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Calculator · E-504

Standby generator sizing calculator

Add up the loads a standby generator must carry, allow for power factor and the largest motor starting, and see the kVA to plan around.

Generator sizing · E-504Metric · SI
kW
Sum of the loads on the backed-up panel(s).
PF
kW
×
An assumption: direct-on-line starting draws several times running current; soft starters and drives far less. Use the motor or drive data.
%
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The inputs start with example values: replace them with your loads. Final sizing uses the manufacturer’s software and the designer’s load schedule.

CCT 01How it works

Running kVA = running kW ÷ power factor. When the largest motor starts with everything else running, the generator sees the running kVA of the other loads plus that motor’s starting kVA. The planning size is the larger of running kVA with the margin added, or the starting case.

CCT 02Assumptions and limits

  • Generator manufacturers’ sizing software models voltage dip, alternator capacity, step loading and harmonics from drives and UPS. Use this to size the conversation, not the machine.
  • Emergency power systems that the building code requires (as opposed to optional standby) have their own installation and testing standard; see generators and transfer switches.

Questions people ask

What size generator does a commercial building need?

It depends on what it must carry: the whole building, or only critical loads through an automatic transfer switch. Most owners back up a defined set of loads. Enter those above.

Why does motor starting matter?

A motor started across the line draws several times its running current for a moment, and the generator’s voltage dips while it does. Soft starters and drives reduce that, and so reduce the generator size.

Does a standby generator need a permit in Calgary?

Yes: an electrical permit for the connection, and depending on the unit, fuel and location, other approvals. See commercial electrical permits.