A generator has to do two things: carry the total running load for as long as the outage lasts, and survive the moment each load starts without the voltage and frequency falling so far that equipment drops out. Sizing for the running load alone is the most common mistake. Motor starting, the order in which loads come on, and non-linear loads such as drives and UPS systems often decide the size.
Start with what the generator must carry
The first step is a clear list of loads, divided into the categories that matter:
- Emergency loads required by the building code, such as egress lighting, fire alarm, fire pumps and firefighters' elevators.
- Standby loads the owner wants backed up, such as heating, sump pumps, IT, refrigeration or production.
- Loads to leave off, because the cost of backing them up is not worth it.
The distinction affects both the size and the design. See emergency power vs standby power. For each load, the designer needs its running power, its type (resistive, motor, electronic), and for motors, how it starts.
kW, kVA and power factor
Generators are rated in both kW, the real power the engine can deliver, and kVA, the apparent power the alternator can carry. They are commonly rated at a power factor of 0.8, so a generator rated in kVA delivers less in kW. The load has to fit within both limits. Loads with a low or leading power factor, including some UPS systems and drives with filters, can cause problems for the generator's voltage regulation even when the kW total looks fine.
Motor starting and load steps
An induction motor starting across the line draws a large inrush current, several times its running current, for a few seconds. On the utility this barely matters. On a generator it causes a sharp dip in voltage and a drop in frequency while the engine and alternator respond. If the dip is too deep, contactors drop out, lights flicker, drives trip and the motor may not start at all.
The way loads are added after a transfer matters just as much. If every load transfers at once, the generator sees the total inrush in one step. Designers commonly split loads into steps, with emergency loads first and larger motors brought on afterwards, using separate transfer switches, time delays or the building automation system.
Ways to reduce the starting burden
- Start the largest motors first, while the generator is lightly loaded, or last after the rest has settled, depending on the analysis
- Use soft starters or variable frequency drives on large motors. See motors, starters and VFDs
- Stagger transfers with time delays
- Choose an alternator with good motor starting capability, which may be larger than the engine needs
Non-linear loads
Drives, UPS systems, LED drivers and IT equipment draw current in pulses rather than smoothly. This harmonic current distorts the generator's voltage more than it would distort a stiff utility supply, which can upset the generator's voltage regulator and other equipment. The usual responses are to choose an alternator with low reactance, oversize the alternator relative to the engine, use drives and UPS units with harmonic mitigation, and specify a generator control system designed for these loads. See power quality and harmonics.
Site and operating conditions
- Altitude and temperature. Engines lose output at high altitude and high intake temperature. Calgary and much of Alberta's foothills are high enough that the manufacturer's derating should be checked.
- Cold weather. Block heaters, battery heaters and enclosure design affect whether the generator starts and picks up load in winter.
- Rating type. Generators carry standby, prime or continuous ratings. A standby rating assumes limited hours and variable load. Long-running or base-load use needs a different rating.
- Fuel. Fuel storage is sized for the required run time at the expected load.
Don't oversize it either
A diesel generator that runs at a small fraction of its rating for long periods can suffer from wet stacking, where unburned fuel builds up in the exhaust system. Oversizing also costs more to buy and maintain and takes more space. If the expected load is much lower than the size needed for motor starting, consider using drives or soft starters to reduce the starting requirement, and plan regular exercising and load bank testing.
A typical sizing sequence
- List all loads to be served, by category, with running power, type and starting method.
- Decide the load steps and the order of transfer.
- Calculate the running total in kW and kVA, with an allowance for future load.
- Calculate each step's starting demand and the expected voltage and frequency dip, using the manufacturer's data or sizing software.
- Check non-linear load content and alternator selection.
- Apply site derating for altitude and temperature.
- Select the generator, then check the result against the code requirements for emergency loads and against fuel storage.
What to give your designer
- The list of loads you want backed up, and any you are unsure about
- Utility interval data or measurements showing the actual load on the circuits involved
- Nameplate data for large motors, and whether they have starters, soft starters or drives
- Details of UPS systems, including their rating and battery recharge behaviour
- How long you need to run without utility power, which drives fuel storage
- Plans for future equipment over the next several years
- Any requirements from insurers, tenants or contracts
A typical case
A small water pumping station needs a standby generator. The running load is modest, but the station has two large pumps that start across the line. A first estimate based on running load suggests a small generator. A motor starting check shows that starting one pump would dip the voltage far enough to drop out the control panel. Two options are compared: a much larger generator, or soft starters on the pumps with the pumps started one after the other. The second option allows a generator closer to the running load, with less risk of wet stacking, and improves the pumps' starting on utility power as well.
Adding load later. Generators are often sized correctly at installation and then loaded up over years as equipment is added. Before connecting anything new to a generator, have an engineer check the running total and the starting step it creates.
How ehZee can help
Our emergency and standby power design work includes generator sizing based on your actual loads, starting methods and site conditions, and designs the transfer switches, load steps and controls that make the size work.
Common questions
Can we size the generator from the main breaker rating?
No. The main breaker reflects the service capacity, not the loads you need to back up or how they start. That approach usually leads to a generator that is much too large, or occasionally too small for motor starting.
Is a natural gas generator sized the same way?
The method is similar, but gas engines usually respond differently to large load steps than diesel engines, and that has to be accounted for. The fuel supply pressure and reliability during an emergency also need checking.