. When a block of load switches on at once, a generator’s frequency and voltage dip and then recover. The engine slows because the governor cannot add fuel instantly, and the alternator voltage dips under the sudden current. Whether the step is acceptable depends on its size as a share of the rating, its power factor, the engine type, and how far the connected loads can tolerate a dip. Screen each step against the maker’s load-acceptance data, and split it into smaller blocks when it is too big.
Who this page is for: For consultants, commissioning engineers and facility managers planning how loads come onto a generator after a transfer or start.
Key facts
- Step as a share of rating = step kW ÷ generator kW × 100.
- Voltage dip screening: dip ≈ step kVA ÷ (step kVA + generator kVA ÷ X′d).
- Turbocharged diesels accept a smaller first step than naturally aspirated ones; gas engines smaller still.
- The maker’s load-acceptance table gives the largest step for a given frequency dip.
- Recovery to within limits typically takes one to several seconds.
- Large steps are split into blocks with delays, or softened with soft starters or storage.
What happens in a step
The new load demands torque the engine cannot supply until the governor adds fuel and the turbocharger builds boost, so speed and frequency fall. At the same moment the alternator voltage dips, by an amount set by the step’s kVA, its power factor and the alternator’s transient reactance. The governor and voltage regulator then restore both. A deep or slow dip can drop contactors, trip drives and UPS inputs, or stall the set.
Screening inputs
| Input | Why |
|---|---|
| Step size in kW and kVA | Sets the disturbance |
| Power factor of the step | The reactive part drives the voltage dip |
| Load already running | Changes the engine’s response |
| Allowed voltage and frequency dip | Set by the most sensitive loads |
| Allowed recovery time | Before the next step |
| Engine type and alternator reactance | Set the set’s step capability |
Worked example: a 300 kW block on a 1000 kVA set (illustrative)
A 1000 kVA (800 kW) diesel set already carries 400 kW. A 300 kW block at power factor 0.8 is about to switch on. The alternator’s X′d is about 0.25.
| Check | Working | Result |
|---|---|---|
| Step as a share of rating | 300 ÷ 800 | About 38 percent |
| Step kVA | 300 ÷ 0.8 | 375 kVA |
| Voltage dip estimate | 375 ÷ (375 + 1000 ÷ 0.25) | About 8.6 percent |
| Frequency dip | From the maker’s table for a 38 percent step on this engine | Compare with the loads’ limit |
| If the frequency dip is too deep | Split into two 150 kW blocks a few seconds apart | Each about 19 percent |
The voltage dip is acceptable for most loads; the frequency dip decides. Splitting the block needs only a timer or controller setting and costs a few seconds. See load-step and frequency response.
Do not let all loads return in one step after a transfer. Even a correctly sized set can stall or trip on a single step larger than its engine can accept. Stage the loads with delays and test the sequence.
Ways to reduce a step
- Split loads into blocks with time delays.
- Start the largest motors first, while the set is lightly loaded.
- Use soft starters or drives on large motors. See motor starting.
- Add battery storage to take the first part of the step.
- Choose a set with better load acceptance, or a larger set.
Performance classes
Generator standards define performance classes for allowed voltage and frequency dips and recovery times. Data centres and hospitals often specify a stricter class than construction sites. Ask the maker which class the set meets for the steps you plan.
Load step plan
| Step | Loads | kW | % of rating | Delay after previous | Within limits? |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 |
Common mistakes
- Checking total kW but not the step.
- Ignoring the power factor of the step.
- Assuming a gas engine accepts steps like a diesel.
- No delays between restart groups.
What to send us
Send the generator model, the loads in each restart group and the most sensitive equipment. WhatsApp 98852 01608 or use the enquiry form.
Evidence note. The dip formula is a screening estimate; the maker’s load-acceptance data and a transient study govern.
Questions engineers ask
What is the largest step a generator can take?
It depends on the engine, alternator and allowed dip. The manufacturer’s step-acceptance data gives the answer.
Why do gas engines handle steps worse?
They typically have less torque reserve, so the allowed first step is often smaller.
How does a BESS help?
It can take the step quickly while the engine responds, cutting the dip.
What is the performance class?
A classification in standards such as ISO 8528-5 for acceptable voltage and frequency deviation and recovery.
Figures are illustrative. Use manufacturer data for design.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.