A Guide to Screw-in Fuses and How They Work

A fuse box is a type of electrical service panel, which is a sort of control board for 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 provide over-current protection for the household circuits.

A fuse box has a series of 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.

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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 catches fire.

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 to socket adapters and Type S fuses makes the panel much safer.
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Type-W Fuses

Type-W fuses are an 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
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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 contain a 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 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
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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 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

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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

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