Understanding Fuses and Fuse Boxes

Electrician working at a fuse box
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The electrical system in every home has some form of circuit protection to shut off circuits in the event of an overload, short circuit or ground fault. In homes built after about 1965—or in older homes in which the electrical service has been updated—this protection is usually provided by a series of circuit breakers in the main service panel. Circuit breakers are mechanical devices that sense the amount of current flow and "trip" when the current flow exceeds the safe capacity of the circuit wires. However, if you have a home built before 1960 and the electrical service has not been updated, there is a good chance that you have a different of circuit protection—screw-in fuses found inside a main fuse panel.

How Fuses Work

Fuses are relatively simple devices. The fuses that protect individual 120-volt circuit are typically ceramic screw-in plugs that fit into threaded sockets in the fuse panel. A thin metal strip inside the fuse conducts all electrical flow through the circuit and if the current flow exceeds the current-carrying capacity of the metal strip, it overheats and melts, thereby interrupting the flow of current and shutting off the circuit. The fuse is a kind of early-warning system, which senses overloads and "blows" before the circuit wires themselves can overheat and possibly cause fire.

Larger 240-volt circuits, as well as the main fuse that controls the main power flow, use a different type of fuse design. This type of fuse is a cylindrical cartridge that fits into a fuse block that slides in and out of the fuse panel. The principle is the same—the metal conducting strip inside the fuse burns through if the current flow exceeds the safe capacity of the circuit.

Unlike modern circuit breakers, fuses cannot be rerest. Instead, blown fuses must be unscrewed (or unplugged) and replaced. It is quite important that the fuses be properly matched to the amperage of the circuit. There is a distinct danger, for example, if a 20-amp fuse is used with a 15-amp circuit, since this creates the potential for the circuit to draw more power than the circuit wires can safely handle.

The Fuse Box

Fuses are housed in a fuse box—the precursor to the main service panel found with modern circuit breaker systems. The fuse box is usually located away from main living areas, such as the garage, laundry room, or basement. If you’re unsure whether you have a fuse or breaker box, locate the panel and open it up. Breakers are rectangular units with on-off toggles. Most breakers are arranged in banks or rows. In a fuse box, on the other hand, you will see a group of round screw-in plugs with small glass windows.

Fuse Types and Sizes

Your fuse panel may include several different types of fuses. The most common include:

  • Edison Base (Type-T) fuses. Type-T fuses are designed to handle no more than 125 volts and have an ampere rating of no more than 30 amps. This is the standard fuse for most 120/125-volt household circuits. The face of the fuse is printed with the amperage rating. If you are confident that the fuses have been properly matched to the amperage of the circuit, then you can safely use replacement fuses of exactly the same amperage. If you think there is a chance the circuit has been "over-fused"—that someone may have installed a fuse that is larger than the circuit rating—then you should consult an electrician to determine the proper fuse size. A 15-amp fuse should be matched to 14-gauge circuit wires, while a 20-amp fuse is designed for 12-gauge wires.
  • Type-S fuses. Type-S fuses consist of two components: an adapter and the fuse itself. Each level of amperage has a matching socket adapter with a unique threading to prevent mismatching fuses. Where practical, it is a good idea to install Type-S fuses and bases, since it will prevent you from installing the wrong fuses in the future.
  • Cartridge fuses. Cartridge fuses are cylindrical ceramic fuses with metal sleeves or blades on both ends. They are typically used for 240-amp circuits. Cartridge fuses fit inside a fuse block that inserts into a slot in the fuse box. Removing them involves extracting the fuse block by pulling on a handle, then replacing individual fuses in the block. They are generally found in pairs inside the fuse block—each one controlling 120-volts of the combined 240-volt service. Cartridge fuses are used not only for 240-volt appliance circuits, but also for the "main fuse" that controls power to the entire fuse panel. Like screw-in fuses, cartridge fuses also have amperage ratings printed on them. The main fuses are often 60-amp, while fuses for appliance circuits are more typically 30-amp or 40-amp devices.

Identifying a Blown Fuse

The most common sign of a blown fuse is a power outage in one or more areas of your home. Fuses, unlike breakers, do not have on-off switches. Instead, most fuses have a small glass window that allows you to examine the fuse itself. When the fuse blows, you will either see the melted metal strip inside the window of the fuse, or you will see cloudiness or scorch marks in the glass. This indicates the metal strip inside has melted through.

Changing a Fuse

Changing a fuse is usually a simple matter of identifying the blown fuse, then carefully unscrewing in and screwing in an exact replacement.

Match Fuse to Circuit Amperage

It is very important that you install fuses that match the amperage capacity of the circuit wires. Installing a fuse that is oversized for the circuit creates a risk that the circuit will draw more power than the wires can safely handle. For example, if a 20-amp fuse is plugged into a circuit served by 14-gauge wire (which is rated to handle only 15-amps of power), you create a serious risk of overheating the circuit wires. NEVER install a fuse that is larger than the blown fuse you are replacing.

The replacement procedure is different with a 240-volt circuit. Here, you will need to carefully pull the fuse block from its slot and examine the individual cartridge fuses. A small tool known as a fuse puller is helpful in extracting cartridge fuses from the block.

How to Replace a Blown Screw-In Fuse

Tools and Supplies You Will Need

  • Rubber mat
  • Flashlight
  • Replacement fuse

Instructions

  1. Locate the Blown Fuse

    Lay a rubber mat on the floor in front of the fuse panel, then open the door of the panel. (The rubber mat is a safeguard against the possibility of shock.)

    Use a flashlight to examine the glass windows on each fuse. The blown fuse is likely to show scorch marks on the glass, or you might see the metal filament inside melted through.

    Shut off lights and unplug appliances connected to the circuit. This will reduce the chances of overloading the circuit again after you replace the fuse.

  2. Remove the Blown Fuse

    Carefully holding the blown fuse by the ceramic rim, unscrew it counterclockwise and extract it from the socket. Examine the face of the fuse for its amperage size, and select an exact replacement.

    Caution

    Be very careful not to touch any metal parts as you remove or insert a fuse. Be especially careful not to touch the threaded metal on the fuse as you unscrew or screw in the fuse. There is a danger of contracting live current if you happen to touch the threads as they come in contact with the live bus bar in the panel.

  3. Install a Replacement Fuse

Insert the new fuse into the socket by screwing it in clockwise until it is firmly seated in the socket. Close the fuse panel then test the circuit by turning on lights and plugging in appliances.

How to Replace a Blown Cartridge Fuse

If an electrical appliance such as a range suddenly stops working, it is likely that a cartridge fuse serving the appliance circuit has blown. These are often 30-amp or 40-amp circuits. If power to the entire house suddenly stops, it is possible that one of the main fuses has blown.

Tools and Supplies You Will Need

  • Rubber mat
  • Flashlight
  • Fuse puller (optional)
  • Continuity tester or multi-tester (optional)
  • Replacement fuse

 

  1. Remove the Fuse Block

    Lay a rubber mat on the floor in front of the fuse panel, then open the door of the panel. (The rubber mat is a safeguard against the possibility of shock.)

    Locate the fuse block controlling the circuit. This may be labeled "Range" or "Dryer." The fuse block usually has a small metal handle attached to the front. If the blown fuse is a main fuse, the main fuse block is usually located at the top of the fuse box.

    Carefully grip the metal handle on the fuse block and pull straight outward to remove the block from the fuse box.

  2. Remove and Test the Fuses

    With most cartridge fuses, it is not obvious when they have blown, so you will need to test them.

    Use a cartridge fuse puller to extract the fuse from the fuse block. Then, use a continuity tester (or multi-tester set to continuity mode) to test the fuse by attaching one probe to each metal sleeve on the fuse. If the tester does not light up, it indicates there is no continuity and that the fuse has blown.

  3. Insert a new Fuse

    Insert an exact replacement fuse into the fuse block, pressing it firmly into place to anchor it to the contract brackets.

  4. Replace the Fuse Block

Insert the fuse block into its slot in the fuse panel and push it straight in until it snaps into place. Take care not to touch any metal parts in the fuse box as you do this.

Turn on the appliance served by the cartridge fuse to make sure it operates correctly.

Upgrading Your Electrical Service

If you have a fuse box, it means your electrical service is quite old and likely is insufficient for the power demands of a modern home. Fuse panels typically provide 30 or 60 amps of power, and the bare minimum for a home with modern appliances is now considered to be at least 100 amps, with 150 or 200 amps preferable.

Homeowners with fuse panels experience blown fuses on a regular basis, and this is a sign that the electrical service needs to be updated. Installing a new electrical service with circuit breakers is a job for a professional electrician and is not something a homeowner should attempt.