"The most important factors are the size and type of load," Brown adds. "You should also know the voltage; the amps; whether the load is single phase or three phase; if the load is purely resistive; the duty cycle of the load; and the operating environment. Altitude and ambient temperature are also important."
Once these factors are known, you can find a variety of sizing tables, charts, graphs and software to assist in determining the correct size of generator for the job.
You can also get help from generator dealers and rental centers. "Most manufacturers work with dealers who have been trained by the company, so they can educate contractors on the proper use, sizing and application of generators on the job," Howe says. "The 'big box' stores and online shopping don't offer that. Dealers are consultants who can guide you to what's best suited for the application and optimize your investment."
One other important consideration is startup draw or surge rating (standby rating on towables). Tools with motors draw higher amps at first, and this is the hardest load placed on a generator.
"Starting wattage isn't typically listed on power tool data plates, but can be calculated or estimated from a table," notes Howe. "Compare starting wattage needs with the generator's surge rating, and running wattage with the continuous rating. Ensure both values fall within the generator's ratings."
The other manufacturers mention that their generators have suitable surge ratings built in. "The amount of surge rating depends on the power of the alternator and design," Pepper explains.
This leads to additional considerations, such as fuel tank size so you don't have to refuel too often.
Brown points out, "When purchasing a generator, contractors should pay attention to life-cycle costing. The more sophisticated buyers are evaluating initial cost, operating and maintenance costs, resale or salvage value, the amount of time they plan to operate the equipment and financing costs."
He adds, "Understand that in generators you get what you pay for, so compare alternator and engine specs, the controls and the sound levels of the generators you are considering to be sure you are buying quality." Also bear in mind the newest models now offer significant noise reductions.
Extension cords affect performance
A proper extension cord must always be used. However, use the shortest one possible for your application.
"The reason is voltage drop," says Brown. "Voltage drop describes the inherent reduction in available voltage associated with the resistance in the extension cord. This effect is directly proportional to the distance from the generator."
"The basic rule is the longer the cord you use, the thicker the wiring (gauge) needs to be," Pepper says. "If the gauge isn't high enough, the wiring will heat up, as well as the tool's motor. Both are potential safety problems, and shorten equipment life."
According to Brown, "If a cord is not heavy enough for the load it must carry, it could burn or there could be a severe reduction in power available at the end of the cord."
The Extension Cord Gauge chart shows the minimum extension cord wire size for the amount of the expected load. Note that the wire size increases with the length of the cord as the anticipated load increases.
Often, construction sites have a system of cables, cords, distribution panels and distribution "spider" boxes to get the power to where it is needed. "Each of these components can have an impact on the performance of the equipment being powered," Brown says. "Extension cords from the spider box to the equipment being powered must meet the requirements of the load. These extension cords, too, should be rated for the voltage and amount of current that is being carried to the equipment. Consider the length of the extension cord, as well as the distance of the distribution box from the generator."
Avoid using a longer cord than you need, as well. "Coiling a 100-ft. cord next to the generator also causes lower voltage at the tool, even if you're using just 10 ft. of it," Howe notes. "While portable units usually don't, towable generators often have a voltage output adjustment. If you measure the voltage at the end of the cord and it's insufficient, you can adjust the generator to provide optimum voltage to the tool."