Power Up: Tips on How to Pick the Right Size Generator and Safely Use It

Doosan Portable Power G25 generator

Working with electrically energized equipment like generators requires careful selection and safety training. Operators should size out a generator to best fit their application and set aside time for safety training without the distractions and limitations of a working jobsite. Taking time to ensure your employees understand the potential hazards of this type of equipment and how to exercise safe troubleshooting procedures is an investment in the future of your people and your business. Following Occupational Safety and Health Administration (OSHA) standards and National Fire Protection Agency (NFPA 70E) guidelines, as well as local regulations, can also assist in assuring safe working conditions.


Generators that are too large for an application are forced to operate below capacity and may experience engine problems as a result, including wet-stacking. When a generator operates a light load for an extended period of time, unburned fuel and soot floods into the exhaust system forming a thick, wet substance on the exhaust pipes or “stacks,” and the engine is said to be “wet-stacking.” Signs of a wet-stacking engine are black smoke from the exhaust, excessive oil consumption and poor performance.


An undersized generator will consistently fail to produce the appropriate output, resulting in machine shutdown and overloaded breakers. In addition to engine and alternator damage from overheating, an undersized generator can also damage the equipment it is intended to power. If the generator pulls too much horsepower from the engine, and in turn slows the engine speed, the voltage and frequency output may become unstable, which can damage the equipment connected to the generator.

Sizing a Non-Motorized Application

Once the equipment and applications have been identified, the voltage and amperage for each piece of equipment are needed. This information can typically be found on the equipment data plate. Knowing the voltage and amperage (amps) allows you to determine kilowatts, which dictates the amount of electrical power needed to operate the load. In North America, most portable/rental/mobile generators are rated in prime kVA rather than kW. To convert kW to kVA, divide by the power factor (PF). A single-phase application has a power factor of 1.0, and a three-phase application has a power factor of 0.8.

• volts x amps = watts/1,000 = kW/PF = kVA

If the generator will be powering more than one application at a time, the amperage for each piece of equipment should be added up to determine the total amperage and multiplied by the volts. For example, if you will be powering 10 light fixtures and each light requires 230V and 10 amps:

• 10 lights x 10 amps each = 100 amps

• 230 x 100 = 23,000/1,000 = 23 kW/1.0 = 23 kVA

Sizing a Motorized Application

It’s important to note that powering a load that operates by motor will require more power to start the electric motor than to keep it running. To size a generator for motor starting applications, it’s necessary to know the motor code, starter type and motor horsepower. There are several motor code and starter types. The most common is motor code G and “across the line” starter type. The common code and starter type can be used when the information is not readily available on the data plate. The motor code and starter type have associated values that are required to calculate the starting kVA (SkVA) for the application motor.

Check Connections

Doosan G240 Generator

When a mobile generator arrives on site, be sure to inspect the equipment per the manufacturer’s instructions, including load connections and electrical wiring. A generator operating at peak performance one day may be inoperable at the next jobsite after bouncing over potholes or traveling down a makeshift dirt road.

Electrical connections need to be secure to maintain a low resistance across the connection, ideally 0 ohms. When voltage flows through a loose connection, power is dissipated in the form of heat, which can burn the connection, scorch wiring and damage the generator. More importantly, any time a connection becomes loose it increases the potential for the energized circuit to come in contact with an unintended surface, which creates the risk of shock.

Electrical wiring should also be inspected before operating. Wires have the potential to rub through insulation during transportation. Exposed wires can charge a piece of sheet metal, energizing the frame of the generator and creating a potential shock hazard for anyone who comes in contact with the machine.

Safe Grounding

Stationary generators are grounded upon site installation, but mobile generators must be grounded upon arrival at each jobsite. Operators should follow all local electrical regulation codes for properly grounding equipment. Grounding a piece of equipment involves creating a path of least resistance to carry current traveling outside its intended path, to ground, typically through a copper rod or stud. If a fault occurs without grounding a generator, current flowing outside its intended path can create a potential shock hazard.

Manage Moisture

Doosan Portable Power G70

Generator equipment should be inspected for moisture between jobsites. When water enters electrical components, it can cause an electrical short circuit. When a generator experiences an electrical short, it can cause permanent damage to the generator but can also result in sparking, fire or arcing.

Inspecting a mobile generator for moisture between jobsites reduces those risks, but it also presents an opportunity for training. Generator doors should be closed during operation to prevent moisture from entering the unit in the form of rain or condensation buildup. Enclosures are sealed to prevent moisture penetration in wet environments.

Load Banking

For occasions when downtime between jobs is more than a day or two, there are additional actions that can be taken to improve generator performance. Mobile generators should be connected to a load bank between jobs to validate that the alternator is producing voltage and electrical current to its rated capacity. Load banking, as it’s often called, may also identify problems that could hinder generator performance at the next jobsite but can be resolved before leaving the shop. Operating a generator on a load bank cleans up any emissions issues that the engine may be experiencing due to light loading from a previous job.

Craig Wilkens works with key accounts for Doosan Portable Power.

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