Selecting the Right Size Generator
"What size generator do i need?"
This is a very common question and it is easier to answer or figure out on your own than you may think. The following article was written in Rental Managment Magazine by Vince Groberg a Wacker Neuson Corp product specialist. We thought he explained how to select the right size generator very well, making it as simple as possible.
By Vince Groberg
Vince Groberg is a regional product specialist, utility for Wacker Neuson Corp., Menomonee, Wis. The information in this article covers basic portable generator sizing information.
Generators are a necessity for most job sites. Portable power units can power everything from small tools to portable offices. Selecting the right generator for the job doesn’t take rocket science, but it does take planning. It is important to understand that customers are asking for reliable, temporary power for their application when they rent a generator. Sometimes the customer knows his/her power requirements, so this is not an issue. More often however, a customer only has a general idea of how much power is needed. In this case, you must determine from the customer the amount and type of power required and where and for how long that power is needed in order to pick the right size generator for the job.
Three factors play a part in sizing a generator for the job: type of load, total watts to be connected and power of the generator.
Type of load
The first factor to consider when sizing a generator is the type of load being connected. There are two types of electrical loads: resistance and induction loads. Incandescent lights and heating units are examples of resistance loads. Most construction equipment requiring generator power has induction loads for electric motors. There are two types of induction load motors. Universal motors, which use brushes, power hand drills, saws and concrete flex shaft vibrator motors. Capacitor motors, which are brushless, usually power submersible pumps, air compressors and table saws.
When sizing a generator to these motors, the main difference is that the capacitor motors need about a third more starting current than universal motors.
Wattage requirement.
After determining the type of load, the next step is to find the total watts of the load to be connected. The most accurate method to determine wattage is to check the nameplate on the machine. However, if this information is not available, wattage can be figured by the simple formula:
amps x volts = watts
Load requirements to electric motors differ, but all motors require more power to start than run. Capacitor motors require up to six times more starting power for longer periods of time. Universal motors draw 1½ to 2 times the running current for shorter periods of time.
Generator power.
Once you determine the wattage, you can choose your generator. The basic guide to follow is if the generator has enough power to start the load — its surge rating — it will have enough power to continually run it. There are three power ratings for generators, including maximum or nameplate rating, which has a duration of 30 minutes and 10 percent duty; continuous rating, which has a 24/7 duration and 100 percent duty; and surge rating or motor starting rating, which has a duration of two seconds and instantaneous duty. Generator designs have different surge capacities that vary between 1.2 to 2.5 times their continuous run rating.
Next, determine what kind of generator you have. A premium construction-grade generator offers 100 percent duty and with an electronic automatic voltage regulator (AVR) can surge 2 to 2.5 times its run rating. A value construction-grade generator offers 100 percent duty with capacitor or rectifier excitation and can surge 1.5 to 2 times its run rating. A low-cost homeowner generator offers temporary duty with capacitor or rectifier excitation and can surge 1.2 to 1.5 times its run rating.
For example, the surge rating of Wacker Neuson’s GP5600 is equal to two times 5,000W or 10,000W.
If you cannot determine the design of the generator, then assume surge capacity is equal to 1.5 x continuous rating. It is better to oversize the generator to be used by your customer.
Let’s say a contractor wants to use a portable gasoline generator to power his temporary job shack. He needs to connect seven 100W light bulbs (700W), a coffee maker (850W), small refrigerator (induction, 600W to run and 2,000W to start) and a 1,500W radiant heater.
The light bulbs, coffee maker and radiant heater would run constantly, consuming 3,050W. With the refrigerator cycling between off and start/run, the total wattage demand on the generator will vary between 3,050W and 5,050W, which can be handled by a 5,600W portable generator.
For another example, let’s say a 2-in. submersible pump requires 771W to run (6.7 amps x 115V). A 2-hp flex shaft vibrator runs on 2,000W. The pump and vibrator motor require a total of 2,771W to run, however, the pump has a capacitor motor and will require 2,313W for starting, while the universal vibrator motor requires 2,600W. You should size the generator by adding the largest start load to the remaining running loads, which in this case would be 4,313W (2,313W + 2,000W). To run the pump and vibrator, a 5,600W generator will be needed. This will give the customer enough power to also add another tool, such as rotary hammer, which needs 1,100W to run and 1,600W to start.
Another way to size a generator is to calculate amperage instead of the wattage. You can compare the maximum amps of the tool to the generator, the same way you compare the wattage. If you need to calculate the maximum amps of a generator, simply divide the maximum wattage by 120: Maximum amps = maximum wattage/120.
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