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Electrical Service Panels & Circuit Breakers: How They Do Their Job

Every home has at least one electrical service panel. Some may also have subpanels that serve energy-intense areas of the home, such as a new addition or an upgraded kitchen. Most homeowners know to look at the service panel when they experience a sudden loss of power to see if a breaker has tripped or a fuse has blown. But ideally, you should know more than that. Service panels should be routinely inspected, because certain issues require more than resetting a breaker or replacing a fuse.

Typical Electrical Service Panel parts

[/media-credit] Typical Electrical Service Panel

Getting to Know Your Service Panel

The service panel’s job is to distribute electricity flowing from a source—in most cases, the electric utility company—to branch circuits within the home. Most branch circuits serve a number of lights and outlets. Sometimes, a single circuit is dedicated to serve a single appliance. This is a code requirement in most locales and good idea when the appliance has a motor (clothes washer, air conditioner or microwave oven) or heating coils (electric range, electric dryer, toaster, clothes iron or blow dryer).

Power purchased from the utility company is fed into the home through a meter at the service entrance. From there, the electricity is routed to the service panel, which is essentially a metal or plastic cabinet installed in or against a wall. Inside the cabinet there are “hot” and “neutral” bus bars—relatively large metal strips designed to conduct substantial amounts of electrical current while dissipating heat—and an array of overcurrent devices, the circuit breakers or fuses. Panels have one “main,” which controls the power supply to the panel, and individual controls for the branch circuits.

The breakers/fuses are rated, in amps, for the safe limit of power that each circuit is designed to serve. They prevent fires by cutting off power when an overload or other fault in the wiring could cause the circuit to overheat.

When a panel is installed correctly, the bus bars and wire connections to the controls are concealed by a cover, which allows only enough access to reset breakers or change fuses. Don’t remove a panel cover unless you are a licensed electrician.

The Evolution of Electrical Service Panels

In the beginning, circa 1900, there was the fuse: either a cartridge or a simple glass vial with a metal filament and base that plugged into the service panel. The filament was designed to melt when it got too hot, cutting off power to the circuit. Prior to 1950, most fuse-protected service panels had the capacity to deliver only 120 volts and 30 to 60 amps.electrical service panels timeline

By the 1960s, circuit breakers were introduced and became the norm for houses ever since. The principal advantage of circuit breakers over fuses is that they can simply be reset, rather than needing to be replaced, after responding to a fault. Service panels designed for circuit breakers were designed to deliver 120 and 240 volts with enough breaker mounts for 100-amp capacity or greater. If you are building a house this year, you’d be wise to install a panel with 200-amp capacity.

Some would argue that fuses are more reliable than circuit breakers because they’re less complicated. Standard breakers installed during the era from the ‘60s through the ‘90s incorporate two separate switching mechanisms—a bimetal switch and an electromagnet. The bimetal component trips the switch when it heats up as the result of an overload. The magnetic component breaks the circuit when it senses a short. In a certain sense, circuit breakers are less sensitive than fuses. They’re intentionally designed to disconnect circuits only when heat buildup persists over a period of time.

Research conducted during the mid-1990s led to the development of arc-fault circuit interrupters (AFCIs) that trip when they sense a high-power discharge between two conductors, which can occur, for example, when connections between wires and switch terminals become loose. By 2005, the National Electric Code required combination-type AFCI breakers for all new construction.

Why the History Is Relevant

This history of service panel and overcurrent protection technology is important for two reasons: First, it should keep you from feeling that you have to change out old equipment that’s working properly for new just to keep up. For example, if you have an old house with an electrical system protected by fuses, you don’t necessarily have to replace them with a breaker panel for safety reasons. If you need to add service, have a licensed electrician add what you need with a modern panel, but don’t let him or her talk you into fixing what’s not broken.

The second reason is that if you need to repair or replace components in a service panel, it’s important to know that they must be replaced with parts of the same brand, technology and amperage as the original. If your electrician can’t find compatible components, you might have to start over with a new panel. Knowing when the panel was installed can facilitate that decision.

What Goes Wrong with Panels and Breakers

Electric service panels should be inspected routinely for signs of corrosion. A recent report based on the observations of professional home inspectors asserts that evidence of rust was found on approximately 10 percent of service panels observed—even with the covers in place. If there’s rust on the outside, there’s sure to be corrosion on the inside, which can wreak havoc on connections and damage breakers.

The inspectors found that the source of moisture in panels is usually poor sealing at the service entrance, which should be corrected before the rusty panel is repaired or replaced. Panels in damp, unheated garages and basements are also subject to corrosion.

A breaker can also malfunction and trip more often than necessary. When one or more breakers trip frequently, the cause could be one of three issues:

  • The circuit is inadequately designed for the load
  • An appliance on the circuit has a fault
  • The breaker is worn or otherwise damaged

It’s easy to verify or eliminate the first two potential causes. A circuit overload is the most common problem, which usually becomes obvious when the over-current device controlling it shuts the circuit down only when two or more appliances are in use at the same time—say, the microwave oven and the toaster.

If you suspect that a faulty appliance might be an issue, there a few steps you can take. When a breaker trips, unplug everything on that circuit, then reset the breaker. Plug in one appliance at a time and turn it on. Wait 15 minutes or so. If the breaker doesn’t trip, unplug the appliance and test the next one. If you find the culprit, repair or replace it. You don’t have to worry about the breakers.

If you eliminate both circuit overload and a faulty appliance as the cause for a circuit shutting down frequently, suspect trouble at the panel. Call a licensed electrician to remove the panel cover and perform an inspection. Electricians know how to work safely in an open, energized panel and can diagnose problems through visual inspection and appropriate test procedures.

The electrician will look for evidence of corrosion and overheating inside the cabinet, at terminal connections and on wires themselves. It’s not uncommon to find burnt connections at breakers either because connections weren’t made properly during installation or because they loosened due to excessive vibration. Breakers that are damaged by corrosion and/or overheating can be replaced, as long as the source of the problem is also addressed.

The electrician will also assess whether the current service and system design is adequate for your home’s demand. He or she may recommend a larger panel with more breaker slots to supply more power and/or reduce loads on certain circuits.

Remember that the service panel’s main job is to keep you and your home safe. The best way to ensure that a panel does that job well is to keep a close eye and consult with a qualified electrician to access and correct anything that you find worrisome.


Michael Chotiner is a former construction manager who writes about DIY projects for The Home Depot. He provides interesting facts and advice to help you make informed decisions. You can view here a wide selection of breaker panels from The Home Depot.


10 Easy Ways to Slash Your Electric Bills

Saving energy—and money—on your utility bill is simple if you follow these guidelines.

Save energy; save money.

Thanks to extreme weather conditions and poor public utility policies in some states, home electric bills have shot through the roof. Unfortunately, forecasters predict no end in sight to even higher electricity prices.

What can you do to take the strain off your electric meter? Here are 10 easy ways to slash your electric bills:

1Turn off lights you’re not using. This is a no-brainer and certainly one of the easiest measures of all, and yet it is a key step toward acting with a conservation mindset. Just flip a switch to stop wasting hundreds of watts of power every day.

2Cut back on the lights you use. Indoors, use dimmers or decrease the wattage of bulbs. Opt for task lighting instead of general room lighting where possible. Outdoors, put motion sensors on light fixtures so bulbs only turn on when and where they are needed.[GARD align=”right”]

3Adjust your thermostat. An air conditioner uses a tremendous amount of electricity when it runs. During the summer, you can realize major savings by raising the set temperature to 78 degrees F. or higher. Conversely, turn the set temperature down during the winter (as a rule of thumb, every degree you lower a thermostat’s set temperature in the winter will save 3 percent of energy costs over a 24-hour period.)

A programmable electronic thermostat

4Use an electronic thermostat. If you don’t already have a programmable electronic thermostat for your heating/cooling system, strongly consider replacing your existing thermostat with one. This device can be set to automatically adjust temperature settings to energy-saving levels when you’re sleeping or away from the house.

5Run the forced-air system’s fan. Run this fan instead of the air conditioner—to maintain a comfortable temperature during the summer. Simply flip the thermostat to “Fan Only” to recycle air throughout the house. (Of course, in a hot, humid climate, running the fan alone won’t cool and dehumidify the house.) Also, regularly replace the heating/cooling system’s air filters to improve efficiency.

6Maintain your refrigerator. Because it is one of the biggest energy users in the home, a refrigerator should be operated at maximum efficiency. This means opening the door only briefly and adjusting the “Cold” control according to the manufacturer’s directions. Vacuum refrigerator coils to remove dust. Defrost it if necessary (though most refrigerators today are frost-free). To reduce the amount of power the refrigerator’s motor uses, consider plugging it into an electronic induction motor control such as the Power Planner, available in home improvement centers.[GARD align=”right”]

7Use electrical appliances less. Only run an electric dryer with full loads or, even better, air dry your clothes. Opt for cooking in the microwave instead of with an electric oven. And don’t forget to turn off the coffee maker when the coffee is done. When buying new appliances, check the EnergyGuide label to be sure they are energy-efficient models.

8Turn down the water heater temperature. Reduce the water temperature to a maximum of 120 degrees F. An electric water heater devours electricity! Consider switching to a gas water heater.

9Replace standard incandescent lightbulbs. Use highly efficient compact fluorescent bulbs instead, which use from about one-quarter to one-third the energy to produce the same amount of light (a 20-watt compact fluorescent will give you about the same light as a 75-watt incandescent). Though most cost considerably more than conventional bulbs, they last up to 13 times longer, which makes them a long-term good buy.

Use natural light to brighten a room.

10Use natural light. Longer days and a higher summer-sun angle afford you an opportunity to let Mother Nature help with the energy bills. The trick is to admit light without summer heat gain or glare (or winter heat loss). Fortunately, you can control heat gain and glare with window coverings and window films, available at home improvement centers. To increase the amount of natural light that floods a room, bounce it off the ceiling and walls with the aid of louvers or operable blinds (light-toned ceiling, walls, and floors work best). For more about using daylight, see Daylight Techniques for Light & Warmth.

Though some of these methods may appear as though they will give you only minor relief from staggering electric bills, adopting several or all of them can result in significant overall savings. Many utilities charge higher rates for the portion of your electrical usage that exceeds baseline requirements. When you trim away the excess fat, you can dramatically lower your bills.

How to Calculate the Electrical Usage for Your Home

Knowing your electrical usage and service rating are crucial if you plan on adding new loads.

After mapping out your home’s wiring circuits (see How to Map House Electrical Circuits), the next step is to determine your present usage, or electrical load. This would be a time-consuming task if you had to go around the house and add up all the wattages of the lights and appliances; however, the National Electrical Code (NEC) has established certain values that represent typical electrical usage.[GARD align=”left”]

Three watts per square foot of existing living space and space for future use is used to figure electrical load for general-purpose circuits (lighting and receptacles). A nominal value of 1,500 watts is used for each 20-amp small-appliance circuit (circuits that power receptacles in the kitchen, dining room, family room, breakfast room, and pantry) and for a laundry circuit.electrical_meter1

By applying these values to your home and using the actual nameplate values affixed to major appliances, you can use a handy formula to calculate your electrical load. Consider the example of a house with 1,800 square feet (based on outside dimensions) of finished living space and space adaptable for future use. The house has the usual two small-appliance circuits (3,000 watts), a laundry circuit (1,500 watts), a hot water heater (5,500 watts), a clothes dryer (5,600 watts), a dishwasher (1,500 watts), a garbage disposal (600 watts), a range (15,000 watts), and a central air conditioner (5,000 watts).

The first step is to multiply 1,800 square feet by 3 watts per square foot. The total is 5,400 watts for lighting and general-purpose circuits. Add 3,000 watts for the two small-appliance circuits and 1,500 watts for the laundry circuit for a total of 9,900 watts. Next, add the values of all the major appliances, except the central air conditioner, for a total of 38,100 watts.

The next step is to figure 40 percent of the amount over 10,000 watts (0.40 x 28,100 = 11,240 watts). Adding the 10,000 watts to the 11,240 watts gives a subtotal of 21,240 watts. Then add the 5,000 watts of the central air conditioner for a grand total of 26,240 watts. This is your estimated load in watts.

To figure the current needed to carry that load, divide 26,240 watts by 240 volts. The total comes to 109.33 amps; therefore, of the standard service ratings (60, 100, 125, 150, and 200), the rating for this sample house should be 125 amps or higher.

Now try it yourself for your own home. Then compare the total load in amps with your present service rating. If the two values are close together, your present service cannot handle the addition of many new loads.

If your service rating is less than 100 amps, you can’t use this formula to calculate your load. You can, however, use a different formula that incorporates the same NEC values for typical electrical usage. The general-purpose circuits, small-appliance circuits, and laundry circuits are computed exactly as they are in the first formula.[GARD align=”left”]

Once you’ve figured the general-purpose circuit load (3 watts x number of square feet of living area), add 1,500 watts for each 20-amp small-appliance circuit and laundry circuit. Using this total, add 100 percent of the first 3,000 watts and 35 percent of the balance over 3,000 watts [3,000 + 0.35 (total – 3,000)].

Add to this value the nameplate ratings of all major appliances (space heater, garbage disposal, dishwasher, etc.). This gives you your estimated load in watts. You can find the current by dividing the total wattage by your voltage—120 volts for two-wire service and 240 volts for three-wire service.

How to Trace a Short Circuit or Overload

Diagnosing the cause of a tripped circuit can be tricky; fortunately, the repair is usually easy.

Home electrical circuits may have a number of problems: faulty wiring within the house, too many lamps or appliances on one circuit, defective wall switches or receptacles, defective cords or plugs, or defective circuits within appliances. Short circuits happen when a hot wire touches a neutral or ground wire; the extra current flowing through the circuit causes the breaker to trip or a fuse to blow.

Although it’s often easy to tell when you have a short or overloaded circuit—the lights go dead when you plug in the toaster oven—it isn’t always as simple to tell where in the electrical system this has occurred.

Resetting a Breaker

Changing Fuses

To diagnose the problem, start by turning off all wall switches and unplugging all lights and appliances. Then reset the circuit breaker. Pull the lever to off and then to on again to reset a circuit breaker that has a lever switch.

If a fuse is blown, it must be replaced. Unscrew the fuse to replace it with one that has exactly the same amperage rating (both circuit breakers and fuses should be sized according to the wire used in the circuit they protect).

If the breaker trips immediately, the problem may be a short in a receptacle or switch. If the breaker does not trip again, turn on each switch one at a time and check if and when the breaker trips again.[GARD align=”left”]

If turning on a switch causes the breaker to trip, there’s a short circuit in a fixture or receptacle controlled by the switch. If turning on the switch makes no difference, the problem is in one of the appliances connected to the switch.

If the circuit went dead when you plugged in the appliance, the problem is probably in the cord or plug. If the circuit went dead when you turned on the appliance, it’s likely that the appliance itself is defective.

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How to Read Your Electric Meter

Old-school mechanical electric meters measure electrical usage with a series of numbered dials.

[/media-credit] Old-school mechanical electric meters measure electrical usage with a series of numbered dials.

A primer on how to read your electric meter so that you can check it against your utility bills

Where power lines enter your house, you’ll find an electric meter, which —in many cases—looks like a large glass jar with a bunch of wheels and dials inside. These four or five dials measure kilowatt hours—the units of electricity you’re billed for.

Newer digital meters are located in the same place, but are easier to use because they display actual numbers to measure your electrical usage by kilowatt hours.

Digital electric meter has easy-to-read numbers to measure electrical usage.

[/media-credit] Digital electric meter has easy-to-read numbers to measure electrical usage.

A kilowatt hour is equal to 1,000 watt hours, the amount of energy it would take to light ten 100-watt light bulbs (totaling 1,000 watts) for one hour.

Each month, a utility company representative records the meter’s readings. The previous month’s reading is subtracted from the current month’s reading to arrive at the usage that appears on your bill.

If you know how to read an electric meter, you can do the same thing. This can be helpful in tracking energy costs or checking the accuracy of your bills.

Reading a new digital meter is easy—you just record the numbers. But the older and far more prevalent type of meter—mechanical with dials—is a bit trickier to read.


A Typical Mechanical Electric Meter

How to Read a Mechanical Electric Meter:

Read from right to left, jotting down the number each pointer has passed or is pointing to. Please note that, with most mechanical electric meters, the numbers circle the dials alternately clockwise and counterclockwise. Don’t let this confuse you.

If the dial hasn’t quite reached a certain number, record the next lowest digit. If you’re not sure whether or not the dial has passed a certain number, study the one to its right to see whether or not it has passed 0. If it has, then the pointer has reached the number you’re wondering about.[GARD align=”right”]

Write the numbers down in the same order that you take them—from right to left—but read the result from left to right. In other words, looking at the line drawing shown here, the meter reads 02798.

Home Electrical Service

An illustrated explanation of how a home is provided with electrical service by a utility

Electrical utilities deliver electricity through a masthead at the roof. Wires continue through the meter to the main panel.

Originally, electrical power was formed by chemical reaction, and that’s still the way that batteries work. This type of current, known as direct current (DC), flows from a negative pole through an electrical device (such as a lightbulb) and on to the positive pole. However, direct current can’t be transmitted over long distances without a debilitating drop in voltage.

Utility companies now provide households with alternating current (AC), which actually pulses—or reverses direction—120 times, or 60 cycles, per second (called 60 hertz power). AC power moves in waves. Lightbulbs actually flicker as power ebbs and flows, but the human eye can’t detect it.

The utility company’s electrical lines may enter a house overhead from a power pole or underground from a buried pipe called conduit. Where the power enters your house, you’ll usually find an electric meter and, either there or on an inside wall behind the meter, the main service panel.[GARD align=”left”]

Called “rough-in components,” wires, cables, and electrical boxes are installed during construction, before the wall and ceiling finish materials are put in place. “Finish components” such as receptacles, switches, and light fixtures are installed after the interior coverings are installed.

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