How Generators Work- Part Two

In this article we discuss NEC guidelines for sizing generators and understanding the significance of power factor in generator sizing. Learn how to convert between watts and volt amps for accurate load calculations. Plus, explore tips for selecting the best generator fuel type based on your specific needs. A must-read guide for anyone dealing with generator sizing and selection.

In part two of how generators work, we will continue our discussion of sizing generators from part 1 of “How Generators Work.” We will also discuss in-depth sizing techniques of generators, and how to best approach sizing techniques.

Big backup generator

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 responsibility for sizing the generator system properly lies with the designer of the generator system or perhaps the generator manufacturer. 

Sizing generators Using Power Factor

In the previous article, we discussed that power factor represents the ratio of real power absorbed by the load (watts) to the apparent power flowing in the circuit (volt amps). When looking at a generator, we need to consider two different power factors. 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 power factor of the load supplied by the generator also requires consideration. 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. 

Check out Electrician U’s YouTube Video, where Dustin explains Volts, Amps, Ohms and Watts.

Electrician U YouTube Video-

Sizing generator example

With theory and concepts covered, let’s integrate everything 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. The recommended safety factor is 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. 

Generator's outside commercial building

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.


Understanding the significance of power factor is crucial as it directly impacts both real power and apparent power within a system. Proficiency in converting between watts and volt amps, and vice versa, holds fundamental importance for both professionals and enthusiasts. This knowledge becomes indispensable when determining the appropriate generator size to match the specific energy demands of an application. Equally important is the careful consideration of the fuel type for the generator. This decision can significantly affect operational costs, reliability, and environmental impact, depending on the intended application. Armed with these insights, individuals can confidently make informed decisions while selecting or recommending a generator, ensuring it not only meets but surpasses expectations.

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