How Do Fuses and Fuse Boxes Work? (2024)

Home Improvement



Bob Formisano

Bob Formisano

Bob Formisano is a licensed architect and builder with nearly 40 years of experience building new homes and restoring older homes. One of his specialties is repairing old systems dating back to the 1920s, including galvanized water pipes, knob-and-tube wiring, and more. His home repair articles for The Spruce span more than 10 years.

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Updated on 03/16/23

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Larry Campbell

How Do Fuses and Fuse Boxes Work? (1)

Reviewed byLarry Campbell

Larry Campbell is an electrical contractor with 36 years of experience in residential and light commercial electrical wiring. He worked as an electronic technician and later as an engineer for the IBM Corp. He is also a member of The Spruce Home Improvement Review Board.

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Jillian Dara

How Do Fuses and Fuse Boxes Work? (2)

Fact checked byJillian Dara

Jillian is a freelance journalist with 10 years of editorial experience in the lifestyle genre. She is a writer and fact checker for TripSavvy, as well as a fact-checker for The Spruce.

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How Do Fuses and Fuse Boxes Work? (3)

Afuse box is a type ofelectrical service panel, which is a sort of control boardfor the entire electrical system of a house. While any home built around 1960 or later has a service panel full of circuit breakers, panels in older houses used fuses to provideover-current protectionfor the householdcircuits.

A fuse box has a seriesof threaded sockets into which the fuses are screwed in like light bulbs. Each circuit in the home is protected by a fuse, and each fuse must be the correct type and have an appropriate amperage rating for its circuit. Using the wrong type of fuse for a circuit can pose a serious fire hazard, so it's important to identify the correct fuse for each circuit.

  • 01 of 05

    Screw-in Fuse Bases

    Fuses for standard circuits (not high-voltage appliance circuits) are called plug fuses and have screw-in bases. There are two different types of bases and screw-in fuses: the Edison base (found on Type T fuses) and the rejection base (found on Type S fuses).

    The Edison base (Type T)looks like a light bulb base and fits the standard sockets found in old fuse boxes. Rejection base (Type S) fuses will work with Edison-type sockets only when combined with an adapter base that screws and locks into the Edison socket. The Type S fuse then screws into the adapter.

    Rejection bases are also known as "tamper-proof," and they were developed to prevent homeowners from using the wrong type of fuse for a circuit.Each Type S fuse of a specific amperage rating has a matching base adapter with a specific size of thread that prevents mismatching the fuses. For example, it stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially serious mistake. This condition is called over-fusing and can result in the fuse failing to blow before the circuit wiring overheats and potentially catchesfire.

    A 15-amp Type S fits only a 15-amp base adapter. By contrast, a Type T fuse can fit into any Edison socket, regardless of the circuit's amperage. If you have an old fuse box with Edison sockets, switching tosocket adapters and Type S fuses makes the panel much safer.

  • 02 of 05

    Type-W Fuses

    Type-Wfuses arean older style of fuse utilizing an Edison base and are all but obsolete today. They are general-purpose plug fuses and are fast-acting—that is, they have no time-delay fuse element and quickly interrupt the circuit once the fuse's rated amperage is exceeded.

    These fuses are designed for use in general lighting and power circuits that do not contain electric motors. Electric motors draw additional current at startup and will blow a Type W fuse if the motor is of any significant size. Because of this, time-delay fuses are used much more commonly than type-W fuses.

    Type-W fuse rating: 120 volts; up to 30 amps

  • 03 of 05

    Type-SL and Type-TL Fuses

    SL and TL fuses are medium-duty time-delay fuses and are now the most commonly used plug fuses found in home electrical systems. The only difference between SL and TL fuses is the type of base: the SL fuse has a rejection base, and the TL fuse has an Edison base.

    SL and TL fuses containa plug of heat-absorbing solder that's attached to the center of the fuse element (the part that burns out, or blows, during a circuit overload). This allows the fuse to absorb a temporary circuit overload, such as that caused by a brief surge in power demandwhen a motor starts up. Without a time-delay feature, simply starting your garbage disposer or refrigerator would cause a fuse to blow.

    Type SL and TL fuse rating: 120 volts; up to 30 amps

  • 04 of 05

    Type-S and Type-T Heavy-Duty Time-Delay Fuses

    Heavy-duty time-delay fuses are used for circuits withcritical or high motor loads or circuits serving motors that frequently cycle on and off (such as a sump pump motor). These fuses have a longer time-delay feature than the SL or TL fuses. However, just like the SL and TL fuses, the only difference between the S and the T heavy-duty fuses are the bases: type-S has a rejection base; type-T has an Edison base.

    Heavy-duty time-delay fuses contain a spring-loaded metal fuse link attached to a solder plug. If the overloaded circuit condition continues for too long, the solder plug melts and the spring pulls the fuse link free, cutting power to the circuit. This allows the fuse to absorb a longer temporary circuit overload than with other time-delay fuses.

    Type S and T heavy-duty fuse rating: 120 volts; up to 30 amps

    Continue to 5 of 5 below

  • 05 of 05

    Mini-Breaker Fuse

    Mini-breakers fuses are retrofit circuit breaker fuses that screw into Edison-base fuse sockets. They essentially replace a fuse with a push-button circuit breaker. Mini breakers have a little button that pops out when the circuit is overloaded. All you need to do is push the button back in to reset the breaker. Mini-breakers are also designed for time delay, so they do not trip unnecessarily when motors or appliances start up.

    Mini-breaker fuse rating: 120 volts; up to 20 amps

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Cordero, Ismael. Checking and replacing fuses.Community eye healthvol. 25,78 (2012): 37. PMID:23139454

How Do Fuses and Fuse Boxes Work? (2024)


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