When putting on an event, customers may know exactly how they want the linens to look and what kind of music they want, but knowing how much power they will need to run the swound board, lights and catering equipment is probably not in their top five concerns.

Sizing a generator for an event requires obtaining a lot of information from the customer and then crunching numbers to ensure the generator provides the right amount of power for the electrical needs of the event.

Dan Thomsen, regional manager, PowerSource Generators, Doosan Infracore Portable Power, Statesville, N.C., says the first step is to identify the type of event you are doing. “With a concert, you’ll need the generators to power up the band and a generator to power up the lighting. Usually you don’t put those on the same generator. With sound equipment, you will need to make sure your generator has an electric governor or you can cause harm to the sound equipment,” he says.

In sizing, the most common problems occur when event planners or end customers who rent the equipment don’t fully understand their electrical needs, says Tom Smith, rental sales manager, Cummins NPower, White Bear Lake, Minn. “It is common for non-experts simply to add up the total amperage of the devices they need to power up and to fill out a power sheet by saying, ‘I need 100 amps.’ Someone knowledgeable in power should read through the sheets and contact the vendors or customers with questions about the voltage needs and connection requirements. For example, 100 amps at 120 volts is 12kW, 100 amps at 120/208 three phase is 35kW, and 100 amps at 480V three phase is 65kW.”

In addition, says Wissam Balshe, sales application engineer, Cummins Power Generation, Minneapolis, many electrical loads require more current or volt-amp (VA) to start. “For example, a typical single phase motor may require up to 10 times its rated VA to start. Also, nonlinear loads, such as battery chargers and computer loads, generate high levels of harmonics that distort the voltage output of a generator and could trip circuit breakers and other protection devices onsite. To minimize the effect of these harmonics, oversizing the genset to lower the impendence of the power source may be necessary.”

As a result, there are times when rental stores will need to account for misinformation. “When in doubt, always err on the ‘bigger is better’ side,” says Kent McLemore, director, Power Distribution Group, Trystar, Faribault, Minn. “Actually you always want to add at least 20 percent to the estimated size because that is exactly what it is, an estimation. Getting all the exact power requirements typically never happens. Most people do not understand electricity and you will get a lot of incorrect information.”

Ken Cannella, product manager, climate control, Wacker Neuson Corp., Norton Shores, Mich., says rental stores also need to consider how large their heat and/or air conditioning unit needs to be in the first place, so they are not adding units mid-event and overloading the power source.

“The biggest problem is sizing the actual structure for heat or air conditioning load,” Cannella says. “If the historical data says it could be 96 degrees and it’s been 82, size it to 96 degrees. Similarly, for heat, always size to the lowest possible ambient temperature to be on the safe side. Lighting and people also give off heat, so that’s a subtraction for cooling and a positive for heating. If it’s a wedding reception and everyone’s dancing, each person will give off 1,600 Btu of heat.”

Another element to consider is secondary distribution from the generator or distribution panel, Cannella says. “The biggest problem you run into is when you heat a large tent structure, and you have eight heaters along a 160-ft. tent wall. You have to figure secondary distribution to get the power to those plugs.”

How much load, or power, is being used has a direct effect on the generator, Thomsen says. “If you do not properly size a generator, you could have catastrophic failure of the engine. If the generator is oversized, you can get wet stacking on a diesel engine. If the generator is under-sized, he says, it can cause the breaker to throw or the engine to slow down or stop,” he says.

“For proper sizing, it’s also important to know that some rental companies and manufacturers rate their equipment in kilowatts (kW), while others use kilovolt amps (kVA). Typically, the kW rating number will be 20 percent smaller than the kVA rating for the same power generation equipment,” Smith says.

“This 20 percent rule of thumb is based on a 0.8 power factor that most motors and inductive loads are rated at. However, with the increasing use of digital equipment, it is common to see loads with 0.9 power factors and higher. Therefore, if the power factor of the load is unknown, we recommend sizing based on the kVA rating of the load, not kW,” Balshe adds.

When fielding questions from rental customers, McLemore says, keep the contact consistent. “Try to have at least one employee who is designated to handle electrical projects. They don’t have to be certified electricians, but should able to understand basic electricity, communicate with the customer, be flexible enough to change with the requirements of a project and able to work late hours and weekends.”

As far as setting up, Thomsen says a rental company should not hook the load to the generator. “They should advise their customer to get a registered electrician.” McLemore adds that rental stores also can provide “color coded cam-lok outlets mounted on the generator to guarantee safe and easy hookup.”

One way to take the confusion out of hooking up generators to various appliances is to use an electrical distribution box, which hooks up to the generator, and breaks down power from the generator into separate plugs for various appliances. “The generator produces three-phase power to the distribution panel and the panel breaks it down to single phase. Large air conditioning units are going to be on three-phase power. A cash register is going to be single phase,” Thomsen says.

“Generators now have selector switches to turn from single phase to three phase. However, if you put the switch in single phase, you lose one-third of the power. So, a 20 kW generator can only provide 14 kW of power in single phase. If it is in three phase, you can pull the full 20 kW,” he says.

No matter what generators a company owns, Thomsen says, a load bank is essential to make sure generators are running properly. “You put the generators on the load bank to check the generator’s performance level,” he says.

Another element to consider is safety. “Safety is the most important concern,” Smith says. “Renters should use ramping to protect cables in traffic areas, and be sure that the load and cord or cable is protected by the breaker on the generator or in the panel.”

Also, “remember that generators’ exhaust output is hot and loud. Ensure that the genset exhaust is directed away from walking areas and directed upwards,” Balshe says.

When in doubt, Smith says, rental companies should encourage their customers to consult an expert. “Not having enough power or the right connections could ruin the event and nothing would be worse than someone getting hurt.”