Vehicle electrical loads are like vehicle weights. Some departments keep adding individual components one after another and are surprised when the resulting total is so high. Some departments don’t even consider electrical loads and end up having problems.

To avoid overloading the alternator and batteries, departments need to understand that some electrical devices draw a lot more current than others. This is especially important for devices that are normally operated when the vehicle is stationary and the engine is at idle. If the total electrical load exceeds the alternator output, the electrical load manager may have to turn off certain components or the batteries may become discharged.

Here is a list of the biggest amp eaters commonly found on emergency vehicles, along with a few alternatives. All values are based on 12-volt electrical systems.

Electro-mechanical sirens. They sound great, but they suck amps like there’s no tomorrow. Every time you punch the button on a big electro-mechanical siren, it can pull as much as 100 amps from the vehicle electrical system. On the plus side, most electro-mechanicals have a free-rolling feature that requires no current and produces a long-lasting sound as the rotor coasts down. The alternatives are electronic sirens, which draw only 10-30 amps and produce a variety of sounds.

Warning lights. Figure a total of 30 to 60 amps load for a modern NFPA 1901–compliant warning light system using halogen or strobe light sources. Light-emitting diode lights draw significantly lower currents in the range of 5 to 10 amps total, which is one reason they are popular despite their higher costs. LEDs also resist vibration better and can last up to 10 times longer than other lights.

Foam proportioning systems. Most departments don’t realize it, but some direct-injection proportioners that use electric motors to control the foam concentrate can eat 25 to 80 amps of current depending on the model. Bypass eductors, pressurized bladders and some other foam systems require little or no current to operate.

Area lights. Area lights powered by the vehicle’s electrical system can draw a fair amount of current. Focused, high-intensity spotlights can draw 20 to 35 amps. Broader area lights can draw 5 to 15 amps each. The more lights you have, the greater the load. The best alternatives are either the careful placement of fewer lights or the specification of lights that run off a separate generator. (See “A Practical Guide to Area Lights” for a discussion of the various types of area lights.)

Others. Additional amp eaters include electrical winches, electric-rewind hose reels, pump priming motors and more.

Like anything else in life, specifying a vehicle’s electrical system requires a balance between many factors. To avoid problems, make sure you understand how each component’s load affects the total system.