This guide explains how designers and facility managers usually rank loads, how the transfer and shedding scheme puts that ranking into practice, and how to prove it works before a real outage tests it.
Why priorities matter
When mains power fails, the generator must start and pick up load quickly. A single large block of load can overwhelm the engine, causing a frequency dip that trips equipment or stalls the set. Staging the load in priority order lets the generator accept it in manageable steps.
Priorities also protect the most important loads if the generator is overloaded later, for example when an extra chiller starts or a unit in a multi-set system fails.
Typical load categories
Most codes and owners recognise three broad groups. Life safety loads include emergency lighting, exit signs, fire alarm systems and fire pumps. Legally required standby loads include smoke control, certain ventilation and communication systems. Optional loads cover everything the owner chooses to back up for business reasons, such as IT rooms, refrigeration or comfort cooling.
Local codes define these groups and their time limits differently, so the design must follow the rules that apply at the site.
Separate transfer switches and circuits
Codes often require life safety loads to be fed through their own transfer switch and wiring, separate from other loads. That keeps a fault in an optional circuit from affecting emergency lighting or fire systems. Larger buildings may have several transfer switches, one for each priority group.
Clear labelling of each switch and its loads makes testing and maintenance easier and reduces the risk of mistakes during an incident.
Transfer sequence and timing
Life safety loads usually transfer first, within the shortest time allowed by code. Legally required loads follow, then optional loads after a further delay. The delays give the engine time to recover speed and voltage between steps.
Set the time delays based on the generator’s tested load acceptance, not just on default controller values. Our article on emergency power for high-rise buildings shows how this works in a tall building with fire pumps and lifts.
Load shedding on overload
If the generator approaches overload, a load shedding scheme drops optional loads first, then legally required standby loads if needed, while keeping life safety loads connected. Shedding can be triggered by power measurement, frequency decay or the loss of a unit in a paralleled system.
Restoring shed loads should also be staged and only happen when enough capacity is available, to avoid repeated overload cycles.
Motor starting and step loads
Fire pumps, chillers, lifts and large fans draw high starting currents. Each step in the transfer sequence must be checked against the generator’s ability to start the largest motor in that step without unacceptable voltage dip. Soft starters and drives reduce the impact.
When a fire pump must start at any time, the generator must be sized to accept it even when other loads are already connected.
Capacity margins
Size the generator for the connected priority loads plus a margin for growth and for real-world conditions such as high ambient temperature. A set that is barely adequate on the day of commissioning may be undersized after a few years of building changes.
Review the load list whenever tenants change or major equipment is added, and update the shedding scheme to match.
Multi-generator systems
Paralleled generators add flexibility. The first set online can pick up life safety loads while the others synchronise and take the remaining load. If one set fails to start, the scheme should automatically limit the connected load to what the available sets can carry.
Test these scenarios deliberately, including the failure of a set to start, so the logic is proven before it is needed.
Testing the scheme
Routine monthly runs often only test the engine. At least once a year, test the full sequence: a simulated mains failure, transfer of each priority group in order, load shedding and restoration. Record the timing of each step against the design values.
Keep these results with the records discussed in our post on test records and compliance, so inspectors and insurers can see the scheme works.
Common mistakes to avoid
Several problems appear again and again in site surveys. Optional loads end up wired to the life safety transfer switch after a renovation. Time delays are left at factory defaults that are too short for the engine to recover. A new chiller or server room is added without anyone checking the generator capacity or the shedding order. Load shedding relays are installed but never tested, so nobody knows whether they work.
A short review after every significant building change, combined with an annual full-sequence test, prevents most of these issues. It costs far less than discovering the problem during a real outage, when lighting, alarms or fire pumps are at stake.
Documenting priorities
A one-page load priority schedule, listing each load, its group, its transfer switch, its time delay and its shedding order, is invaluable during an outage and for future design changes. Post it in the generator room and keep a copy with facility management.
For commercial buildings, the approach in our guide to commercial emergency generators shows how priority planning fits into the wider system design.
About the authors
Our emergency power engineers wrote this page from designing and commissioning standby systems for commercial, industrial and public buildings. USP&E delivers and maintains these systems as the contractor itself, not as a broker, drawing on 150+ power projects in 45+ countries, 24+ years of practice and 25,000 MW contracted since 2002. Our full delivery scope is described on the EPC projects page.
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Load priority questions
Which loads come first on an emergency generator?
Life safety loads such as emergency lighting, fire alarms and fire pumps, followed by legally required standby loads and then optional loads.
Why stage loads instead of transferring everything at once?
A single large block can overload the engine and cause a frequency dip. Staging lets the generator recover between steps.
What happens if the generator becomes overloaded?
A load shedding scheme drops optional loads first, then legally required loads if needed, while keeping life safety loads connected.
How often should the priority scheme be tested?
At least annually with a full simulated outage, checking each transfer step, shedding and restoration against the design.
Review your load priorities
Send the single-line diagram, load list and generator data, and our engineers will check your priority groups, transfer timing and shedding scheme.
Phone +27 10 822 2324 during office hours, or write to info@uspeglobal.com at any time; the emergency power enquiry page also routes straight to our project engineers. Standby sets that could add capacity are listed in our diesel generator stock list.