Your Home Fuse Box and How Fuses Work

By Bob Formisano, About.com Guide

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The Fuse Box

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Fuse Box and Sockets

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The fuse box is the equivalent of the circuit breaker's electrical service panel in that it is a metal box with a hinged cover that houses and controls the incoming electrical service and distribution to branch circuits within the house. It provides overcurrent protection through the use of fuses.

The fuse box will have threaded sockets into which the fuses will be screwed. These large threaded sockets look like light bulb sockets and are called Edison sockets. They are named after Thomas Edison who, like everything else we take for granted, invented them.

The types of fuses that go into these sockets however are several. There are fuses that have Edison bases, and fuses that have a socket adapter that screws into the Edison base, but the fuse itself screws into the adapter base. These are called "S" fuses and are also called "tamper-proof" fuses with Rejection bases.

Picking the right fuse can be confusing but once you understand what the various codes mean, you can pick the right fuse for the job. The codes on a fuse relate to two things, the type of base and the degree of time delay of the fuse.

Fuses have two types of bases, the Edison Base (Type T fuses) and the Rejection Base (Type S fuses), also known as a "tamper-proof base".

The Edison Base Type T fuse works with the typical socket used in the fuse box seen in older homes and the fuse base looks like a light bulb base.

A Rejection Base Type S fuse actually consists of two components, an adapter base that screws and locks into the Edison socket in the fuse box, and the fuse itself. Each different amperage rating of a Type S fuse has a matching socket base adapter with a different sized thread that prevents mismatching (rejection base). It stops a person from putting a 20 Amp rated fuse in a 15 Amp circuit.

Putting in higher rated fuses than a circuit was designed to handle was a problem with older homes. Once the wrong amperage fuse is installed you really can't tell it is improper because the wire size is the determinant of circuit capacity. So what happens is that people replace the wrong size fuse with another one since they assumed the blown fuse was correct. This resulted in many circuits being dangerously unprotected. Using a Type S fuse with its rejection base adapter prevents tampering or mistakes from being made.Type "W" fuses are an older style fuse utilizing an Edison base and are all but obsolete. They are general purpose plug fuses and "fast-acting", that is they have no time-delay fuse element but will quickly interrupt the circuit once the rated amperage is exceeded.

The fuses are designed for use in general lighting and power circuits that do not contain electric motors. Electric motors draw additional current at start up and will blow these fuses if the motor is of any significant size. Because of this, time-delay fuses are now almost exclusively used.

Rating: 120 volts; up to 30 Amps

SL and TL fuses are medium duty time delay fuses and are now the most commonly used plug fuses found in the home. The only difference between the "TL" and the "SL" fuses are the types of bases. The "TL" fuse uses an Edison base and the "SL" fuse uses a Rejection base.

These fuses work by having a plug of heat absorbing solder attached to the center of the fuse element. This allows the fuse to absorb a temporary circuit overload such as may be caused by a surge power draw when a motor starts up (e.g., garbage disposal). This ability to handle overloading starting currents of a motor without unnecessarily interrupting (blowing) is a key benefit of these types of fuses.

 Type "TL" and "SL" Fuses

Rating: 120 volts; up to 30 Amps

SL and TL fuses are medium duty time delay fuses and are now the most commonly used plug fuses found in the home. The only difference between the "TL" and the "SL" fuses are the types of bases. The "TL" fuse uses an Edison base and the "SL" fuse uses a Rejection base.

These fuses work by having a plug of heat absorbing solder attached to the center of the fuse element. This allows the fuse to absorb a temporary circuit overload such as may be caused by a surge power draw when a motor starts up (e.g., garbage disposal). This ability to handle overloading starting currents of a motor without unnecessarily interrupting (blowing) is a key benefit of these types of fuses.

 Type "TL" and "SL" Fuses

Rating: 120 volts; up to 30 Amps