Rent the Right Generator

Portable generators are available in a wide range of sizes, referring both to the physical dimensions and capability—the output—of the unit. Consumer units that are light enough to be lifted or wheeled by hand certainly provide unique benefits to the home and small jobsite. But as demand grows, so must the generator. The next “nominal” footprint is that of the towable diesel generator. These units are a more appropriately scaled solution for large events, busy construction sites, and critical systems in need of emergency power.

Towable diesel generators are engineered for continuous operation, and many have the fuel tanks to support such use. But in addition to large fuel tanks and a variety of power receptacles, these units can offer three-phase output in the voltages needed by industrial equipment such as portable lighting, compressors and pumps, welding equipment and more. Using welding equipment as an example, three-phase power may improve equipment output and duty cycle.

 But is a 60 kVA generator sufficient? An insufficiently sized generator will leave your operation suspended while you search for something with greater output. Should you “err on the side of caution” with a 250 kVA unit? Bigger is not always better because operating a generator for long periods of time without sufficient load (as is the case with a significantly oversized generator) can be electrically inefficient, demands a higher rental rate or purchase cost, and requires accelerated fuel consumption. Instead, renting the right generator “size” is critical to meeting your needs, minimizing the drawbacks to off-grid power, and maximizing operational efficiencies.

 

The Kilovolt Amp

It is common to search for and specify the capacity of a diesel generator by using a unit of measurement known as the kilovolt-amp. A kilovolt-amp is 1000 volt-amps and both are used to convey “apparent” power. In short, the maximum energy that the generator can produce cannot be harnessed due to reactive losses in the system. To understand the “real” power of the generator—in kilowatts—multiply the kVA rating of the generator by its power factor. This information is typically provided in the equipment’s list of specifications in its documentation.

”The maximum energy that the generator can produce cannot be harnessed due to reactive losses in the system.”

As an example: the specifications of the MMD PowerPro 65 Towable Diesel Generator shows that it can continuously provide 63 kVA of three-phase power with a power factor of 0.8. Multiplying these two values provides a value of 50 kW; this is the “real” usable output of the machine.

Once you understand the maximum generator output in watts, you can begin “sizing up” the best generator for your application.

The Steps to Sizing a Generator

Step 1: Create a list of all the equipment that you think you will power with the generator.

Step 2: Locate the manuals, specification sheets, or similar documentation (such as machine placard information) for the equipment you want to power. This will come in handy for the next step. Take note of the running wattages or kVA.

Step 3: Add up the wattages of everything you want to plug into the generator. Here’s some tips to keep in mind:

• A tool or machine (such as an electric motor) may require up to three times its running power to start, so be sure to factor this into your calculation.
• You can find the running power equipment that is rated in amperes (amps) by multiplying the rated amperage by the input voltage.
• Like the generator itself, some equipment is a “reactive” load. In these instances, multiply the running kVA by the equipment’s power factor, which may either be estimated from similar equipment or found in the equipment specifications.

 Step 4: Multiply the total wattage by 1.1 (or 1.2 for a more conservative estimation). Doing this will help cover underestimation of your requirements, under-performance of the generator, or equipment upgrades.

Example

The Red-D-Arc EX360 FieldPro is a popular choice for field welding in the construction industry. The specification sheet shows that the power source requires 13.6 kW at rated load output. Powering 8 of these units for simultaneous use at rated load output would require a minimum of 109 kW, but 131 kW (109 kW * 1.2) is a more conservative estimation. With this power requirement in mind, the Red-D-Arc RDA150T3S 150 kVA diesel generator would be sufficient for the application.

Of course, the example above is a simplification of the information gathering required to power a complex jobsite having a variety of equipment: lighting, compressors, welding power sources, power tools, and more. On job sites such as shipyards or refineries, eight power sources may be a small portion of the required fleet. Remember that towable diesel generators are available in sizes ranging from 24 to 400 kVA of output. A massive jobsite using even larger power sources for processes such as plasma cutting and arc gouging may demand the output of the MQ 300SSK 300 kVA diesel generator.

Although this article outlines the general process of sizing a generator, it can be easy to feel overwhelmed. Equipment rental services can be trusted advisors if you are unsure what generator would be best for your application. Contact our team today to learn more about Red-D-Arc’s extensive offering of generator rentals. Rely on their expertise serving similar applications to ensure that you can stay operational without being inefficient.