A Guide to Screw-in Fuses and How They Work

A fuse box is an older type of electrical service panel that serves as a control board for a house's entire electrical system. While any home built around 1960 or later has a service panel full of circuit breakers, panels in older houses used fuses to provide over-current protection for household circuits.

A fuse box has a series of threaded sockets into which the fuses are screwed like light bulbs. A fuse protects each circuit in the home, and each fuse must be the correct type and have an appropriate amperage rating for its circuit. Using the wrong kind of fuse for a circuit can pose a severe fire hazard, so it's essential to identify the correct fuse for each circuit.

Here's how to understand the difference between screw-in fuses, bases, power rating, and what they're used for.

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 types of bases and screw-in fuses: the Edison base (found on Type T fuses) and the rejection base (found on Type S fuses).
- Edison base (Type T): This base resembles a light bulb base and fits the standard sockets in old fuse boxes. It can fit into any Edison socket, regardless of the circuit's amperage.
- Rejection base (Type S) fuses: Also known as "tamper-proof," these bases 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 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 particular thread size that prevents fuse mismatching. A 15-amp Type S fits only a 15-amp base adapter.

This failsafe stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially grave mistake. Overfusing can result in the fuse failing to blow before the circuit wiring overheats and potentially catches fire.

Tip

If you have an old fuse box with Edison sockets, switching to socket adapters and Type S fuses makes the panel much safer.
02 of 05

Type-W Fuses

Type-W fuses are an older style of fuse utilizing an Edison base and are practically 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 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 that are now the most commonly used plug fuses in home electrical systems. The only difference between them is the type of base: the SL fuse has a rejection base, and the TL fuse has an Edison base.

SL and TL fuses contain a plug of heat-absorbing solder 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 demand when 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 with critical 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 is the base: type-S has a rejection base, and 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 into the circuit. This allows the fuse to absorb a longer temporary circuit overload than 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. You only need to 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

FAQ

How do you tell if a screw-in fuse is blown?

Usually, you can tell a screw-in fuse is blown by looking at it. The fuse will look darkened with ash or broken. You can also tell by testing the fuse with a multimeter tool.
What is a bolted-type fuse?

Bolted-type fuses are used in industrial, electrical, and automotive vehicles. They are "bolted in" to stay fixed in place.
How can I reset my old fuse box?

A fuse box is reset by changing a blown fuse. Circuit breakers have replaced fuse boxes; for those, you flip a switch to reset them. But fuses need to be removed, discarded, and replaced. One exception is if you have a special mini-breaker fuse. It has a little button that pops out when the circuit is overloaded. You only need to push the button back in to reset the breaker.

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.