By Steve Shupert
For many of us, the concept of electrical wattage, either about power produced (as in a discussion about generator output) or power consumed (as in the wattage of a light bulb), can be confusing.
It is also not unusual to find fire department apparatus with “too much” generator. When an electrician is hired to put in an electrical service, he will often do an “electrical” load calculation of how much power you need to do what you want to do. The issue with just buying a big generator is, if you don’t “load” the generator to 80% to 90% of capacity from time to time, you risk the long-term health of the generator. The generator, like a chain saw or any engine, needs to heat up and stretch its legs to maintain reliability and to get your money’s worth. Doing a load calculation is easy, and it may save you from spending money on kilowatt (kW) capability you don’t need.
Wattage is the unit of measure for any electrical device’s energy capacity. For a light bulb, it is a measure of how much energy is consumed; for a generator, it is a measure of how much energy is made. A simple mathematical expression of this is Volts × Amps = Watts. Voltage is a constant at 120 VAC; it’s the amps that are different. Tool to tool, the amount of current a tool/appliance needs to operate is the electrical load. The operator of the tool can change the amperage draw by bearing down on the tool to work harder than it is effectively capable of.
Voltage, current, wattage, and resistance are all interrelated—you cannot change one without changing another. If you decrease voltage or increase resistance (size and length of extension cords), less current will flow, and tool performance will suffer.
Also, part of this relationship is how we work the tool. Feed rate determines a lot of tool performance. If we try to get more cut from a reciprocating saw, chain saw, etc. by pressing it harder against the workpiece (past the blade and electric/gas motor’s ability to operate at efficient rpm) we have overloaded the tool (it gets hot), voltage will be pulled down, and current will go up. This will result in the tool working outside its design, and it will heat up too fast and damage will result. The motor, in addition to moving the bit or blade, also moves fan blades to cool itself off. Keep your hands from covering these vent holes when running the tool.
PORTABLE GENERATORS
An electric generator is a vital part of almost any rescue/fireground operation. A generator’s size is usually expressed as kW, or thousands of watts. In other words, a 2.5-kW generator is advertised/rated to make 2,500 watts of electrical energy.
To figure your power needs correctly, you need to know how much and what kind of loads you are dealing with. A load is defined as the device that you are powering. There are two kinds of electrical loads: reactive and resistive.
Reactive loads contain an electric motor, which requires the needed additional power to start but significantly less power to run once it gets going. A typical rule of thumb is starting power = 1.5 times the amount of power to run the motor. Examples of reactive loads include air compressors and electric motor power tools.
The large startup current draw can overload the generator. The motor is motionless, then we apply electricity to get it moving. It’s like pushing a car stuck in mud—it takes a lot of energy to get it moving but less energy to keep it moving.
Resistive loads are simple: They require the same amount of power to both start and run the equipment. They usually have no moving parts and no startup current. Many resistive loads are involved in heating or light of some kind. Examples of resistive loads include light fixtures and battery chargers.
Figure 1. Watts = Amps × Volts
If the generator is not powerful enough to supply the tools plugged into it, electric circuit breakers will likely trip, and the generator may shut down. These are engineered protective measures because if we put too much electrical load on a generator, its voltage output will drop below the tool’s operating range. Operating tools with motors in this low-voltage situation will damage the tools and generator.
You may experience brownouts in other tools on the same circuit/generator or circuit breakers tripping as other motor/reactive loads stop and start. It’s the reason your lights might momentarily dim when you run your vacuum in your house. Or, when the tool operator bears down on the tool, forcing it to slow down, the generator itself slows down, causing a voltage drop. The electric motor (load) then draws more current from the generator to make up for this reduction in voltage, potentially tripping circuit breakers. Let us examine these concepts in detail, starting with how much electricity you can really get out of a generator and what size generator you need to compensate for in-rush/start-up current demands.
SIZING YOUR GENERATOR
The following will lay out how to size a portable electrical system for your application and how to calculate an electric tool’s wattage. Our power tools are labeled using volts and amps, not watts. So, for example, a 120-VAC reciprocating saw uses 12 amps to operate. We’ll figure out the watts. Use the math formula above to find out how many watts this tool or any appliance will need to operate. Once we are armed with this knowledge, we can successfully determine what size generator we need. 120 VAC × 12 amps = 1,440 watts. This is to “run” the tool.
We also need to take into consideration the startup current. Startup current is 50% of the running current. 1.5 × 12 amps = 18 amps startup current. This is the figure we need to determine generator size. The startup current can cause a voltage drop, similar to when you run a vacuum or microwave oven, and the lights dim.
120 VAC × 18 amps (startup current) = 2,160 watts—that’s almost half a 5-kW generator to properly run just one saw!
Some generator sizing formulas will use much higher figures for startup, but 1.5 has always worked for me. Inventory all the electrical loads on your apparatus, calculate the total wattage demand using the above method, and the result will be the minimum size generator you require. It is wise to add 25% to the final figure for any growth in tool inventory.
MAXIMUM VS. RATED POWER OF A GENERATOR
Generators are advertised by the maximum wattage they can produce, but you will also see the rated or continuous power listed. Rated power is what the generator can deliver all day long; the maximum is the peak (limited) output.
Maximum power is the maximum output that a generator can produce. Maximum power is generally only available for up to 30 minutes. This is the wattage most prominently displayed on generators.
Rated/continuous power is the power that a generator can produce for long periods of time, typically, 90% of the maximum power. For example, 5,000 W is 4,500 W continuous.
If you spec a 10-kW generator on a piece of apparatus and now know you will never “load” this generator up if you plug everything you own into it, you can hire a “load test,” where a contractor will slowly load up your generator and sustain this load to ensure proper operation—not too different from doing a ladder test.
RESOURCE
https://www.fireapparatusmagazine.com/magazine/extension-cords-one-of-the-most-misused-tools-on-the-fireground/.
STEVE SHUPERT is a lieutenant (ret.) from Miami Valley Fire District, Montgomery County, Ohio. He is rescue team manager and training officer for Ohio Task Force #1 US&R. He is a veteran of 11 federal deployments including WTC and Hurricane Katrina, a member of DHS/FEMA Rescue Sub Group, a certified rescue specialist, a structural collapse specialist instructor, and a heavy equipment rigging specialist instructor. Shupert is director of training for Crash Course Village, a 501C3 nonprofit organization.
