And There Was Light and Pumps, Compressors, Saws, Drills…

Sept. 1, 2000
Gx Bug Web

One of the best things about divine decree is that thought and action go hand in hand. For us mere mortals, however, a little more thought is usually required if we want our actions to go according to plan.

When it comes to making the right choice about onsite electrical power generation, it’s a good idea to consider all the variables before you make a decision. A little help here from the Almighty would make the decision easier, but a simple cost-versus-use analysis and a two-sided list should suffice.

Master everything from OSHA regulations, to high-tech safety equipment in this FREE Special Report: Construction Safety Topics That Can Save Lives. Download it now!

Obviously if you don’t have access to utility power, you need to make your own. And that situation creates a host of attendant questions: How much power do you need? Will the generator be used for more than one application? Do you need 240-V or 480-V power? How long will you need portable power? What are the advantages of renting versus owning? Are there potential safety hazards to consider? What about noise and other environmental concerns?

These are all valid questions for every contractor who is considering either renting or buying a generator set for job-site electrical power. Fortunately, whether you are running a 5-kW gasoline-powered generator for a single table saw or a 2,000-kW diesel generator with enough juice to power an electrical substation, the choices about size, power, renting vs. buying, safety, and the environment run along the same lines.

he Right Equipment for the Right Job

It’s a good idea to consider all the possible power uses you might put a generator to before you make the decision to rent or purchase one. “First and foremost, it’s our job to find out what the client wants to use the power for,” points out Dennis Stranathan, rental power representative at Wagner Equipment Company in Denver, CO. “Will they be running 110-volt handheld tooling such as saws, screw guns, and drills or tower cranes that require 480-volt power? Once you’ve figured out what level of power the customer will be working with, then you need to know how much power, which will tell you if customers can just plug directly into Edison plugs on the generator set or whether they need a larger generator with a distribution panel connected to power distribution nodes, called ‘turtles’ and ‘frogs.'”

In order to figure out which generator is the right one for the job, experts depend on proprietary equations that allow them to calculate the kilowatts of power required; this step requires knowing the necessary voltage and the amp draw of the equipment being used. But before we enter into that discussion, there needs to be a brief mention of phased power.

The term phased power comes from the way the electrical energy is generated and supplied to the end user. Steve Losee, rental manager of Wheeler Power Systems in Salt Lake City, UT, looks at three-phase power this way: “Phased power is the most effective way to transport electricity. If all electricity were sent in a single phase, the electrical cabling would have to be so large that it wouldn’t be efficient.”

If you used a standard sine wave to look at power, one complete sine wave would represent single-phase power. Three-phase power looks like three sine waves spaced 120º apart. In simpler terms, single-phase power is like using a pickup truck to move 20 tons of earth from point A to point B. You have to make several trips to move all the dirt. Three-phase power is like using three pickups, hooked one right after the other, to move the dirt between the same points. It takes fewer trips with three trucks because you move three times more dirt each trip. Phased power allows you to deliver more power.

Calculating how many kilowatts are necessary to do the job is an important part of getting the work done. For example, if the power of your generator set greatly exceeds the needs of the job, then you’re wasting money on fuel and on either renting or buying equipment that is too big for the job. Those relatively minor costs are nothing compared to the problems of fried equipment and lost production time that come from trying to squeeze too much power out of a too-small generator.

To combat both of these scenarios, Jason Rogers, rental manager at Cummins Rocky Mountain Inc. in Denver, relies on a couple of tried-and-true equations. When calculating requirements for three-phase alternating current power, Rogers uses

volts x amps x 0.8 (power factor) x 1.732
= n kW

Let’s say you are using 480-V power to run a 200-amp compressor and a 200-amp water pump. What size generator do you need? According to Rogers’ equation,

480 volts x 400 amps x 0.8 x 1.732
= 266 kW

Given that generators larger than 50 kW usually come in 25- or 50-kW increments, the right choice would be a 275- or 300-kW generator, depending on the particular application and any additional uses.

For single-phase, alternating-current power, Rogers simplifies the equation by eliminating the 1.732 and raising the power factor to 1.0.

volts x amps x (1.0) power factor
= n kW

The most important part of calculating the correct amount of power has to do with the fact that the initial amp draw of power equipment, such as compressors, drills, cranes, saws, and pumps, is greater than the running amp draw. Stranathan uses the analogy of driving a car: “Heaters and lights have a constant amp draw, but when you start a motor, it takes more juice to get the motor moving than it does to keep it running. It’s just like a car: It takes more gasoline to get the car going at a stop sign than it does to keep it rolling down the highway.” Regardless of whether you are planning to rent or purchase a generator set, this critical aspect of calculating how many kilowatts you need should be taken into account before making your selection.

To Rent or Not to Rent

“Whether you’re talking about generators or scrapers, the benefits of renting are very similar,” notes Pat Dodd, market development manager at Wagner Equipment. The principal benefits of renting generator power are money, sizing, maintenance, flexible power output, length of use, storage, cabling, and safety.

Determining which application will cost less in the long run and which will give the best service are two concerns when weighing the rent-vs.-buy dilemma. Although each application has a similar set of variables, the cumulative effect of these variables over the course of two or three years is what makes either renting or buying a smart decision. These variables include the cost of utility power, the availability of utility power, and the length of time and how often you need generator power.

The very nature of the grading and excavation business frequently forces contractors to work in places that do not have utility power and might never have it. And considering that the national average cost of relocating utility power a half mile is roughly $7,000, sometimes utility power isn’t the best solution. What complicates the rent-vs.-buy question is that some jobs only require power for a couple of day or weeks, while others require generator power only as long as it takes to get utility power hooked up. Whatever the scenario, it pays to figure out how many similar jobs you might be working on in the next year or so before deciding to buy a generator set.

“One customer of ours was developing a country club, and utility power wasn’t going to be available until the following spring,” recalls Stranathan. “They wanted to put the grass in that fall, so we ran a 275-kilowatt unit, connected to a couple of water pumps, and they started watering.”

An example of the advantages of temporary remote power is a ground-freezing application Stranathan worked on that helped a contractor excavate the foundation for a coal chute at a utility plant in Colorado. The coal chute was about 100 yd. from the Platte River and about 50 ft. from a highway. According to Stranathan, the contractor had to dig 35-40 ft. down and couldn’t use metal shoring because the construction had to be stair-stepped. And when they got down that deep, they were below the water table.

“So they core-drilled around the project, sunk in refrigeration tubes, and hooked those tubes to a 350-kilowatt generator. The moisture content of the ground was high enough to freeze the ground solid, and they were able to excavate and put in a concrete foundation for the coal chute-without the metal shoring,” explains Stranathan. “They didn’t have to tear up the highway, and the ground was solid, so the water from the Platte River didn’t come in and affect the project even though the excavation was below the level of the river.”

Stranathan is currently working with the same contractor on a “tropical freeze” project in the Cayman Islands. “I believe they are building a hotel close to the water and need some way to pour caissons and the rest of the foundation.”

According to Dodd, the classic application of remote power is asphalt batch plants. “They are remote and usually don’t have utility power. The timing of the process is critical: One main generator usually runs all the mixers, conveyors, and motors, and an auxiliary generator is often used to keep the batch oil hot overnight. Paving contractors depend on asphalt from the batch plant because they have set up a train of paving trucks that costs $75 an hour per truck, so you need to have the product ready to go.”

Big operations such as batch plants and rock quarries require large, dependable generators that are part of the everyday operation of the plant, which makes owning a powerful generator more attractive than renting one. But the cost of buying and maintaining a generator can be too much for new companies just getting started.

Wheeler Power Systems rents large-kilowatt generators to rock-crushing and road-construction contractors. “Typically it costs up to $10,000 to bring utility power to the job site from a quarter- to a half-mile away,” says Losee. “Renting a 210-kilowatt generator costs about $2,000 a month.” So the break-even point happens in five months, but because the contractors are creating their own power, they only have to pay for fuel and the rental fee instead of utility power.

Renting generator power is ideal for smaller operations that have only been in business for six months to a year, Losee points out. “They can apply their full rental fee minus an interest charge-3% over prime rate-to the purchase of the generator they are already using. This way they can defer their operating capital to wages and benefits, instead financing the machines. In a sense we’re financing their operating capital, but we find that most new companies don’t want to overextend their credit. And they would rather buy the machine that they’re already paying on.”

Losee estimates that 20% of his generator-rental customers are new companies just getting started. The other 80% are contractors who need power in a hurry because they had an equipment failure, they planned maintenance, they can’t get utility power fast enough, or they overextended their existing generator.

Size Matters

“It’s all about getting the equipment sized right and getting everyone trained on how to operate it,” notes Stranathan.

One of the biggest advantages of renting a generator is the ability to tailor the generator to the job. This flexibility allows contractors to customize their power source to match their needs. “We deliver the proper generator to the proper load,” says Stranathan. “If you can tell me what your load is, then I can tell you what generator you need for the job. I can also tell you if you need single-phase or three-phase power.”

When you purchase a generator, you are married to that machine’s capacity to generate kilowatts. And unless you buy several generator sets, it’s human nature to make do with what you have. “If you need to add a conveyor belt, then you add it. If your job trailer needs air conditioning in the summer and heat in the winter, then you do what you have to do. And even though power plants love to work, your generator is designed to give you just so much power,” observes Stranathan.

“The cost of owning and storing multiple generators is much greater than renting,” states Rogers. “We have several short-term customers who rent generators until they can get utility power. So on a per-use basis, it’s a lot cheaper to rent than it is to buy.”

One of Rogers’ customers is a clean-room manufacturer that ships its products overseas where the voltage is different than it is here in the United States. “They rent a generator from us so they can test everything at 50 hertz and 400 volts.”

A great example of how renting multiple generators saved a contractor time, hassles, and money is a dewatering job for which Stranathan provided power so the contractor could lower the water table and install a sewer line. “The customer came to me because he wanted to power up a 20-horsepower water well,” Stranathan recalls. “Then he came back and said, ‘OK, what if we add two or three wells to the system?’ Then he said, ‘Here’s the deal. We’re going to leapfrog these pumps to keep the water moving as we excavate down the line.'” So what started out as a simple one-generator, one-pump job site became a three-generator, three-fuel tank, two-pump job site and then a three-generator, three-well site.

“They could dig their first hole, lay their pipe, fill it in, and move on to the next section, constantly dewatering as they went. They had to drop the water table 22-25 feet so they weren’t digging in mud,” Stranathan says.

He adds that although pumps, motors, and lights aren’t too particular about the power they receive, computers require a constant 60 Hz. So you’ve got to have enough juice to ensure that your computers work properly. If you’re thinking about adding a computer or two to your job trailer, then the same small generator that gave you enough power for the lights, a heater, or an air conditioner might not be able to keep up with your new energy demands.

Routine Maintenance

“People tend to plug into utility power through a wall socket and do not think about the maintenance that’s getting done on the steam turbine at the power plant,” observes Stranathan. “It’s no different on a diesel-powered generator. You’ve got to watch your power plant to make sure everything is operational. You’ve got to do daily maintenance checks on oil and water on your power plant.”

Maintenance is another big consideration when evaluating ownership versus renting. If you own your equipment, you either service it yourself or take it to a mechanic. If you rent your onsite power, the maintenance is usually taken care of by the rental company. So when you’re working out the numbers, you need to factor in the cost of parts and labor for routine maintenance. In addition to free maintenance, Wheeler Power Systems offers customers free repairs on rented equipment. “All rental repairs are on our bill,” says Losee.

“More of our customers are looking for a turnkey application—fuel, maintenance—so we provide them with everything except the actual hookup,” Stranathan says. “It just depends on what they need. Most dealers these days have a fleet of service technicians.”

Another advantage of renting a generator set from a reputable outfit is that you receive the advice of an experienced technician who probably has provided power for several similar applications before. “Standardization is a process of duplication,” states Stranathan. “If I can create the ideal power use on a particular job site, then chances are that I can do it again and again on similar jobs.”

Safety Is Job Number One

“Unless you know what you’re doing, you’ll burn up your equipment, kill someone, or not get the job done,” warns Rogers.

Education and safety go hand in hand. To reduce the risk of liability, it is in every rental service’s interest to educate its customers as much as possible about the safe operation of its equipment. “We try to show people as much about the generator as we can,” assures Stranathan. “The smaller generators, like Hondas, are pretty simple because they give you a little plug-in and that’s it. It’s all single-phase power, so there’s not a lot to screw up. But when you’re trying to run 480 voltage, you need to power up stadium lights, and you have to run the neutral wire so you can get 277 voltage back from the 480, it gets a little more complicated. It’s all simple stuff once you know what you’re doing, but that’s why electricians go to school and get a license.”


One of the most frequent sources of safety problems with job-site generators is cables. Twist-lock plugs on cables help prevent plugs from working their way out of distribution panels and nodes because you have to physically push them in and twist them. This twist-and-lock procedure keeps the cables from stretching too far, pulling out of the panel, and arching potentially lethal amounts of electricity. Even with stronger, more secure plugs, however, incorrectly installed cables are an accident waiting to happen. “If you hook up your cables yourself, you better know your voltage, because if you hook up a 240-/208-volt panel to 480-volt power or vice versa, your equipment isn’t going to last very long,” states Stranathan.

The cables themselves can also be a significant source of safety problems. “Invariably something gets damaged,” Stranathan points out. “Somebody runs over a cable with a tractor or a backhoe. That’s why every cable gets inspected before it goes out, and we really look it over when it comes back in.”

Newer generator models also have safety features. For example, if the power-plant motor is running and someone opens the door, a door-activated kill switch automatically shuts down the engine.

Environmental Concerns

Excessive noise and diesel exhaust are two quick ways to get your neighbors up in arms. Because of these problems, most larger generators now come encased in sound-attenuating housing and have water-heater jackets to prevent smoky starts, especially in cold weather. “We’re noticing more and more contractors requesting sound-attenuated units not just to run the job trailer, but to be on job sites because of OSHA,” says Stranathan. “They don’t want noise problems with their employees. A prime example is the tower cranes that I’m running down at the new Mile High Stadium [in Denver]. All of our generators on that job are sound-attenuated and sound-suppressed.”

Diesel smoke from a cold generator is another environmental problem for contractors using onsite generator power, especially during hard-starting winter months. You don’t have to be environmentally minded to realize that cold diesel engines without jacketed water heater, glow plugs, or an auxiliary generator produce a lot more exhaust than a preheated power plant. And nothing sours public opinion of your job site like a stinky black cloud of diesel wafting through the aroma of sausage, eggs, and fresh pancakes.

Add Grading & Excavation Contractor Weekly to  your newsletter preferences and keep up with the latest articles on grading and excavation: construction equipment, insurance, materials, safety, software, and trucks and trailers.    

The Bottom Line

So is it better to rent or buy a generator? That depends on many solid factors and perhaps a few intangibles, such as service, established relationships with local dealers, and brand loyalty. “It’s our job to develop trusting relationships with our customers because that’s what brings them back,” remarks Dodd. “If you’ve been renting John Deere equipment for the last 10 years, then when it comes time to buy one, you’ll probably pick John Deere because that’s what you know and that’s whom you’ve developed a relationship with.”

So with a little planning, and perhaps some divine inspiration, your future decisions about how to keep the lights on and the equipment running should be as simple as turning on a switch.