Standard fuse and fixed trip circuit breaker sizes are 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amps, according to 240.6.1 of the National Electrical Code.
The number after each amp rating represents the ampere load or current that can be carried by the breaker before it will activate its internal thermal cutoff mechanism. For example, a 120-volt, 20-ampere load would require a 1500-watt heater. The corresponding circuit breaker size for this application would be 15-amp service. A 10-foot extension cord could supply power to these lights if they were plugged into separate outlets on the cord. But if the lights were connected in parallel with other like items, such as another set of lights or a lamp, then only one light could be plugged into any one outlet on the extension cord. This would cause the overloaded outlet to trigger its protection device and shut off the power to that extension cord.
In addition to the current ratings shown on the box or specification sheet for a product, an important factor in selecting the correct breaker size is knowledge of how many circuits will be used on the installation. For example, if there are three bedrooms and each room has an electrical fixture in it, then there would be 3 circuits required for this use.
3 A, 5 A, and 13 A are the most prevalent. The fuse should be rated to handle slightly more current than the gadget requires. Circuit breakers function as reversible fuses. These are electrical switches that function automatically to safeguard electrical circuits from overloading or short-circuiting. The switch may be reset by an operator who physically moves it to open the circuit. Switches can also be set off by specific events, such as power surges or excessive temperature. Circuit breakers are available in the same sizes as regular fuses.
The most common type of fuse is the screw-type fuse. It consists of two parts: a glass or ceramic tube with several holes drilled through it, and a metal cover attached by screws. When the cover is removed, you can see the elements inside the tube: a carbon arc lamp, two cylindrical electrodes, and some powdered metal. The powdered metal forms the path for the electricity to flow when the light goes out. It does this by melting when enough heat is applied; once molten, it stays that way until cooled down again. Screw-type fuses are easy to replace if needed. They range in size from 1-amp fuses for small appliances to 15-amp fuses for large industrial applications.
Another common type of fuse is the molded case circuit breaker. They are very similar to regular wall outlets, except they can switch on heavy currents without burning.
It is simple to determine the right fuse or breaker size for a branch circuit. For most applications, an overcurrent device should be sized at around 125 percent of the rated amperage of the load. Your load will be safeguarded against electrical short-circuits in this manner. The main switch on the wall controls all the circuits feeding into it. If it's a single-pole switch, then only one circuit is controlled by it. If it's a three-way switch, then each terminal on the switch controls a different circuit. The rule of thumb for sizing breakers is based on how many amps they will have to handle in case of a failure.
For example, if you plan to use four 20-ampere outlets, then you should select a 40-amp breaker. This will provide enough power for up to 80 amps of current through your wiring. Of course, more than 80 amps won't get any bigger breaker, so you'll need additional ones to accommodate the rest of the current. A five-outlet circuit requires a 60-amp breaker because 10 amps times five outlets equals 50 amperes. A six-outlet circuit needs a 75-amp breaker because 15 amps times six outlets equals 90 amperes. You can see that this method ensures that you will have enough power available if any one outlet is overloaded while others are not used at all.