Load Bank Sizing Calculations – Part Four

Today, we will explain the Load Bank components and its basic circuits.

Load Bank Components

Basically any low voltage load bank will be consists of the following basic components:

  1. Enclosure,
  2. Load elements,
  3. Controls & Instrumentation,
  4. Cooling system,
  5. System Protections,
  6. Load and supply Connectors.


Load banks are made in several form factors, depending upon the application and the environment in which it will be installed and as we explained before in Article “Load Bank Sizing Calculations – Part Three” that the enclosure specification will be determined as per site conditions to:

A- Indoor
If the installation is indoors, the load bank can be enclosed in a NEMA-1 cabinet.

B- Outdoor
For outdoors installation, a NEMA-3R enclosure is typically used.

2- Load Elements

As we explained before in Article “Load Bank Sizing Calculations – Part Three” That the Load elements can be one of the following types:

  1. Resistive Load Element,
  2. Reactive Load Element (inductive or capacitive),
  3. Combined (resistive/reactive) Load Element.

3- Controls & Instrumentation

Also, in Article “Load Bank Sizing Calculations – Part Three” We explained that Load bank control can be classified according to type of control to:

A- Mechanical Control:

Decade switch local control panel

The load bank will be controlled via Physical toggle switches, pilot relays, decade switches, indicator lights and alarms. The below table summarize the use of each mechanical control type.

Control Type
Toggle Switches
Basic manual switch for applying and rejecting load locally.
Simple Load Control
Decade Switches
Basic switches similar to toggle switches but provides synchronous load changes.
Synchronous load changes.
Digital Toggle

Toggle switches with the features of electrical control including synchronous load step changes and digital instrumentation.
Synchronous load changes, digital instrumentation.

B- Electronic Control (Digital Control):

Electronic load banks are programmable and can be employed to test all sizes of generator systems in any given industry. They use a Programmable Logic Controller (PLC) to translate digital commands into mechanical switching. The below table summarize the use of each Electronic Control type.

Control Type
Remote Control
(Figure 4)

Load control and instrumentation that can be viewed on a hand-held control.
No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, variable power factor testing.
PC Software
Feature rich control for testing that requires detailed instrumentation and reporting.
No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, data capture, automatic testing, testing to predetermined standards.
BMS Integration
Integrate the load banks into an existing BMS system through PLC.

No pre-calculations, synchronous load changes, digital instrumentation, graphical display, networking, voltage correction, data capture, automatic testing, testing to predetermined standards, customized test parameters.

Also, we explained in the same article that Load bank control can be classified according to operating mode to:

A- Automatic Operating Mode.

B- Manual Operating Mode:

Manual operating mode can be via local and remote manual control:

  • Local control:  Control panel self-contained in Load Bank. The local control is usually a mechanical control type.
  • Remote control: Control panel placed in a location away from the physical load bank. The remote control is usually an Electronic Control (Digital Control) type.

The Hand-held Remote Control

4- Cooling System

In article, Load Bank Sizing Calculations – Part Three” we explained that Cooling methods for Load elements in the load bank can be: 

  1. Natural Convection Air,
  2. Integral Fan Forced Air: it consist of an aluminum fan blade (shrouded for high efficiency) directly driven or belt driven by a motor. The fan motor is energized by a control contactor and protected by a circuit breaker.
  3. Radiator Airflow, or
  4. Water Flow.

5- System Protections

It is advisable to protect the load bank from the potential hazards inherent in high voltage applications or from the malfunction operation. Load banks employ protections specifically for this purpose as follows:

A- Overload and Short Circuit Protection: 
  • Load banks must eliminate the dangers of short circuit through by using of branch circuit fuse protection of the load elements. Per NEC 110- 10, protective devices shall clear a fault without “extensive damage” to the circuit components.  Usually, a Load Bank is divided into branch circuits of not more than 50 KW each.
  • If the input load voltage is over the safe threshold value, the system unloads automatically and gives an alarm.

B- Malfunction Detection System:

Load Banks are equipped with a malfunction detection system which provides automatic load disconnect and alarm on sensing of an abnormal operating condition. The system includes:

  • Cooling air intake temperature switch:  set at 120°F, to sense recirculation of load bank hot exhaust air or an ambient temperature above the rating of the load bank,
  • Cooling air exhaust temperature switch: set 75°F above maximum rise at maximum ambient, to sense restricted cooling airflow, recirculating air, loss of airflow,
  • Air pressure switch: to sense for positive cooling airflow
  • Overheating Protection: temperature is over the safe threshold value, system unload automatically and give an alarm.
  • Smoke Fog Protection: If there is smoke fog in the container, system cut off load automatically and give an alarm.
  • Fan Delivery Protection: insufficient air volume with any fan 2450m³/h, system unload automatically and give an alarm.
  • Overload protection with fan: when any fan cannot normally work, the load bank can unload automatically and give an alarm.
  • Emergency Stop: manually press the Emergency Stop switch on the panel, then load bank is in locked state, and cannot do any operation.
  • Protection Switch: some protection with switch, when misinformation or in special need, user can close the corresponding protection function temporarily.
  • Safety Monitor: through indicator lights on main interface, user can monitor system state. When system abnormally stop and protect, software interface prompt the cause immediately.

6- Load and Supply Connectors

Power sources to be tested can be connected to the load bank in several ways as follows:

  • Cable Set: for Typical of portable units, Cable sets can be connected to the load bank with Cam-Lok plugs, lug style connectors or via bare braded wire ends.
Cam-Lok plug

wire ends

  • Terminal Block: Typical of stationary and duct mounted units, Power terminals for connection of building cables.
  • Plugs: To match commercial wiring devices, aircraft plugs, MIL-SPEC plugs, cam-lock connections.

MIL-SPEC plugs

Essential Load Bank Circuits

Fig-1: Load Bank Control Circuits

Fig-2: One Line Control Circuit Diagram

In any load bank, there are Separate circuits used to operate the load elements, the blower system, and the devices that control them. Figure-1&2 provides a simplified one-line diagram of the relationship of these circuits. The load bank essential circuits are:

  1. Control Circuit,
  2. Cooling Circuit,
  3. Load Element Circuit.

1- Control Circuit

Example Load Bank Control Circuit

The main functions of Basic Control Circuit in any load bank are as follows:

  • It includes a master on-off switch as well as switches that control the cooling system and load elements. The load elements are often switched individually using dedicated switches; this enable operators to apply and vary load in a stepwise fashion.
  • It provides power and signaling for one or more over-temperature sensors and air-fail safety devices. The former are designed to sense overheating in the load bank, regardless of cause. The latter are switches that close only when they sense air flowing past the load elements; if a switch remains open, power cannot flow to one or more load elements, preventing an overheat condition.

A- Power source and protection for the Control circuit:

The control circuit requires a single-phase voltage source, typically 120-Volt at 60 hertz or 220-Volt at 50 hertz. This power can be derived from the power source for the load elements using any necessary step-down transformer, or by supplying power from an external single-phase source. If the load bank is configured for dual-voltage operation, a switch will be provided in the control circuit for user to selecting the proper voltage mode. A fuse on the line side of the incoming power source protects the control circuit.

B- Operation Sequence of the Control Circuit:

  1. When a control power on-off switch is closed, a control power indicator illuminates to show that power is present.
  2. After control power becomes available, an operator starts the cooling system using the blower start switch. After the blower provides proper airflow velocity, one or more internal differential air pressure switches detect airflow and close to place voltage on the load circuits.
  3. If proper airflow is not detected, the air switch will not close and an “air fail” indicator will illuminate.
  4. A Master Load Switch is commonly provided to control the overall function of the specific load elements or element groups switches. This switch can be used to safely drop all applied load, or as a convenient means for providing full or “bulk” load to power source.
  5. Load Step Switches engage individual elements to provide the desired amount of load.

C- Control Circuit Metering:

Digital metering is standard on many portable load banks, and power for meters is derived from the control circuit. The three-phase load voltage is measured directly on the main input bus. In applications served by digital control equipment, the measurements can be monitored, recorded, and subsequently evaluated.

Note For outdoor load banks:

If a load bank is designed for continuous outdoor use, one or more strip heaters are typically installed inside its enclosure to limit condensation and avoid freezing conditions. When heaters are present, an additional circuit is provided to power them. A temperature switch activates the circuit when temperature decreases below a minimum temperature set point, often 32°F.

2- Cooling Circuit

Example Cooling Circuit

The cooling circuit provides the required components for proper cooling fan operation. These components include:

  1. Motor starter contactor,
  2. Three-phase fuse protection,
  3. device to protect from motor overload.

A- Operation Sequence of the Cooling Circuit:

  1. When an operator closes the Blower Start Switch, the motor starter contactor closes to send voltage to the motor.
  2. If phase conductors are correctly installed, the motor will turn in the proper direction, creating positive airflow and closing the air fail switch.
  3. Upon closure, the air fail switch completes the circuit, enabling load application.
  4. If the phase conductors are incorrectly installed, the motor will turn the opposite direction creating a negative air differential that keeps the air switch open.
  5. If air flow slows sufficiently or ceases during load bank operation, the air fail switch opens to remove load from the circuit-under-test.

B- Power Source and Protection for the Cooling Circuit:

Power for the cooling circuit can be derived from either an external source or internally from load bank bus bars. Some manufactures offer an optional control power transformer for the cooling circuit, which converts three-phase blower power to 120 VAC, single phase. This arrangement eliminates the need to provide dedicated cooling circuit power to permanently installed load banks. Three-phase fuse protection is provided to protect the cooling circuit.

3- Load Element Circuit

Example Load Element Circuit

Load Element Circuits typically include:

  1. An input power buss: where the main voltage input is connected to it. Input power buss can be bus bars or quick-connect receptacles.
  2. Branch circuit fuse: for each load element group there will be fuses to limit current damage from phase-to-phase short circuit currents.
  3. Individual contactors: they are used for the load elements and on/off each load step.

Sequence of Operation for Load Element Circuit:

  1. The control circuit sends current through the individual load circuits to the coil of their respective contactors.
  2. After the coils are energized, the contactor will close to load each circuit.
  3. Operators typically apply load in stepwise fashion till reaching the desired load value.

The Relationship of the Control, Cooling, and Load Element Circuits

The relationship of the control, cooling, and load element circuits is reinforced by review of typical load bank shut down sequences as follows:

  • After a load test is complete, the control circuit is used to shut down the load bank.
  • Load is removed from the power source by placing switches for individual load steps in the “Off position. This removes control voltage from the coils, de-energizing contactors serving each load element.
  • The entire load system is then shut down using the Master Load Switch and operating a Blower Stop Switch.

  • Most load banks do not require a cool-down period prior to shut down. However, it is good practice to fist run blowers to remove residual heat.

In the next article, we will explain Power and control drawings of Load Banks. So, please keep following.

The previous and related articles are listed in below table:

Subject Of Pervious Article
What is a Load Bank?
Why we don’t use the actual facility loads to test the power source?
Wet Stacking Problem
Load Bank Applications
Applicable standards for Using load banks with emergency power generating systems

Types of Load Banks:
First: According to the Load Element Type

Second: According To Portability,
Third: According To Cooling Method,
Fourth: According To Method of Control,
Fifth: According To Operating Mode,
Sixth: According To Application,
Seventh: According to no. of Load Steps,
Eighth: According to Load Bank Voltage and Frequency.

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