### Load Bank Sizing Calculations – Part Six

Today, we will explain the Steps for Selection/Calculation of the Proper Load Bank and Standard Load Bank Testing Procedure.

 The (14) Steps for Selection/Calculation of the Proper Load Bank

There are several factors involved in the selection and sizing of the proper load bank used for testing a power source. Inadequately sized load bank may shorten its operating life or cause overloading failures.
These factors can be used to determine if the load bank can handle its operating load and conditions or not. The factors are indicated in the selection/calculation steps for the proper load bank.

Step#1: Assign the Proper Load Bank Type Based on the Type of the Application

 Application Generator Capacity Load Bank Type Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz) < 100 kVA Resistive Only Small Generators and UPS Systems (480VAC, 60Hz) < 200 kVA Resistive Only Single Units (480VAC, 60 Hz) >200 kVA combined Single Units (480/5kV/15kV, 60 Hz) 1MVA – 6MVA combined Multiple Units Combined (480/5kV/15kV, 60 Hz) >6 MVA combined Small UPS and Battery systems (DC voltage) Resistive Only Rotary UPS , large UPS and Battery systems (DC voltage) - Resistive Only - combined

Step#2: Assign the Load Bank Type Based on the Power Source Voltage and Frequency

The following features of the power source affect the selection of the load bank type:
• AC or DC voltage,
• Single or three phases,
• Voltage Rating.

Note:
The operating voltage of the load bank must match the voltage of the Generator or power source.

 Applied Voltage Type Voltage Rating Load Bank Type Low voltage AC systems 120/240v, single-phase, 2 or 3 wire - Resistive - Reactive - Combined Low voltage AC systems 208-240/416-480/575-600v , three-phase, 3 or 4 wire - Resistive - Reactive - Combined Medium voltage AC systems 5kV or 15kV class - Low voltage (LV) load bank with a step-down transformer - Containerized Solution (Integrated Load Bank and Transformer Package) - Direct Connect  Solution DC voltage systems 12/24/32v, 125/250v, or 350/700v - Resistive - Combined

Sometimes, you need to use the same load bank to test different power sources with different voltages and frequencies, so you need to select the load bank in this case to have the following features:
• Single or multiple Voltages,
• Single or multiple Frequencies.

 Voltage Range Selections Single Voltage Designed for a specific voltage, typical of exercise load banks up to 15KV. Multiple Voltages Capable of dual or quad voltages; (Typical voltages—60HZ: 120/240, 208-240, 416-480; 50HZ: 190-220, 380-420; DC: 12-60, 125, 250) to 15KV.

 Frequency Range Selections Single Frequency Designed for a specific frequency. Multiple Frequencies Capable of multiple frequency or AC/DC operation (Typical frequencies: 50-60-400HZ, DC)

Step#4: Assign the Load Bank Type Based on the Power Source Capacity

 Generator Capacity Load Bank Type Application < 100 kVA Resistive Only Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz) < 200 kVA Resistive Only Small Generators and UPS Systems (480VAC, 60Hz) >200 kVA combined Single Units (480VAC, 60 Hz) 1MVA – 6MVA combined Single Units (480/5kV/15kV, 60 Hz) >6 MVA combined Multiple Units Combined (480/5kV/15kV, 60 Hz)

Step#5: Calculate the Load Bank Capacity Based on the Power Source Capacity and Load Bank Type

Step#6: Assign the Required No. of Steps and Load Step Resolution

Step#7: Derate the Load Bank Capacity from Step#5 In Case Of Testing Power Sources with Lower Voltage Rating Than That of the Load Bank

Load banks are designed to provide a specific capacity at a rated voltage. They cannot be operated at a voltage higher than their rating without risking damage to the load bank. However, the load bank can be operated at lower voltages but its KW capacity must be derated. The derating is calculated as follows:

D= (Applied Voltage)2/(Rated Voltage)2

Where D is the Derating Factor

Derated Capacity = D X Rated Capacity KW

Example#1:

Can a 500 KW, 480 VAC Load bank fully load test a generator rated at 300 KW, 380 VAC?

Solution:

First: calculate the Derating Factor
D= (Applied Voltage)2/(Rated Voltage)2 = 3802/4802 = 0.627

Second: calculate the Derated Capacity
Derated Capacity = D X Rated Capacity KW = 0.627 x 500 = 313 KW

Yes, this load bank can test the 300 KW generator because it provides 313 KW load at 380 V which is higher than the 300 KW generator rating.

Also, the derating is affecting the value of load bank steps in the same way.

Example#2:

If we have a load bank rated at 80 KW, 400 V with the following steps 5, 10, 10, 25, 30 KW. Calculate:
The derated capacity of the generator,
The derated steps KW.
When the load bank used to test a generator rated at 380 V.

Solution:

First: calculate the Derating Factor
D= (Applied Voltage)2/(Rated Voltage)2 = 3802/4002 = 0.9

Second: calculate the Derated Capacity
Derated Capacity = D X Rated Capacity KW = 0.9 x 80 = 72 KW

Third: calculate The derated steps KW as follows

 D = 0.9 Rated steps KW 5 10 10 25 30 Derated steps KW 4.5 9 9 22.5 27

Step#8: Derate the Load Bank Capacity Based on the Altitude and Ambient Temperature of the Installation Location

 Altitude and ambient temperature condition Required Derating More than 3300 feet Altitude (The thinner air at higher altitudes effect the proper cooling of the unit) Derate the load bank capacity by (10-20%) Ambient temperatures below -20°F or above 120°F Use special  type “High Ambient” or “Arctic” load bank designs

Step#9: Assign the Load Bank Type Based On the Available Installation Form

 Load Bank Installation Form Load Bank Type Radiator Mount Resistive Portable (DC or AC) Resistive Mobile/Static Resistive / Capacitive / Inductive / Combined Static Only Resistive Container Resistive / Combined Medium Voltage Resistive / Combined

Step#10: Assign the Cooling Methods for Load Bank based on Load Bank Capacity and Space Limitations

 Method Of Cooling Application Natural Convection Air load bank type In case of Portable load banks rated up to a 20 KW maximum. Integral Fan Forced Air load bank type Horizontal Discharge: Horizontal discharge units offer a low profile. By directing exhaust flows downward, their louvers prevent hot air flow from discharging directly towards nearby personnel Large load banks (portable or stationary), indoor or outdoor, where there is no space limitations. Vertical Discharge: Vertical discharge units direct hot air upward, away from personnel. They offer a smaller footprint than comparable horizontal discharge units. Radiator Airflow load bank mounted on the generator set’s radiator In case of space limitations; Space not sufficient to install an Integral Fan Forced Air load bank type Water Flow load bank They used in case of indoor installations in controlled environments where an outdoor air-cooled unit would be impossible to install due to space or noise restrictions.

Step#11: Assign the Method of Control (Mechanical or Digital / Local or Remote/ Manual or Load Shed Control) For the Required Load Bank

Step#12: Assign Blower Power Source (Internal Or External)

 Blower Power Source Rating Internally from load bank bus bars The voltage rating of the blower is usually specified the same as the load bank External power source which is the preferred method which will allow the load bank blower motors to operate if main voltage supply is lost.

Step#13: Assign Enclosure Type for the Required Load Bank Based On Installation Location / Space Limitations

 Installation Location Enclosure Type NEMA Definition Indoor the load bank can be enclosed in a NEMA-1 cabinet Enclosures constructed for indoor use; to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt outdoor the load bank can be enclosed in a NEMA-3R cabinet Enclosures constructed for indoor or outdoor use; to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet and snow; and that will be undamaged by external formation of ice on the enclosure

Step#14: Assign the Required Load and Supply Connectors

 Load And Supply Connectors Type Application Cable Set (Cam-Lok plugs, lug style connectors or via bare braded wire ends) Used with portable units Terminal Block Used with stationary and duct mounted units Plugs (To match commercial wiring devices, aircraft plugs, MIL-SPEC plugs, cam-lock connections) Used with Wide range of load bank application