Engineering of Water Systems

Standby and Backup Systems: Wrapping Up Generator Sizing for Electric Motors

By Ed Butts, PE, CPI

Let’s pick up where we left off in the July column by examining the primary application of generators for use in the water well industry: the sizing of generators for electric motors.

First, a competent and qualified electrician or installation technician who is familiar with applicable and local codes, standards, and regulations should perform all wiring and installation procedures. It is also essential to comply with all applicable regulations established by the Occupational Safety and Health Administration (OSHA), and strict adherence to all local, state, and national codes is mandatory.

Before selecting a generator, it is wise to check all municipal and local governmental ordinances that may dictate requirements regarding placement of the unit (e.g., setbacks from building or lot lines), electrical wiring, gas piping, fuel storage for liquid propane or diesel fuel storage vessels, and permissible sound and exhaust emission levels.

Load Groups and Stepping

The starting sequencing and progressive addition or reduction of single or multiple loads can have a significant impact on the sizing of a generator.

Additive loads for generators with more than a single load are typically assembled in groups. A group for generators is defined as an assemblage of various types and sizes of loads all within the same operational sequence, also known as a step.

A group can consist of a single or multiple motors or a diverse combination of motor, resistive, and non-linear loads. Groups are often based on a time-delayed addition or sequential reduction or stepping of selected loads to lessen the demand and impact on the genset.

One commonly used approach is to assume all connected loads function together or that one group will start in a single step once the unit has attained sufficient operating speed. This is usually not performed in practice as it results in the largest and least economical generator selection. Unless loads are added or removed incrementally, such as using multiple transfer switches, motor starters with staggered time delay or a stepping load controller, a single-step approach for sizing purposes is usually conducted as the default method.

In multiple step applications, the largest motor is generally started first to minimize the generator size. Once all loads are started and placed online, the loads can be periodically stopped and started (i.e., cycled) using automatic controls. These are often referred to as intermittent loads. In this case, the genset should be sized by assuming the largest motor starts last with all other connected loads already functional.

The engine or prime mover is sized to provide the actual or real power in kW, as well as speed (frequency) control using a mechanical or solid-state engine governor. The generator is sized to supply the kVA needed at startup and during normal running operation. It also provides voltage control through use of a brushless exciter and voltage regulator.

The engine and generator together provide the energy necessary to supply electrical loads in many different applications encountered. Determining the stepping procedure can be conducted using manual methods or use of a manufacturer software package that permits the evaluation of inserting or deleting load steps under various conditions and operating sequences.

Generator Sizing Using Pre-Selected Values

Single Motor with Full Voltage Starting

Induction motors have fairly universal starting characteristics. A typical curve of motor current versus speed shows that during starting the motor draws approximately six to seven times its full load current. This current remains high until the motor reaches approximately 80% of full speed and then rapidly drops to full load current.

This high inrush current causes a resultant reduction or dip in a generator’s output voltage. The electric power initially required by the motor at initial energization (with the motor at rest) is about 150% of its rated power. The power required by the motor peaks at about 300% of rated power and 80% of speed with full voltage applied. However, the generator set supplies less than 300% power because starting voltage is lower than full voltage during acceleration and because the genset’s rotating inertia transfers energy to the motor.

Table 1 is from Franklin Electric’s AIM Manual and lists minimum generator sizes for 1/3 to 200 HP. It is based on a typical 80°C rise continuous duty generator with 35% maximum voltage dip during full voltage (across-the-line) starting conditions for single- and three-phase submersible pump motors.

The table provides conservative values and can generally be used for most standard induction motors starting a centrifugal style of pump using full voltage starting methods and with acceptable wire sizing criteria to limit voltage drop to 5% of rated voltage at the motor terminals.

These tabular values should not be used for premium efficiency motors, reduced voltage starting, unusual motor applications or service (i.e., positive displacement pumps, high inertia loads), multiple motors, or diverse single- and/or three-phase loads. These applications warrant individual consideration and calculation of each load’s inrush current, load diversity, and grouping/stepping/sequencing of loads.

The instantaneous voltage dip and frequency dip will also likely vary between manufacturers based on equal ratings of gensets. For a closer estimation of transient (starting instantaneous voltage) performance, always refer to and use the manufacturer’s sizing software and recommendations when available.

There are two types of AC generators commonly available, externally and internally regulated, although most older units are externally regulated. They use an external voltage regulator and engine governor that senses and responds to the output voltage. As the voltage at the generator dips at motor startup, the regulator increases the output voltage of the generator by ramping up the speed.

Internally regulated (i.e., self-excited) generators have an extra winding embedded in the generator stator. This additional winding uses a solid-state sensor that monitors the output current to automatically adjust the output voltage (Figure 1). In all cases generators must be sized to deliver at least 65% of the motor’s rated voltage during starting to ensure adequate starting torque is present.

Besides sizing, generator frequency is also important as the frequency (in hertz or Hz) also varies with the motor speed. Due to the affinity laws, a pump running at 1 Hz to 2 Hz below a motor’s nameplate frequency will underperform and not meet its performance expectations. Conversely, a pump running at 1 Hz to 2 Hz above full speed may blow fuses or trip overloads.

The generator’s response to loads and recovery time is another critical aspect of generator sizing. Upon addition of a load, the response will be a voltage dip. The genset must be capable of recovering this loss of voltage within a few milliseconds to prevent possible drop out of operating units (Figure 2).

Always start the generator before the motor is started and stop the motor before the generator is shut down as the motor thrust bearing may be damaged if the generator is allowed to coast downward in speed with the motor running. This same condition occurs when the generator is allowed to run out of fuel during operation.

Generator Manual/Software Sizing Procedure

Single or Multiple Motors and Loads

Generator/engine sets (gensets) for single or multiple loads can be sized using either manual calculations (preferably using a load inventory and worksheet) or with a software sizing program developed and available from most major genset manufacturers.

Using a genset manufacturer’s software package is usually the better option since much of the required information on typical motor and load characteristics is already available in the software’s database. Most software generally permits accurate generator selection based on voltage drop, frequency dip, and various grouping, stepping, and sequencing scenarios to determine the most effective and economical genset size.

Regardless of the sizing and selection method, the basic process is the same. To start, it’s always best to use actual motor and load data if known. If this is unknown or unavailable, use of typical tabular data can provide a close approximation. As previously stated, gensets must produce adequate voltage to be able to start and run all projected and combined loads. This may consist of a single or multiple motor loads or a combination of various loads.

When sizing a generator using manual methods, several criteria should be met to ensure proper performance, including the following recommendations:

• Obviously, the generator and engine must both produce enough power in RkW and HP to run the full load.
• Upon application of the largest motor load step, the generator voltage and frequency dip must not exceed the specified maximum during the entire transient period (starting surge). This is generally defined as the instantaneous voltage dip (IVD), typically limited to between 20% to 35%, with 30% the most common value. Tighter control of voltage drop to 10% or 20% will require higher generator capacity to lessen the drop.
• With the largest motor load step applied to the generator, the generator output voltage should recover to a minimum of 90% of its rated voltage (i.e., the sustained voltage).
• Use appropriate derating for altitude, ambient temperature, and loads with harmonics and/or unbalance.

When analyzing an electrical load, consulting the nameplate data or typical values for each major item of electrical equipment is needed to determine the starting and running requirements in terms of watts, amps, and voltage to provide an effective but not oversized generator plant. This equipment includes motors, electric heating and cooling, receptacles, and lighting fixtures.

When choosing a generator output for water pumping applications, it is generally suggested the designer select a kilowatt rating that is approximately 20% to 25% greater than the peak load. For example, if the peak design load is determined to be 100 kilowatts (100 kW), selecting a 125 kW genset is often the recommended size.

In most cases, a higher-rated generator will operate well at 75% to 80% of full load capability and work down to 30% of its full capacity rating as well as provide a margin of needed flexibility for different motor starting characteristics and expansion potential, if the load increases in the future. This is particularly important for many deep well pumping applications where the motor horsepower may increase in the future to handle higher capacity or pumping heads.

Using the Manual Sizing Procedure for Motors and Other Loads
Step 1.

Gather information by assembling the following data for each separate motor and other loads. (Note: if power factor and efficiency are unavailable, estimate by using the values in The Water Works July 2025 column.)

• From motor nameplate: HP, voltage, full load current, hertz, code, efficiency, power factor, and phase.
• From other loads: kW or kVA, voltage, current, hertz, phase, and power factor (when applicable).
• Unless otherwise indicated, use full load values for efficiency and power factor.

For motors and other loads, calculate the following:

• Running kilowatts (RkW). Also used for resistive and/or non-linear loads.
• Running kilovolt-amperes (RkVA). Also used for resistive and/or non-linear loads.
• Running motor power factor (PF). Use Table 4 values in The Water Works July 2025 column if unavailable from nameplate.
• Starting motor power factor (PF). Use Table 4 values in The Water Works July 2025 column if unavailable from nameplate.
• Motor efficiency. Use approximate values in The Water Works July 2025 column if unavailable from nameplate.
• Locked rotor kVA/HP. Obtained from code letter on motor nameplate or values from The Water Works July 2025 column.

Step 2a.

Use the above data and following equations to calculate the RkW and RkVA for motors:

Equation 1a: RkW = [Nameplate HP × 0.746 kW ÷ HP] ÷ Motor efficiency (%)

Equation 1b: RkVA = [RkW ÷ Running motor PF] (PF in percent)

Equation 1c: VFD’s: RkW = Drive nameplate HP × 746 ÷ Motor efficiency (%) × 1000

Equation 1d: VFD’s: RkVA = Drive nameplate HP × 746 ÷ Motor efficiency (%) × PF (%) × 1000

To calculate starting kilovolt-amperes (SkVA) and starting kilowatts (SkW) for motors, use these equations:

Equation 2a: SkVA = [Nameplate HP × Locked rotor kVA ÷ HP]

Equation 2b: VFD starting: SkVA = Drive nameplate kVA ÷ Motor efficiency (%)

Equation 2c: SkW = [SkVA × Starting motor PF]. Use Table 4 data in The Water Works July 2025 column for PF if otherwise unavailable.

Equation 2d: Reduced voltage starting: SkVA = Motor SkVA × Reduced voltage factor (refer to The Water Works July 2025 column.)

Step 2b.

The motor loads will then be listed with the other resistive and reactive loads.

Step 3a.

Total the RkW, RkVA, SkW, and SkVA numbers for all loads.

Step 3b.

When warranted or desired, place individual loads into groups and use separate steps to optimize the genset selection and reduce the overall SkVA requirement for motor starting.

Step 4.

Select the genset by comparing the RkW, RkVA, SkW, and SkVA to the ratings on the manufacturer’s specification or datasheets after appropriate derating if needed for ambient temperature and altitude. Although 35% IVD is the generally accepted value for maximum voltage drop, 30% IVD is the typical voltage dip value used for genset selection. This retains 5% for cable loss for submersible pump installations. Most generator manufacturers rate their units for 30% or 35% IVD. Lower values will require larger genset kW sizes. Consult the generator manufacturer or supplier for the applicable SkVA rating for lower IVD or SVD values.

Generator Sizing Design Example

Determine the required generator size for the following
water system loads (refer to Figure 3):

Nos. 1 and 2: Low service booster pumps: (2) 5 HP, three-phase, units (assumed efficiency = 85%. PF = 81%, code letter: H)

No. 3: High service booster pump: (1) 20 HP, three-phase, unit (assumed efficiency = 90%, PF = 86%, code letter: G)

No. 4: Well pump: (1) 15 HP, three-phase, submersible unit (refer to submersible motor data from Franklin AIM Manual) 15 HP, three-phase, submersible motor: efficiency = 81.5%; PF = 82.8%; code letter: H (use 7.0 SkVA ÷ HP)

No. 5: Utilization equipment: 10 kW, 120/208 VAC, three-phase (assume 0.95 PF for fluorescent lighting)

Solution: This is a common application for a water system facility that involves a source well pump, storage reservoir, three booster pumps, and miscellaneous loads. The 10 kW load is a typical demand for heating and low voltage power. For this example, booster pump motor data was not provided; thus, estimated values will be used for sizing.

Scenario 1

Based on a single step in Scenario 1 and using a manufacturer’s selection program, even though the running load is only 49.08 RkW, this application will require a 150 kW genset that can provide up to 394 SkVA to handle the high value of 302.1 inrush SkVA for the simultaneous motor starting requirement.

Scenario 2

If the switchover procedure was slightly varied to provide two steps of load activation by placing the 10 kW utilization power and both 5 HP booster pumps in Step 1, followed by time-delayed starting of the 15 HP and 20 HP pumps in Step 2 just a few seconds later, the peak SkVA will drop to 245.34 from 302.1, a reduction of almost 20% below Scenario 1. This is the sum of SkVA of 224.0 from Step 2 combined with the RkVA of 21.34 from Step 1. This allows use of an 80 kW genset, around one half the original size, although the running kW remains the same at 49.08 kW following the start of motors in Step 2, underscoring the advantages of step starting motors.

Scenario 3

Using solid-state reduced voltage starting methods with a 200% current limit setting on the 20 HP and 15 HP pumps will additionally lower the SkVA to 90.78 (69.44 SkVA + 21.34 RkVA from Step 1), resulting in the selection of a 60 kW genset as the smallest possible size needed to handle the running kW of 49.08,although most experienced designers would likely stick with an 80 kW genset to provide a small margin of a safety factor, possible growth, and harmonic protection from the solid state motor starters and fluorescent lighting.

These examples illustrate the importance of using both values of starting (SkVA) and running (RkW and RkVA) system demands versus the genset ratings when sizing a generator as well as obtaining accurate data and using load stepping and diversity to optimize the generator selection.

Since the booster pumping units will likely cycle and the largest units may not even run, this will result in an intermittent load. This creates an application in which the generator will often fall below the minimum desired load of 30% of the generator rating regardless of the unit ultimately selected. In this case, for prolonged or frequent generator operation, the addition and use of a load bank that can be removed during higher motor loads may be warranted to protect against continual operation of the diesel engine at a severely reduced load.

Generator Sizing Rules of Thumb and General Data and Recommendations

Rule 1: While a motor by itself can usually tolerate 30% to 35% of voltage dip during the starting phase, it can conceivably cause a motor starter coil to drop out. Consideration should be given to the effect on other voltage-sensitive systems on the generator, such as lighting. Therefore, a more realistic value of recommended acceptable voltage dip should be between 15% to 20%.

Rule 2: Use 1 kW of genset rating per each ½ HP to ¾ HP of single motor nameplate rating (50 HP = ~66.6 kW).

Rule 3: Oversize gensets by 20% to 25% to provide reserve capacity and for across-the-line motor starting.

Rule 4: Oversize gensets for unbalanced loading, significant harmonic loads, or low power factor running loads.

Rule 5: For variable frequency drives, oversize the genset by at least 40% for six-pulse technology drives.

Rule 6: Always start the largest motor first when load stepping. Most generators are capable of delivering 300% of their rated current for 10 seconds, which is adequate for most induction motors to accelerate to rated speed.

Thus, for across-the-line or direct online (DOL) starting, a generator rated two to three times the kVA of the motor will generally provide enough starting kVA to the motor. For a star/delta start, even a generator sized just above the rated motor kVA will generally be sufficient.

These general rules will usually result in a genset size that will keep the voltage dip during motor starting within acceptable limits but without significant oversizing.

Estimating a Generator Size

Quickly estimating the size of a generator is needed in some cases for obtaining cost estimates, replacing an existing unit, or electrical load calculations of an existing system. In these instances, the following can be used for single- and three-phase estimates.

It must be noted that this shortcut method cannot replace a complete analysis of the power system, including the number of motors, starting method, and other loads. However, for a quick estimation, the following can be reliably used.

In all cases, use a clamp-on ampmeter or power analyzer to measure the facility load levels. Clamp each leg separately, and for best results, take the measurements during peak usage levels.

For 120/240 VAC Single-Phase (1ϕ) Applications:

To determine the peak usage in kW, add the highest amp readings from the two legs (L1, L2), multiply by 120, and divide by 1000, as: [(L1 + L2) × 120] ÷1000. Size generator 15%-20% larger than the peak measured load.

For Three-Phase (3ϕ) Applications:

Add the highest amp readings from all three legs and then divide by 3 to determine the average amps. Multiply the average amps by the voltage, multiply the result by 1.732, and then divide by 1000 multiplied by the power factor to convert the amps to kW.

Average Amps = (L1 + L2 + L3) ÷ 3

kW = [(Average Amps × Volts) × 1.732] ÷ 1000 × PF⁽¹⁾

⁽¹⁾ In lieu of or lacking the actual system or motor power factor, assume a power factor of 0.80. Size the generator 20% to 25% larger than the measured load.

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I hope this has been helpful. In the next installment of The Water Works in the January 2026 issue, we will begin to wrap up this multi-part series on standby and backup engines and generators with a look at designing and laying out equipment rooms for engines and generators.

Until then, keep them pumping!

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Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.