In short. Not every load needs backup. Classification ranks loads by what happens if they lose power, then sets which are carried, which are shed and in what order they restart. It sizes the generator, shapes the single-line diagram and drives the load-shedding logic.
Tier model
| Tier | Consequence of loss | Typical supply |
|---|---|---|
| Life safety | Risk to people | Dedicated, tested, often separate circuits |
| Critical process | Safety, major damage or large financial loss | UPS then generator |
| Essential | Operations degrade, recoverable | Generator after delay |
| Deferrable | Convenience or comfort | Shed when capacity is short |
Method
- List loads from the load list. See load list.
- Ask for each load: what happens if it stops for a second, a minute, an hour?
- Define the maximum tolerable outage time, which sets whether it needs UPS, short-break or a delayed start.
- Assign a tier and a start sequence.
- Sum demand by tier to size supply and N+1. See multi-DG capacity.
- Define the shed order when capacity falls. See load shedding.
- Review with operations and update when loads change.
Dependencies
A critical process often depends on auxiliaries: cooling, ventilation, pumps, controls and communications. Include those in the same tier. Many failures come from an unprotected auxiliary supporting a protected load.
Restart logic
- Return power in steps, respecting load-step limits. See load-step response.
- Prevent the restart of heavy loads until enough capacity is online.
- Define manual and automatic restarts for each load.
Illustrative example
A hospital wing has 1,200 kW of load. Life-safety and critical clinical loads total 380 kW, essential loads 420 kW, and the rest is deferrable. The classification shows that 800 kW covers the first two tiers, so the plant is sized to that with N+1 and the deferrable load is shed automatically when one set is out.
Why classify loads
A generator rarely carries everything. Classification decides which loads must stay on, which can be delayed and which shed, so that the generator is sized to purpose and the right loads survive a shortage.
| Class | Meaning | Examples |
|---|---|---|
| Critical | No interruption tolerated, or only for seconds, with UPS | IT systems, life support, control systems |
| Essential | Short interruption acceptable, must restore quickly | Lifts, fire pumps, emergency lighting, selected process |
| Important | Restore if capacity allows | HVAC for key areas, pumps |
| Non-essential | Shed during generator operation | General HVAC, water heating, convenience loads |
Method
- List loads from the load schedule.
- Assign class with the operator, based on consequence of loss.
- Define restoration order and time for each class.
- Check the generator can start and carry the sequence, including motor starts.
- Define the shedding order if capacity falls.
Document the decisions
Record the class, owner of the decision and justification for each load. Review when the facility changes.
Link to design
Classification drives the switchboard split into essential and non-essential sections and the ATS arrangement. See load list and load shedding.
Common mistakes
- Everything on the generator without ranking.
- Forgetting auxiliaries.
- A list that is never updated.
What to send us
Send your load list and your outage tolerances. Send your application details.
Questions engineers ask
Who decides the tier?
Operations and safety owners decide the consequence. Engineering turns it into design.
Why classify at all?
It controls cost and makes shedding predictable instead of random.
How often should it be reviewed?
After any significant change and at least yearly.
Do UPS loads need generators?
Yes, unless the UPS battery can cover the full outage.
This guide gives general principles. Tiers must be agreed with the site owner.