Your home is powered by the electric company via the service entrance. Wires feed an electric meter and quite often a disconnect before entering your home's electrical panel. This panel may be a fuse panel or a circuit breaker panel. These panels have certainly changed over the years. Either way, its job is to protect the home's wiring from overload via either fuses or circuit breakers. The fuses are designed to take a predetermined amount of current and then the fuse link will melt. That's why people say the fuse blow and circuit breakers trip. Circuit breakers, on the other hand, have predetermined limits also, but trip when they exceed the limit and can be reset. This makes circuit breakers reusable, while fuses are a one-time thing.
Adding a Subpanel
Sizing subpanels to the need of your anticipated load can be tricky. You'll need to consider the amount of available power load you'll need and what the main service has to offer. For example, if you have 200-amp main service, you'll have no problem adding a 100-amp subpanel to feed a shed, garage, or barn. A 60-amp subpanel can power lighting and general-use outlets in another section of your home. But if you only have a 60-amp service and want to add a 60-amp subpanel, you'll have to upgrade your main panel first to allow such a distribution addition.
However, many modern-day homes can already use close to 200 amps. That means there isn't necessarily capacity to add an additional 100-amp subpanel. Consult with a licensed contractor if you are at all unsure of the size of your subpanel.
When adding a subpanel, it's best to add at least a 12-slot circuit breaker panel. This should provide ample room for lighting and general circuits. However, if you plan to add many 240-volt appliances such as central air conditioning, baseboard heaters, water heaters, ovens, ranges, or 240-volt window air conditioners, then a circuit breaker panel with more openings may be required, as well as a circuit breaker panel with more openings and a larger main breaker rating.
Subpanels provide the convenience of lessening circuit wiring runs to a minimum by centrally locating the panel, which also lessens the voltage drop that would occur on smaller wires over a long distance. You can see the advantage of running a larger set of panel feeder wires to the area where the power distribution is needed, rather than run multiple wires long distances.
Subpanels and main panels alike have specific rules that need to be followed according to the National Electrical Code (NEC). All electrical panels must have a minimum of 36 inches of clearance in front of the panel, 30 inches of clearance across the face of the panel, and a minimum of 78 inches above the floor from the top edge of the panel. If you can picture an invisible phone booth with an electrical panel on one wall, you'll have a good idea of what is required. Now the NEC goes a little farther in requirements than that. The panel must be mounted in a dry location and have easy access to it. Only attach a panel in an area that is not exposed to flammable materials and never mount it where it is exposed to moisture like a bathroom or indoor swimming pool area or the like.
One last tip about adding a subpanel: whenever dealing with electricity, always turn off the power before you begin any project. If the power is off, you won't get shocked. It only takes a minute to shut off the power, but it only takes a split second to get shocked and possibly injured from contact with electricity. Don't become a statistic. Think about it, it will take you longer to say why you won't shut off the power to the panel than to actually turn it off, walk back to the project, and be safe.
This is a project that would likely require permits and a degree of experience, so don't hesitate to reach out to a licensed contractor for help.