Load Bank Sizing Calculations – Part One

Today, we will start explaining what the Load Bank is and How to Size it correctly.

Introduction to Load Banks

What is a Load Bank?
Any device which generates electricity can be called as a power source and it may require testing from time to another for the purposes of product development, production line testing, product demonstration, commissioning of a new installation or for periodic service, maintenance or trouble shooting.
To test a power source, we need to use a load bank which can be simply defined as follows:
A Load Bank is an electrical device which acts as an electrical load to an electrical power source for the purposes of testing, supporting or protecting this power source.
A load bank test ensures that your power source will run property when it’s needed so that you can depend on its proper operation during an actual emergency situation.
We can control, measure and record the electrical load provided by a load bank for determining performance characteristics of any power source like:
  • Engine generators,
  • UPS,
  • Batteries,
  • Power supplies,
  • Fuel cells,
  • Solar cells,
  • Wind turbines,
  • Hydro-generators, etc.

Why we don’t use the actual facility loads to test the power source?
If the performance of the power source like a generator is not known, it is better to evaluate the performance with a load bank than with actual facility loads for the following reasons:
  1. A load bank provides a stable, known, controllable and relatively “harmless” load for determining performance characteristics of a power source.
  2. While the actual facility loads is variable from time to another and not fully controllable which may be overload the power source or being under the minimum running load of this power source “Wet Stacking condition” which may cause harmful effects on the power source in both cases.
  3. Also, the load bank can test the power source at its peak specified kW output for a specified amount of time while actual facility loads may not do the same.

Wet Stacking Problem

What is wet stacking?

The term ‘Wet Stacking’ originates from the fact that fuel is still ‘wet’ in the exhaust system when in operation. Wet Stacking occurs in diesel gen-set engines operating under no or lightly loaded conditions (less than 30% of rated load) for extended periods. The low running load means the engine does not reach optimal operating temperature for peak performance leaving unburnt fuel and carbon deposits within the engine.

Consequences of wet stacking:

Wet stacking conditions cause unburnt fuel deposits to collect on the combustion chamber, injector nozzles, piston rings, turbocharger, and exhaust system. Some issues associated with wet stacking include:
  • Increased emissions: In areas of stringent emissions standards, the effects of wet stacking can cause reduced efficiency of the engine and excessive pollution.
  • Increased cost: Without maintenance, wet stacking can reduce the working life of a diesel generator by years increasing potential capital expenditure for replacement units.
  • Reduced Power: Wet stacking reduces efficiency and power of the engine and may not be able to cope with power demands required causing a stall or failure.
  • Maintenance costs: A correctly loaded gen-set will need much less maintenance than a wet stacked unit.

National fire protection agency (NFPA) standards for wet stacking

The NFPA 110 has guidelines to reduce the effects of wet stacking on backup power systems. The NFPA 110 8.4.2 guidelines in Level 1 and 2 applications require exercising the unit, at least monthly, for 30 minutes under either of two methods:

  • Method#1: Loading that maintains the minimum exhaust gas temperatures, as recommended by the manufacturer.
  • Method#2: Under operating temperature conditions and at not less than 30 percent of the Emergency power supply EPS nameplate kW rating

How to prevent wet stacking?

The best way to mitigate the effects of wet stacking is to apply additional load to the gen-set, increasing operating temperatures to burn off accumulated unburnt fuel and carbon. The amount of minimum load varies per engine manufacturer, but the typical range is 30% to 50% of the kilowatt rating. It is proven diesel engines operate more efficiently in the 70% to 80% range of rated kW output.

Load banks are the best practical means of applying load to gen-sets and preventing wet stacking.

Load Bank Applications

Load Banks can be used for several purposes, including:

1- Manufacturing Testing

a- Generator Testing
Manufacturers of standby power generators commonly use load banks to test and tune newly assembled units. A test engineer uses the load bank to apply a discrete, selectable electrical load to the generator and measure the response stability and endurance. During this process, the test engineer calibrates and adjusts the generator performance to ensure it meets the intended product specifications and tolerances.

b- Engine Testing
Manufacturers of small gas engines also use load banks to test newly manufactured engines under various mechanical loads. This is accomplished by connecting the engine to the load bank via a device called a dynamometer. Again, engine performance is monitored and, if necessary, adjustments are made to ensure the product performs within intended specifications.

c- Demonstration
Load banks are often used as tool for demonstrating to buyers and users of a new generator set that the unit meets the performance specifications outlined in the purchase contract “Customer witness testing”.

2- Field operation and Testing

a- Periodic maintenance
Generators installed in the field must be periodically tested and exercised to ensure that it will operate as intended when a power outage actually occurs. Service groups use load banks to apply a load that mimics the load of the facility the generator is protecting. Any problems with the generator can then be identified and rectified in a non-critical environment.
Load Banks can be used to satisfy the requirements of the National Fire Protection Association (NFPA) for emergency stand-by power systems. These requirements concern acceptance testing and periodic on-site testing and maintenance of emergency and stand-by power systems.

b- Elimination of ‘Wet Stacking’
Load banks may be installed in a diesel powered generator set to apply the additional load required to ensure the engine fully consumes the fuel in the combustion process. If a diesel engine is not operated under adequate load, unconsumed fuel will collect in the engine’s exhaust stack and form an oily coating.

Applicable standards for Using load banks with emergency power generating systems

It is important for consulting engineers and facility managers to understand the specific code requirements for installation, performance, and testing of emergency power systems.
There are many codes and standards have requirements to use load banks with emergency power generating systems like NFPA, NEC, and ISO standards. These codes are further described as follows:

The National Fire Protection Association (NFPA)

NFPA 37: Standards for the Installation and Use of Stationary Combustion Engines and Turbines
This standard establishes criteria for minimizing the hazards of fire during the installation and operation of stationary combustion engines and gas turbines. 
This standard provides minimum fire safety requirements for the installation and operation of stationary combustion engines and gas turbines.

NFPA 70B: Recommended Practice for Electrical Equipment Maintenance
This recommended practice applies to preventive maintenance for electrical, electronic, and communication systems and equipment and is not intended to duplicate or supersede instructions that manufacturers normally provide. Systems and equipment covered are typical of those installed in industrial plants, institutional and commercial buildings, and large multifamily residential complexes. 
Consumer appliances and equipment intended primarily for use in the home are not included. 
Also, the purpose of this recommended practice is to reduce hazards to life and property that can result from failure or malfunction of industrial-type electrical systems and equipment.

NFPA 99 - Healthcare Facilities Code  
Healthcare facilities must exercise Emergency and Standby Power Systems under load and operating temperature conditions for at least 30 minutes at intervals of not more than 30 days. 

NFPA 101 – Life Safety Code
NFPA 101, Article requires that emergency generators be installed, tested, and maintained in accordance with NFPA 110. Provisions dealing with maintenance and testing of emergency generators can be found in NFPA99, Article 4.4, which deals with issues such as:
  • Test criteria
  • Test conditions
  • Test personnel
  • Maintaining and testing circuitry
  • Battery maintenance. 

NFPA 110 - Standard for Emergency Generator Systems

This standard sets safety standards to protect commercial building occupants by making sure generator-powered backup lighting will operate as expected. Monthly testing is performed on generators whose failure could result in injury or death. If a generator fails a monthly test, it should be tested annually for two continuous hours using a load bank. Under the continuous test, the generator should be operated at 25 percent of the nameplate kilowatt rating for 30 minutes, at 50 percent of the kilowatt rating for 30 minutes and at 75 percent of the kilowatt rating for 60 minutes. 

The National Electrical Code (NEC) 

NEC Article 700 - Emergency Systems
Emergency systems are required to receive an operating permit as determined by the local code enforcement authority. This requirement is a lifeline for occupants, ensuring that lighting and life safety loads take priority over other building loads. Should the main electrical power supply fail, backup emergency power for life safety systems must be available within 10 seconds. 

NEC Article 701 - Legally Required Standby Systems
It requires standby power to be available to legally required systems within 60 seconds of power loss. While NEC 700 is designed to ensure that people can exit a building, NEC 701 responds to the needs of firefighters and other personnel responding to an emergency. 

NEC Article 702 - Optional Standby Systems
Applies to situations where standby generators are optional. In these cases, the systems may be put in place to protect against economic loss or business interruptions. For instance, data centers may elect to install backup power because an outage could result in large revenue losses. 

NEC Article 708 - Critical Operations Power Systems
This article was developed following the 9/11 World Trade Center, Hurricane Katrina, and Hurricane Rita disasters in the United States. It requires a commissioning plan for on-site backup generation, baseline testing, and periodic witness testing, as well as a documented preventive maintenance program, written test records, and a method for testing all critical power systems for maximum anticipated load conditions. 

ISO8528 (BS 7698) - Reciprocating internal combustion engine driven alternating current generating sets

ISO 8528 (BS 7698) Part 6 is the standard for testing engine-driven generating sets. It details general test requirements and defines functional and acceptance load bank testing. Functional tests must always be performed, and usually occur at the manufacturer’s test cell. 

ISO 8528 (BS 7698) Part 6 defines three performance classes - G1, G2, and G3. An additional class, G4, is reserved for performance criteria which are agreed upon between the supplier and the buyer. Each performance class has different criteria depending on the characteristics of the generator set: 

Table-1 below lists the performance class and their criteria and application examples.

The performance class relevant for the application must be followed to be within the standard and achieve maximum performance.


Generator’s Performance Class

Performance Class Criteria

Application Examples



Required for applications where the connected loads are such that only basic parameters of voltage & frequency need to be specified.
General purpose
applications Lighting & electrical loads


Required for applications where the demand on voltage is very much the same as for the commercial power system.
When load changes, temporary deviations in voltage and frequency are acceptable.
Lighting systems, pumps, fans and hoists

Required for applications where the connected equipment may make severe demands on voltage and frequency and waveforms.

Required for applications where the demands on voltage, frequency, and waveform are extremely severe.
Data-processing &
Computer equipment

Table-1: Generator’s Performance Class

Table-2 shows the acceptance (dip) and rejection (overshoot) parameters identified by ISO 8528-5. Class G4 is reserved for limits that are unique and must be agreed upon by the manufacturer and customer. ISO 8528-5 also sets limits on recovery times for each class and identifies how recovery time is measured.


Frequency %





Frequency %





Voltage %





Voltage %





Time Seconds





Table-2: ISO 8528-5 acceptance (dip) and rejection (overshoot) Limits


AMC:  Agreed between Manufacturer and Customer

In the next article, we will explain the different types of Load Banks. So, please keep following. 



  2. Great job Eng. Ali as usual. Thanks for such an interesting topic and looking forward to the next article.
    God Bless You