DANA R. BREWER
Workplace Safety Division
Directorate of Assessments and Prevention
U.S. Army Combat Readiness Center
Fort Rucker, Alabama

According to the National Fire Protection Association, 47,700 home fires in the U.S. are caused by electrical failures each year. The fires leave behind staggering results: more than 400 deaths, 1,500 injuries and $1.4 billion in property damage. Overloaded electrical circuits are a major cause of residential fires. Getting to know the limits of your house’s electrical system will help prevent overloading your circuits, lower your risk of electrical fires, and keep your home and family safe.

The two ways that we access power inside of our houses are through lighting and power circuits. These circuits are made up of wiring, circuit breakers, outlets and switches. A circuit overload can occur when too much power is drawn through a circuit. Preventing a circuit overload starts with knowing the limits of each circuit and how much power can be drawn through it safely. Get to know your circuits. Familiarize yourself with the outlets, switches, circuit breaker ratings and even the wire sizes of the circuits around you.

Single-family houses and apartments typically have 15-amp circuit breakers and outlets with 14-gauge wires installed. By exception, bathrooms and kitchens are required to have 12-gauge wire with 20-amp circuit breakers and ground fault circuit interrupter (GFCI) outlets installed to meet the National Electrical Code. A kitchen and utility room are often wired with 220 volts to accommodate appliances like stoves and dryers, which require larger wires, circuit breakers and outlets. Figure 1 below illustrates how wire size and circuit breakers match up by use.

Circuit breakers are designed switches that operate automatically to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. The warning signs of an overloaded circuit may vary in severity and frequency but will be obvious compared to normal operations. Common electrical problems range between harmless and hazardous. Harmless faults include loose outlet plugs, broken light switches, simple short circuits, and flickering and dimming lights. Hazardous faults include light bulbs burning out frequently; dead outlets; a frequently tripped circuit breaker; mild shocks/tingles from appliances, receptacles or switches; a cracking or burning odor; and cut or damaged wires or cords.

Electricians calculate circuit loads with a 20 percent safety margin, making sure that the maximum appliance and fixture loads on the circuit are no more than 80 percent of the available amperage and wattage provided by the circuit. For example, a bathroom with a 20-amp circuit providing 2,400 watts of power can quite easily handle 1,800 watts of demand with a 25 percent safety margin.

First, find the main electrical panel (typically in the utility room) and determine the voltage and amperage available (i.e., 120/240 voltage, 200 amperes service). Second, count how many circuit breakers are in the main service panel. The current is divided into individual branch circuits, each controlled by a separate circuit breaker. A typical house will have a dozen or more circuits, each supplying power up to the circuit breaker and wire-rated limits (15 to 20 amps). Third, determine where each circuit goes to in your house. The label on the circuit breaker should describe the circuit location (i.e., “master bathroom”). Fourth and final, calculate the total power required by the devices plugged into each circuit. Figure 2 below illustrates a partial house wiring diagram.

Figuring the electrical power used by an appliance begins with an understanding of the relationship between amps, watts and volts — the three key means of measuring electricity. A relationship principle known as Ohm's law states that amperage (A) x volts (V) = watts (W). Using this simple relationship principle, you can calculate the available wattage of any given circuit size:

15-amp, 120-volt circuit: 15 amps x 120 volts = 1,800 watts

20-amp, 120-volt circuit: 20 amps x 120 volts = 2,400 watts

For example, think of a simple hair dryer rated at 1,500 watts running on a 120-volt, 20-amp bathroom branch circuit. Using the W ÷ V = A variation of Ohm's law, you can calculate that 1,500 watts ÷ 120 volts = 12.5 amps. Your hair dryer running at maximum heat can draw 12.5 amps of power. Consideration should be given to the remaining power (900 watts or 7.5 amps) on that 20-amp circuit to avoid overloading the total capacity of 2,400 watts. Beware of adding a power strip to an existing outlet. This adds additional outlets but does not add amperage to the circuit.

The house wiring diagram below illustrates a section of a house or apartment with several rooms and circuits. Let’s use a sample bathroom with a vent fan that draws 120 watts of power, a light fixture that has three 60-watt bulbs (180 watts total), and an electrical outlet where that 1,500-watt hair dryer is plugged. The load on that circuit could reach 1,800 watts, as all of these could easily be drawing power at the same time on the 20-amp circuit (providing 2,400 watts). A circuit overload could occur if you plugged in an additional appliance such as a small space heater drawing 750 watts of power. The total power demand would be 2,550 watts, exceeding the 2,400 watts available and causing the circuit breaker to interrupt (trip) the circuit.

Overloading electrical circuits can happen easily and be the cause of a residential fire. Help prevent circuit overloads by knowing the power capabilities of your house. Know the total power consumption of your devices and limit the amount of power demand you place on each circuit. Never exceed the capacity of your circuits and be aware of the warning signs of an overloaded circuit. Getting to know the limits of your house’s power will help lower your risk of electrical fires, prevent overloading your electrical system and keep your family and home safe.