Understanding Fuses and Fuse Boxes

Fuses and fuse boxes have long provided homes with essential protection against system temperature increases and excessive current flows. Though largely supplanted by circuit breakers and service panel boxes, fuses and fuse boxes can still be found in some older homes—still operating and often code-compliant.

What Fuses and Fuse Boxes Are

Fuses

A fuse is a device that protects electrical systems against potentially dangerous power surges and excessive temperatures. When subjected to extreme conditions, the fuse will blow or burn out. Unlike a circuit breaker, a fuse cannot be reset. It must be replaced.

Fuse Boxes

A fuse box is a metal box that contains screw-in fuses and cartridge fuses. Smaller than the electrical service panels found in most homes today, fuse boxes contain between six and 12 fuses. Most older fuse boxes are rated at 60-amp total capacity. By comparison, residential electrical systems today are 200-amp total capacity.

How Fuses Work and What They Do

A fuse is classed by the National Electrical Code as an overcurrent protective device (OCPD). This puts fuses and circuit breakers in the same category, though they are different devices.

Any fuse must contain a fusible section: that is, a piece that is capable of being melted. The fusible part is called the fusing element and it is sealed inside of the glass fuse body. The fusing element is a thin strip of metal.

When the fusing element receives more power than it is built to handle, it heats up, melts, and severs. Thus, the circuit is broken, rendering the circuit dead and safe.

After a fuse blows, the fusing element is permanently broken, so the fuse cannot be reused. It must be replaced with a fresh fuse.

Types of Fuses

A typical fuse box setup is a 60-amp box, with two main cartridge fuses in fuse blocks and four screw-in fuses.

Fuses for residential use are of two types: screw-in or cartridge. 

A screw-in fuse is small and round, with a glass body that protects the fusing element (a thin metal strip) and a conductive metal base that screws into the fuse box, much like a light bulb. 

A cylindrical cartridge fuse, which fits into a metal fuse block, handles greater loads such as for dryers or ovens.

How Fuses Blow

By design, fuses are the weakest link in the home's electrical system. All other wires and components in the electrical system are more robust than the fusing elements within fuses. 

When there is enough current or temperature present that could threaten the rest of the system, the fuse’s fusing element is instead the first to be damaged. 

Why Fuses Blow

Overload Fuse Blows

A closed circuit is one that works properly. Blowing a fuse opens the circuit. Electrical overloads can be up to 6 times the normal current level and usually will open the circuit.

Overloads usually happen when too many devices are plugged into the same circuit. For example, turning on a microwave, hairdryer, and lights all at the same time may blow a fuse.

Short Circuit Fuse Blows

Short circuits happen when a path of lesser resistance is offered to an electrical circuit. Electricity travels freely along copper wires and conductors. But when a pathway develops that makes it easier for the electricity to travel, the electricity naturally follows that path. 

Short circuits can be up to thousands of times greater than the normal operating current. They can melt metal, damage wire insulation, cause arcing fires, and even vaporize conductors.

Nails or screws piercing electric cables, water entering an electrical box, or wires being loosened can all result in a short circuit.

Ground Fault Fuse Blows

Ground faults are the result of a powered, hot wire touching anything grounded: a metal electrical box, a metal pipe, an outlet, a bare ground wire, or even your hand. Ground faults invariably will cause fuses to blow.

How to Know If a Fuse Is Blown

You can test if a fuse is good or blown either visually or by checking it with a testing device such as an ohmmeter or a continuity tester.

Visual Testing

The glass window on screw-in fuses will sometimes be cloudy, brown, or black as a result of the metal fusing element melting away. Or if the glass is clear, you might be able to see the severed fusing element inside. 

With a Testing Device

Continuity testers and ohmmeters both are inexpensive electronic devices that measure electrical resistance. To check a screw-in fuse with a tester, touch one test lead to the end of the fuse and the other test lead to the threaded side of the fuse. 

To check a cartridge fuse, hold the test leads at opposite ends of the fuse. If the tester indicates a current, then power is flowing through the fuse.

How to Change a Fuse

Your fuse box will have two sets of fuses. At the bottom will be 15-amp and 20-amp screw-in fuses with glass faces that are exposed and visible. These fuses will service branch light circuits and outlet circuits. At the top will be 30-amp or 40-amp cartridge main fuses for ovens, laundry rooms, or stoves. Cartridge fuses are hidden in fuse blocks that can be pulled out with handles.

Tools and Supplies You Will Need

• Flashlight
• Replacement screw-in fuse
• Replacement cartridge fuse
• Ohmmeter or continuity tester

1. Open Door

   Open the door to the fuse box. Make sure that you are standing on a dry floor and that you are wearing rubber-soled shoes.
   

2. Find Blown Fuse

   With a flashlight, look at the glass-front screw-in fuses. If you find one that’s burned or clouded, that is likely the blown fuse.
   

3. Unscrew Fuse

   Unscrew the blown fuse by turning it counter-clockwise. As with unscrewing a lightbulb, do not touch the metal threaded section to avoid shock.
   

4. Add New Fuse

   Screw the replacement fuse into place by turning it clockwise. Again, avoid the metal threads on the fuse.
   

5. Locate Fuse Blocks

   To replace a main fuse, first locate the fuse blocks. For most 60-amp fuse boxes, there will be two fuse blocks, each with metal handles.
   

6. Remove Fuse Blocks

   Grasp the metal handle or plastic T-shaped handle of the fuse block. Firmly pull the fuse block straight out. Set it on a table upside-down to expose the fuses on the back.
   

7. Remove and Test Fuses

   Remove the cartridge fuses from the fuse blocks. Since blown cartridge fuses are not visibly obvious, test them with an ohmmeter or continuity tester.
   

8. Replace Fuse in Fuse Block

   Put the new cartridge fuse into the fuse block, fitting it the same way you would a battery: two end contacts touching the metal conductors on the fuse block.
   

9. Replace Fuse Blocks

   Slide the fuse blocks back into place in the fuse box. Test the power by turning on a light or another device on the circuit.
   

Are Fuses and Fuse Boxes Code-Compliant?

The National Electrical Code (2020) provides for fuses and fuse boxes as overcurrent protection devices. Consult with your municipal code and permitting office to see if your area allows these systems and, if so, under which conditions.

Whether or not your code allows fuses and fuse boxes, it’s recommended that you switch to a conventional electric service panel with circuit breakers.

Circuit breakers are standard, so all electricians are experienced at working with them. Circuit breakers offer safety features such as GFCI and arc-fault reduction technology that fuses and fuse boxes do not offer.

Fuses and fuse boxes are not inherently unsafe. But context can make them unsafe. For one, fuse boxes are often attached to older knob-and-tube wiring systems—exposed wiring that usually should be replaced. As 60-amp service, fuse boxes cannot handle the increased energy demands of most contemporary homes.

Another issue with fuses is that some of the older fuse boxes lack rejection features. It is possible to screw a 30-amp fuse in error into a hole meant for a 15-amp circuit. This could possibly allow for overload of the circuit and result in a fire. Some later types of fuse boxes do have a rejection feature that requires that both the fuse and the fuse box threads match.

When to Call a Professional

Have a qualified, licensed electrician do the work when updating your fuse box to 200 amp service with circuit breakers. For work on fuses and fuse boxes other than replacing fuses, it’s a good idea to call an electrician. Simple, fuse-based systems are unfamiliar to most do-it-yourselfers—if you don't feel confident in your ability to replace a fuse yourself, consider calling an electrician to complete that task, too.