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  • Writer's pictureJim Gunshinan

Anatomy of an Electric Panel

The post was originally published on Sense.com.


The modern electrical panel is like a highway interchange for your house. It performs two essential functions: it carries electricity to the outlets and fixtures in your home and prevents overloads that could cause house fires.


The power to run your appliances typically begins at a power plant. It travels over long distances through high voltage wires—the tall towers that disturb many a homeowner’s views out their windows—through a transformer at a substation that steps down the voltage, and then through another local transformer, one of those big “cans” you may see on telephone poles along the road, where it steps down again. Finally, the electricity passes through the power lines to the meter at your home and into the breaker panel. The voltage that could be measured in the thousands at the beginning of its journey is rated at 120V or 240V at your panel—just the right amount to keep your home running.


Figure 1 shows a schematic of the typical breaker panel you will find in the basement, utility closet, or garage in most homes today. It is a gray metal frame with a door and panel cover inside the door. The panel cover protects from the breaker and wires inside. The panel cover should only be opened by a licensed professional. The panel door allows the homeowner access to the breaker switches. The main circuit breaker is on the top, which can shut off your home's power supply. The power from your meter feeds into the main circuit breaker. Below the main circuit breaker are two vertical rows of branch circuit breakers that feed into sections of your home. Yours might be labeled “Dining Room,” “Kitchen Lights,” “Master Bedroom,” and so on.

Figure 1. Inside a typical, modern breaker panel. The door is opened, as well as the panel cover, which should only be opened by a professional.


How Home Electricity Works

To understand home electricity, let’s start with basic terminology.

Voltage: The electrical force that pushes electricity through a wire, measured in volts.

Current: The flow of electrons through a wire, measured in amperes (amps).

Power: The electrical power in a circuit equals voltage multiplied by current. The higher the voltage (the push) and the higher the current (the flow of electrons), the higher the power, measured in watts. 1,000 watts = 1 kilowatt.

Energy: The power an electrical device uses over time, expressed as kilowatt hours (kWh).

Ohm’s Law describes the relationship between voltage, current, and resistance.

Voltage = Current (amps) x Resistance (ohms)

The human cardiovascular system provides a good analogy. The blood flow rate is current, the vessels and capillaries provide the resistance, and the heart provides the voltage to keep the blood flowing. In other words, if you are “pushing” a high current through a circuit with high resistance, you need to pump up the voltage.


It’s helpful to know the terminology because the capacity of electrical panels is described in amps and volts. In contrast, the power draw of appliances and lights are expressed in watts, and your utility bills you for the power your home consumes in kilowatt-hours.


How a Panel Controls Electricity

You may remember when circuits were protected by round, screw-in fuses that had to be replaced each time they were “blown.” New breaker panels have “bus bars” that don’t need to be replaced for the life of the breaker box. In older homes, the maximum current was set at about 100 Amps. Newer homes that are bigger and have more electric devices can require 200 Amps or more. The current divides into circuits in your house. Each circuit covers an area of the house and all the electric appliances. When the total draw from all your appliances is greater than 200, then the breaker box automatically shuts off the overloaded circuit. This minimizes the possibility of sparking and fire.


When a circuit breaker does its job of shutting off an overloaded circuit, the vertical breaker switch is in the middle, between the on and off sides. It makes a distinctive clicking sound that is familiar to most homeowners. To reset your system—after turning off a couple of your appliances to relieve the circuit—you flip the breaker switch to the off position and back to on.

If you expand your house, you may need to upgrade your breaker panel to meet a higher load. It can cost $3,000 to $5,000 to upgrade a 100-amp capacity panel to 200 amps.


Sense System Operation

The orange Sense Home Energy Monitor at the bottom of the panel gives your electrical system a Ph.D. in Electric Engineering—making it a brilliant home. The Sense sensors clip around the main circuits inside the panel and read the electrical current 1 million times per second. At the same time, machine learning analyzes the patterns of devices turning on and off. The optional Sense Flex sensors can track up to two circuits of your choice, such as an HVAC system or washing machine.


As you see electrical patterns in the Sense app, you can make practical changes to your home energy use that will save you electricity and money while minimizing the greenhouse gasses you create.

The Sense Home Energy Monitor can be installed in various panel configurations, including these:

Split service systems up to 400A, where service is provided via dual panels.

Subpanels provided that the monitor is installed in the main panel and the subpanels are downstream of this panel.

Busbar panels, using Sense’s re-designed slimmer sensors.

Recessed or flush-mounted panels, using a specific installation process.

Three-phase panels, where Sense can monitor two of the three legs of a three-phase 208V service.


Efficiency Vermont has been helping Vermonters and anyone who will listen save energy and money and decrease emissions for over twenty years. The nonprofit points to three benefits associated with the Sense monitor. First, you can make more informed choices if you can understand how much energy each electrical device uses and when it uses. For example, it can make you an energy “vampire” slayer. A vampire is any device that uses standby power even when “off.” Devices are now losing less standby power, but you can still save energy and money by using “smart strips” that shut off devices completely when they are not in use, like a TV set-top box or an audio power bar. A study done in 2015 by the Natural Resources Defense Council showed that you can save as much as a quarter of your energy use by slaying your vampires.


Second, you can find out which devices use the most. Is it the fridge? You can swap out an older model for a highly efficient Energy Star model or adjust how it functions to use less energy. Or you can find out where the lights are on when not needed and start paying attention to the off switches when you leave a room.


Finally, a Sense monitor can give you peace of mind while away from home. The Sense monitor will let you know if you left the fridge door open or forgot to turn off your stove.

Using Sense will make your home smarter. But even more important, it will make you and your family smarter and wiser about your energy use at home. At the very least, if you read this article, you will know much more about breaker panels than most people. It’s a start.







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