Understanding Fuses and Fuse Boxes (2024)

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 containing 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 have a 200-amp total capacity.

Tip

Circuit breakers and service panels are sometimes called fuses and fuse boxes, though this is technically incorrect. Fuses have an element that burns out, while circuit breakers have thermomagnetic switches that can be reset.

How Fuses Work and What They Do

The National Electrical Code classifies a fuse 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 is sealed inside 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 will use two types: screw-in or cartridge.

A small and round screw-in fuse has 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.

Tip

Look for the fuse box in closets, hallways, mudrooms, crawl spaces, basem*nts, utility rooms, or laundry rooms. Fuse boxes in more prominent locations might be covered with a wood panel or picture meant to disguise the appearance of the box cover or door.

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 too much current or the temperature threatens the rest of the system, the fuse's fusing element is the first to blow.

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 six 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 simultaneously 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, it 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 loosening wires can all result in a short circuit.

Ground Fault Fuse Blows

Ground faults result from 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 visually or by checking it with a testing device such as an ohmmeter or a continuity tester.

Perform a Visual Test

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

Use a Testing Device

Continuity testers and ohmmeters 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, power flows 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 you are standing on a dry floor and 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 counterclockwise. 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. Most 60-amp fuse boxes will have 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 do not have a visual clue they are blown, 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.

Things to Consider When Choosing a Fuse Box

When choosing to change your fuse box, the primary considerations are size or circuit rating, type, cost, and capacity considerations (the amount of power needed by all electrical devices).

Size or Circuit Rating

The size of the fuse box needed for an individual household depends on the number of circuits you have. Every circuit requires about 20 amps, so multiply by the number ofelectrical outlets. Most homes operating on a fuse box have a 60-amp total capacity.

However, most modern homes have many higher-load devices requiring more amps. The average home operates on 100 to 200 amps. If your electric load has exceeded 60 amps, you might consider upgrading to a circuit breaker.

Type

There are two types of fuse boxes—Type-S and Type-T. The main difference between them is the base. Type-S fuses use a different-size base with a rejection feature that will only allow the right-sized Edison-type socket with an adapter. Type-T fuses have Edison bases resembling a light bulb base. These are typically found on older panels. The different size bases prevent the wrong fuse from being used in the wrong fuse box. For example, a 20-A fuse in a circuit rated 15-A may cause an overload.

Price

Fuse boxes are less costly to buy and replace blown bulbs. Replacing a fuse box will run between$200 to $2,000.

Capacity Considerations

Most modern homes operate on more than 60 amps, the usual maximum capacity of a fuse box. If the fuse box needs replacement, it might be a good time to consider updating to a circuit breaker system. Replacing a fuse box with a breaker box runs from$1,500 to $2,000. If the wiring needs a complete overhaul, you might spend over$4,500.

Fuse boxes are cheaper to have and maintain, but if something overloads in a fuse box, wires can melt, and all the wiring will need replacement. Conversely, this doesn't happen with circuit breakers; if a circuit overloads, it trips a switch, stopping the issue.

Are Fuses and Fuse Boxes Code-Compliant?

The National Electrical Code 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 provide.

Fuses and fuse boxes are not inherently hazardous. 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 a 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 allow for an overload of the circuit and result in a fire. Later types of fuse boxes have a rejection feature that requires that 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 replacing a fuse, consider contacting an electrician to complete that task.

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Understanding Fuses and Fuse Boxes (2024)

FAQs

Understanding Fuses and Fuse Boxes? ›

Fuse boxes use disposable fuses to safeguard the circuit from overloading and shorting out. To shut off power to a particular circuit, you must physically remove the fuse. It's also possible for fuses to blow, requiring you to replace them before the power will work on that circuit again.

How to understand fuse box? ›

Understanding the Fuse Box Anatomy

If it's a breaker panel, you'll see columns of circuit breaker switches with levers marked ON/OFF. Whenever the breaker trips, it falls back to the 'OFF' side, so you'll simply flip it back to 'ON'. For a fuse panel, you'll basically have a column of fuses lying side by side.

How to know which fuse is for what? ›

Check the inside of the lid for a diagram of the devices each fuse controls. Your manual may also include a diagram but, if all else fails, you can also find information on your specific vehicle online. The diagram will also list the fuse's amperage, which you'll need to know to purchase a replacement.

What do the letters mean on fuses? ›

Here is a helpful guide to common fuse markings used with instrumentation, taking a common fuse type as an example: T 6.3A L 250V The first letter, “T” in this case, is the fuse acting speed. Fast-acting fuses disconnect the circuit power more quickly than slow-acting. The current rating is next. In this case, “6.3A”.

How do you know if a fuse is blown in a fuse box? ›

Remove the fuse from its holder. In some cases you may need a small screwdriver to unscrew the fuse holder cap. Look at the fuse wire. If there is a visible gap in the wire or a dark or metallic smear inside the glass then the fuse is blown and needs to be replaced.

How to read a house fuse box? ›

Read and Check the Labels

Once you open the box you'll notice two columns of switches. Power from each switch is sent to a specific circuit. Each circuit should be labeled clearly. The circuits could be numbered; the numbers will correspond to the switches and will be listed inside the breaker's side panel.

What do numbers on fuses mean? ›

The ampere rating is almost always shown on the fuse and is usually expressed in amperes (eg. 10A or 0.5A), but it could also be expressed in milliamps (eg. 100mA). Sometimes the ampere rating is incorporated into the fuse part number eg. FNQ-5, which is a 5A fuse.

How to identify correct fuses used with electrical appliances? ›

Choosing the Correct Fuse

The way the code of practice indicates this should be done is by looking at the appliance power rating. This is generally written in Watts. If the appliance is under 700watts then a 3amp fuse should be fitted. However if the appliance is over 700 watts then a 13 amp fuse should be fitted.

What amp fuse do I need? ›

The fuse is intended to protect the appliance. An appliance that's rated up to 700 watts should be protected with a 3 amp fuse, whilst and appliance rated above 700 watts needs a 13 amp fuse. Some appliances (such as your TV) may need a 5 amp fuse, but always check the appliance manufacturer's instructions first.

What is the most commonly used fuse? ›

The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs: if the device works at 3 A, use a 5 A fuse. if the device works at 10 A, use a 13 A fuse.

What type of fuse for AC? ›

Typically, AC systems use “TR”-type fuses to ensure safe system operation. However, if you're in a pinch and need to replace a fuse quickly, you can use a fuse that is rated for a lower amperage than what the system is rated.

How do I know what fuses I need? ›

First, you will need to determine the maximum fuse rating in amperages by applying this formula for calculation: P (Watts) ÷ V (Voltage) = I (Amps). Next, you will need to calculate the minimum fuse rating or amperage by multiplying that total (the maximum amperage rating) by 125%.

What does f mean on a fuse? ›

Alternatively, “F” stands for “Fast” or “fast acting fuses” with the “FF” rating for ultra-rapid fuses such as the one shown here. The ultra-rapid fuses operate at higher temperatures so are ideal to provide circuit protection to semiconductor devices.

What does L stand for in fuses? ›

In this case, “6.3A”. “L” is the fuse breaking capacity (or short circuit rating) which is defined as the amount of current that will cause the fuse to open if a fault or short circuit occurs. Low indicates very little over-current will cause an open.

What do the numbers on a fuse box mean? ›

The ampere rating is almost always shown on the fuse and is usually expressed in amperes (eg. 10A or 0.5A), but it could also be expressed in milliamps (eg. 100mA). Sometimes the ampere rating is incorporated into the fuse part number eg.

How do I know how many amps my fuse box is? ›

Label on the electrical panel: You may also find a label that indicates the panel's amperage rating. The label may be affixed by the manufacturer, or by an inspector when your panel was first installed or inspected.

How do I calculate which fuse to use? ›

Divide the power of the appliance (in watts) by 230V (standard UK mains voltage). Imagine if an appliance rating was 1900W. Dividing this by 230V gives us a rating of 8.3 Amps. Fit the next highest fuse to the appliance which would be a 13 amp fuse.

What should I look for in a fuse box? ›

Look for danger signs

Heat is a sign that danger is afoot. If the door to the fuse box is abnormally warm to the touch, you should contact an electrician who can examine it further. You should also contact an electrician if you smell burning plastic or can hear audible crackling.

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