Generators Part Two

We will continue our discussion of sizing generators from part 1 of generator basics. In this part we will discuss in-depth sizing techniques of generators, and how to best approach sizing techniques.

What does the NEC say about sizing generators?

The NEC doesn’t necessary provide a calculation for sizing generators, however there are some important sections to refer to: NEC 700.4, 701.4, and 702.4 (life safety, legally required standby, and optional standby) state that the system shall have adequate capacity in accordance with Article 220, or another approved method. Essentially, the NEC is telling us how to calculate the loads connected to the generator, but does not specifically state how much load a particular generator can handle. The designer of the generator system, or perhaps the generator manufacturer, would be responsible for making sure the generator system is sized properly. 

Sizing generators Using Power Factor

As discussed in the previous article, power factor is the ratio of real power absorbed by the load (watts), and the apparent power flowing in the circuit (volt amps). There are actually two different power factors that need to be considered when looking at a generator. One is the power factor of the alternator on the generator itself. This is usually .8 or 80% (sometimes 1, or 100 percent, on smaller single-phase generators). The other power factor to consider is that of the load being supplied by the generator. Power factor can be either leading or lagging. Lagging power factor, when current lags behind the voltage, is caused by inductive loads such as motors or transformers. Leading power factor, when the current leads, is caused by capacitive loads such as battery chargers, capacitor banks, and other electronics. In most buildings, most loads will be of the lagging type. 

Understanding how to use Volt Amps and Watts

The concept of apparent power (volt amps, or VA) versus real power (watts, or W) is a crucial one to grasp for generator sizing. Thankfully, there are a few easy tips and tricks we can use to better understand this.

Let us review how to convert from watts to volt amps and vice versa:

To get volt amps if we know voltage and current Single Phase: Voltage x Current = Volt Amps

To get volt amps if we know voltage and current Three Phase: Voltage x Current x 1.732 = Volt Amps

To get watts if we know volt amps and power factor: Volt Amps x PF = Watts

To get volt amps if we know watts and P=power factor: Watts PF = Volt Amps

To get power factor  if we know volt amps and watts: Watts Volt Amps = Power Factor

When calculating loads, it is important to make sure we are working in volt amps, or VA, rather than watts. Working in watts does not always consider power factor, which could lead to an undersized generator. 

Sizing generator example

Now that theory and concepts have been covered, let’s put everything together and apply it to a real world situation. 

Example: A customer has requested for a new generator to go into their brand new 20 story office building. They only want to have one generator for all the loads required in the building. The generator must be a three phase 480V generator. Natural gas and diesel are both options. 

Load breakdown:

100kW @ .9PF of life safety loads (emergency lights, fire alarm etc).

50 HP fire pump (65A, 480V, 3 phase, @ .8PF). 

500kW of optional standby load from tenants @ .9PF (lights, receptacles and computer loads)

200kW of legally required standby @ .9PF (elevators, stair pressurization fans etc). 

Step 1: Convert all loads to volt amps

Fire Pump:

80V x 1.732  x  65A  x  .8pf =  43,230VA or 43.2kVA life safety loads=100kW .9pf = 111.1kVA

Optional standby load = 500kW .9pf = 555.6kVA

Legally required standby load = 200kW .9pf= 222.2kVA

Total sum of all loads in kVA = 932kVA. 

Step 2: Choose a generator size

Since the load is 932kVA, we want to make sure we have enough buffer room for future expansion as well as motor startup. It is recommended to provide a safety factor of about 25 percent. 

932kVA x 1.25 = 1165kVA.Consider a 1000kW or 1250kVA @ 80% PF for this application.

Remember that we are looking at the volt amp values, even though most generator manufacturers provide only the watt or kilowatt rating. Working in volt amps, we are 100 percent certain that this generator will work. 

Step 3: Consider generator fuel source

Since this application has life safety loads connected to the generator, the fuel source becomes an important consideration. As per NEC 700.12: “Current supply shall be such that, in the event of failure of the normal supply, to or within, the building or group of buildings concerned, emergency lighting, emergency power, or both shall be available within the time required for the application but not to exceed 10 seconds”. This code section requires that life safety loads are up and running within ten seconds of a power failure. That means the generator needs to be able to start up and get to a steady state extremely quickly. As mentioned previously, one of the downfalls of natural gas generators is a slow start up time at the larger sizes. Since this is a 1000kW unit, a diesel generator will most likely be the best choice. However, as of recently some generator manufacturers are providing generators of up to 1000kW that are ten-second capable, while running on 100 percent natural gas. 

Some helpful tips and tricks

There will be many instances where the power factor is unknown. A helpful tip is to use .9, or 90PF, on most modern buildings or new construction buildings. The reason for this is that the biggest contributing factor to power factor in a building is usually large, inefficient motors. Most modern-day motors are highly efficient and are controlled with variable frequency drives or even soft-start motor starters. This is common when installing new mechanical equipment such as chillers, pumps, or air handlers. Also, many LED drives and computers have active power factor correction and/or operate at .9 or great power factor. .8 can be considered for older buildings with outdated equipment and motors. 

Load shedding is also something that can be an option if increasing a generator size is not possible. Load shedding means that not all loads are running simultaneously – those that are unused are ‘shed’, meaning that a smaller generator can be used. For example, when a fire pump is active on generator power, optional standby loads can be shed as the building will be evacuated at that point and the standby loads will not be needed. Both loads would not run at the same time, so we would only consider the larger of the  two loads when sizing the generator.


Let’s review and summarize some things we discussed about generator sizing in particular:

  1. Working with power factor
  2. Converting from watts to volt amps  and vice versa
  3. Calculating and applying generator sizing
  4. Choosing the generator fuel type based on application

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