### Generators Sizing Calculations – Part Fourteen

Today, we will explain Rules Of Thumb for Generators Sizing Calculations for Existing and New Installations.

 Fifth: Rules Of Thumb For Generators Sizing Calculations

 First: Rules Of Thumb For Existing Installations

 In article “ We explain how to calculate the peak load of an existing installation by one of the following methods:   Measurement Method, Billing History Method, Load Summation Method, NEC Load Calculation Methods, Estimate based on square footage Method.     Then we can size the generator(s) of an existing installation based on the calculated/estimated peak load and based on the next steps explained in the following articles:     Also, We can size the generator(s) of an existing installation for each of the above methods by using Rules Of Thumb as will be explained in below.

 1- Measurement Method   Use a clamp-on Amp meter or power analyzer to measure facility load levels. Clamp each leg separately and take the measurement during peak usage levels.   1.A- For 240V 1ø Applications:   To determine peak usage in kW, add the highest Amp readings from the two legs then multiply by 120 and divide by 1,000.   Step#1: Peak Amps = (L1 + L2) Step#2: Peak kW = (L1 + L2)120 / 1000 Step#3- Rules Of Thumb: Size the generator 10 to 20% larger than the peak measured load.   1.B- For 3ø Applications:   Add the peak Amp readings from all three legs and divide by 3 to determine peak Amps. Multiply peak Amps by volts, multiply the result by 1.732 (square root of 3), then divide by 1000 to convert Amps to kW.   Step#1: Peak Amps = (L1 + L2 + L3) / 3 Step#2: Peak kW = [(Peak Amps x Volts) x 1.732] / 1000    (Assumes power factor of 1.0) Step#3- Rules Of Thumb: Size the generator 20 to 25% larger than the peak measured load.

 2- Billing History Method   Many commercial customers have a utility rate structure that has a peak demand charge. Using a year's worth of electric bills, find the Peak Demand. Then Verify motor and UPS load compatibility.   Step#1: Peak Demand = largest peak demand from Billing History Step#2- Rules Of Thumb: Size the generator 25% larger than the largest peak demand

Step#1: Enter running kW for all motor loads (except the largest) expected to run during peak load levels into Table-3. Refer to Table-1 for typical motor load sizes and electrical requirements.

 Table-3 Motor Load Table (refer to Table 1) Device HP RA LRA kW Running (= HP) Starting kW

Notes:

For HP < 7.5; starting kW = HP x 3
For HP > 7.5; starting kW = HP x 2
Starting kW for loads with no listed HP; calculate HP based on running amps in Table-4 below.

 Table-4: How To Calculate kW for loads with no listed HP 120 V 1ø Amps x 120/1000 = kW 240 V 1ø Amps x 240/1000 = kW 208 V 3ø (Amps x 208 x 1.732 x PF) /1000 = kW 240 V 3ø (Amps x 240 x 1.732 x PF) /1000 = kW 480 V 3ø (Amps x 480 x 1.732 x PF) /1000 = kW PF is the load power factor - Typical application power factor is 0.95.

Step#2: Enter kW for all non-motor loads expected to run during peak load levels into Table-5. Refer to Table-2 for typical residential loads and rules of thumb.

 Table-5 Non-Motor Load Table (refer to Table-2) Device Amps Kw

From Table-3, the Motor load running total - minus largest motor = X kW
From Table-5, Non-motor load total = Y kW
From Table-3, Starting load from largest cycling motor = Z kW
Total electrical loads = X+Y+Z kW

Step#4- Rules Of Thumb: Select generator:

Step#5- Rules Of Thumb: Select Generator Rating from Standard Sizes/Manufacturers Catalogs

Step#6- Rules Of Thumb: Confirm that voltage dip is within acceptable limits by comparing motor LRA to generator surge capability (see Table-3). If not, select the next higher standard generator size.

Step#7- Rules Of Thumb: Confirm UPS compatibility according to UPS type as in Table-7 in below.

 UPS Type Generator Sizing Recommendation Passive (also referenced as standby or off-line) and Line-Interactive Limit the total UPS loading to 15% - 20% of the generator capacity. Double-Conversion Single phase models Limit the total UPS loading to 25% of the generator capacity. Single phase Minuteman UPS models Limit the total UPS loading to 50% of the generator capacity. Three phase models without filters (current distortion > 30%) Limit the UPS loading to 35% of the generator capacity. Three phase models with filters (current distortion < 10%) Limit the UPS loading to 80% of the generator capacity.

Table-7: UPS compatibility according to UPS type

If UPS compatibility not verified, select the next higher standard generator size.

You can know the UPS type, if you know the supplier name from the below table:

 Supplier(s) Passive (Standby) Line-Interactive Double-Conversion Minuteman UPS Enspire Enterprise Plus Endeavor APC Back-UPS Series Smart-UPS Series Symmetra Series Liebert PowerSure PST & PSP PowerSure PSA & PSI UPStation & Nfinity Powerware 3000 series 5000 series 9000 series
Note: Ferrups and Delta-Conversion UPS technologies not included in discussion

 4- NEC Load Calculation Methods   No Rules Of Thumbs will be used just follow the NEC rules explained in article “

Step#1- Rules of Thumbs: Calculate the square footage of the building. (For typical floors multiply the square footage of one floor by number of typical floors).

Step#2- Rules of Thumbs: Calculate the estimated KW by multiply the total square footage of the building by the application factor indicated in below table.

 Applications Estimated kW Fast food, convenience stores, restaurants, grocery stores applications 50 kW + 10 watts/sq. ft. Other commercial applications 30 kW + 5 watts/sq. ft.

 Facility Power Usage Watts/sq ft kWh/ft2/yr Schools   Class Rooms Locker Rooms, Auditoriums, Halls and Corridors 4-5 average (43.06-53.82 W/m2 (Total) 5-6 (53.87-64.58) 2-3 (21.53-32.29) 20 Watts per running foot (65.62 W/m) 11 to 17 (1.02 to 1.58 kWh/m2/yr) Shopping Centers Stores, Large Department and Specialty Stores Show Windows 5 average (Total)   5-6 (53.87-64.58) 500 Watts per running foot (1640.5 W/m) 28 to 34 (2.6-3.2) Office Buildings Private and General Offices Professional Offices Dentist, Drafting Rooms, etc. 5-6 average 4 (43.06) 6-7 (64.58-75.35) 7 (75.35) 28 to 34 (2.6-3.2) Hotels and Motels   Lounge Rooms Dining Rooms Exhibition Halls, Shops, Lobby, Kitchen 3-4 average (32.29-43.06) (Total) 2 (21.53) 3 (32.29) 4 (43.06)   3 (32.29) 12 to 17 (1.1-1.58) Hospitals   Lobby, Wards, Cafeterias   Private Rooms, Operating Rooms   Operating Tables: Major Surgeries Minor Surgeries 1.5 to 2.5 kW per bed average 3 Watts/sq ft (32.29 W/m2) 5 Watts/sq ft (53.82 W/m2)   3000 Watts each 1500 Watts each 8500 to 11400 kWh per bed per year Apartment Houses Lobby   Apartments   Small Appliances 2-3 kW per unit (Total) 2 Watts/sq ft (21.53 W/m2) 3 Watts/sq ft (32.29 W/m2) 1.5 kW/unit 11 to 17 (1.20 to 1.58)

Step#3- Rules Of Thumb: Select Generator Rating from Standard Sizes/Manufacturers Catalogs.

 Second: Rules Of Thumb For New Constructions

 We have two methods as Rules Of Thumb for generator sizing in New Constructions as follows:   By using conservative rules of thumb, By using Load Form,

1- By Using Conservative Rules Of Thumb

Here are some conservative rules of thumb for generator sizing include:

1. Oversize generator 20–25% for reserve capacity and for across the line motor starting.
3. Use 1/2 hp per kW for motor loads.
4. For variable frequency drives, – 100% oversize unless pulse–width–modulated oversize the generator by at least 40%.
5. For UPS systems, oversize the generator by 40% for 6 pulse and 15% for 6 pulse with input filters or 12 pulse.
6. Always start the largest motor first when stepping loads. For basic sizing of a generator system,

The steps of generator sizing by this method will include the following steps:

• Step#1: Calculate Running Amperes,
• Step#2: Calculating Starting Amperes Using 1.25 Multiplier,
• Step#3: Selecting kVA of Generator.

Example#1:

Size the generator to supply the following loads:
• 200 hp motor,
• 100 hp motor,
• 60 hp motor,
The system voltage is 480 V and PF is 0.77.

Solution:

Step#1: Calculate Running Amperes

 Motor loads Load in KW (from rule#3 above) Running Amperes 200 hp motor 100 KW = (100x1000)/(1.732x480x0.77)= 156 A 100 hp motor 50 KW = (50x1000)/(1.732x480x0.77)= 78 A 60 hp motor 30 KW = (30x1000)/(1.732x480x0.77)= 48 A Normal Loads Lighting load = 68 A Miscellaneous loads = 95 A Total Running Amperes = 445 A

Step#2: Calculating Starting Amperes Using 1.25 Multiplier

 Motor loads Running Amperes Starting Amperes 200 hp motor 156 A = 156 x 1.25 = 195 A 100 hp motor 78 A = 78x 1.25 = 98 A 60 hp motor 48 A = 48 x 1.25 = 60 A Normal Loads Lighting load = 68 A Miscellaneous loads = 95 A Total Starting Amperes = 516 A

Step 3: Selecting kVA of Generator

 Running kVA = (445 A x 480 V x 1.732)/1000 = 370 kVA Starting kVA = (516 A x 480 V x 1.732)/1000 = 428 KVA Generator must have a minimum starting capability of 428 kVA and minimum running capability of 370 kVA.

# 2- By Using the Load Form

The load form is prepared by the generator manufactures to help and facilitate the process of generator sizing. A copy of the load form is in below image.

The steps of generator sizing by using the load form will include the following steps:

# Step#1: Write down all resistive loads on column 12 and 13, KW = KVA so entries are the same on the rows of resistive loads.

Note: Resistive loads consist of incandescent lights, water heaters, electric heaters, stoves, and electric furnaces. Power factor = 1, KW = KVA. Running and starting KW are the same.

Step#2: List all motor loads with the worst case (largest load applied last) in order on the load form.

Note: Motors are inductive loads with KVA always larger than KW. The power factor running usually is between 0.6 and 0.85.

Step#3: List the following items for each motor load (If the motor is existing, use nameplate data. If this is a new installation, use Table-2):

• Column-1: HP =
• Column-2: Starting Code Letter =
• Column-3: nos. of phases =
• Column-4: Volts =
• Column-5: reduced voltage motor starting =
• Column-6: Start KVA =
• Starting power factor =
• Column-7: Start KW = start KVA x start P.F. =
• Column-8: Run KVA =
• Column-9: Run KW =
Table-2

Special cases for step#3:

A- Only locked rotor amps are known:

To find starting KVA if only locked rotor amps are known;
• For 1-phase: KVA start = (locked rotor amps) (voltage)
• For 3-phase: KVA start = √3 (locked rotor amps) (voltage)

B- For submersible water pumps only:

Submersible water pump motors have a higher normal locked rotor code letter and running current than Onan tables shown for average motors. The running current is higher than NEC 430-148 when run at their service factor amps. Use the following two tables:

Step#4: calculate the values of columns 10, 11, 12 & 13 as follows:

• Max. KVA - Column 10: Add column 6 to previous line column 12.
• Max. KW - Column 11: Add column 7 to previous line column 13.
• Continuous KVA - Column 12: Add column 8 to previous line column 12.
• Continuous KW - Column 13: Add column 9 to previous line column 13.

Step#5: Look at the largest numbers in columns 10, 11, 12 and 13. Generator output must be greater than these numbers.

Note:
These calculations do not take into account voltage drop on generator. Consult manufacturer at this point.

Example#2:

For the following Loads, calculate the suitable generator size.

• Motor: 2HP, 230V, 1-phase, code J
• Resistance Loads: 4.5 KW, 230V, 1-phase

Solution:

# Step#1: Write down all resistive loads on column 12 and 13, KW = KVA so entries are the same on the rows of resistive loads.

Step#2: List all motor loads with the worst case (largest load applied last) in order on the load form,

Step#3: List the following items for each motor load (use Table-2):

• Column-1: HP = 2
• Column-2: Starting Code Letter = code J
• Column-3: nos. of phases = 1
• Column-4: Volts = 230V
• Column-5: reduced voltage motor starting = Not Applicable
• Column-6: Start KVA = 16.4 KVA
• Starting power factor = 0.8
• Column-7: Start KW = start KVA x start P.F. = 16.4 x 0.8 = 13.12 KW
• Column-8: Run KVA = 2.47 KVA
• Column-9: Run KW = 2 KW

Step#4: calculate the values of columns 10, 11, 12 & 13

• Max. KVA - Column 10: Add column 6 to previous line column 12 = 20.9 KVA
• Max. KW - Column 11: Add column 7 to previous line column 13 = 17.62 KW
• Continuous KVA - Column 12: Add column 8 to previous line column 12 = 6.97 KVA
• Continuous KW - Column 13: Add column 9 to previous line column 13 = 6.5 KW

Step#5: Look at the largest numbers in columns 10, 11, 12 and 13. Generator output must be greater than these numbers.

 Column 10 Column 11 Column 12 Column 13 Max. KVA Max. KW Cont. KVA Cont. KW 20.9 17.62 6.97 6.5

In this case, a 20 KW generator would handle the continuous KVA and KW, and maximum KVA and KW.

We can use for example, Onan generator 20ES which has the following ratings:

 Max. KVA Max. KW Cont. KVA Cont. KW 42 22 25 20

In the next article, we will explain the special cases for Generators Sizing Calculations. So, please keep following.

The previous and related articles are listed in the below table: