Screw-in plug fuses are found on old home wiring systems that include a fuse panel instead of a modern circuit breaker panel. Most of the fuses in a fuse panel are screw-in type, while circuits for large electric appliances, like ranges and clothes dryers, may have cartridge-type fuses. When properly installed and maintained, fuse panels effectively protect circuits from overloads and other hazards, but panels that accept the older type of fuses can be dangerous if homeowners aren't careful. For this reason, these panels should be fitted with fuse adapters that accept only tamper-proof screw-in fuses.
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How Screw-In Plug Fuses Work
Screw-in plug fuses are the most common type of fuses used in a home's fuse panel. They come with ratings of 15, 20, or 30 amps (amperes), indicating how much electrical current they can safely handle before the fuse blows and shuts off the circuit.
Each fuse controls the amount of amperage that is allowed to flow through the circuit it protects. Plug fuses control 120-volt circuits that normally feed lighting and receptacle (outlet) circuits throughout a home.
Plug fuses have a metal-alloy ribbon inside that carries the current of the circuit. In the case of a short or overload causing too much current to flow, the ribbon will melt and open the circuit. When this happens, no current can flow and the circuit is disconnected from the power supply. This protects against short circuits and overloads that may damage electrical wiring and cause house fires.
Plug fuses have a clear glass window through which you can see the metal-alloy ribbon that carries the current. When a fuse blows, the window usually is blackened so you can easily identify which fuse must be replaced.
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20-Amp Plug Fuse
A 20-amp plug fuse is a screw-in fuse that can handle electrical current up to 20 amps without the fuse blowing. These are generally used for outlet and appliance circuits.
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30-Amp Plug Fuses
A 30-amp plug fuse is the largest-capacity plug fuse and is most commonly used for circuits supplying electric clothes dryers or window air conditioners.Continue to 5 of 6 below.
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Tamper-proof fuses are the current standard and the most commonly sold type of fuse today. Also called Type-S fuses, they were developed to prevent homeowners from using the wrong size of replacement fuse and creating a potential fire hazard.
Type-D fuses, the precursors to Type-S, had a one-size-fits-all base, making it possible to screw a 20- or 30-amp fuse into a 15-amp socket in a fuse panel. However, the wiring in a 15-amp circuit is designed for a maximum of 15 amps; if you use a 20- or 30-amp fuse to protect the circuit—called "over-fusing"—the wiring could overheat and possibly start a fire without the fuse ever blowing.
Tamper-proof fuses have a specific size of base for each fuse size, so that a 20-amp fuse won't fit in a 15-amp socket, for example. Many fuse panels are designed to accept only Type-S fuses, while very old panels may accept only Type-D fuses.
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Plug Fuse Adapters
If you have an older-style fuse panel made for Type-D plug fuses, it's a very good idea to convert the panel to Type-S (tamper-proof) fuses. This is easy to do with plug fuse adapters. The adapters come in 15-, 20-, and 30-amp sizes, and they simply screw into the panel sockets. Once the adapters are in place, you screw a tamper-proof fuse into the socket, and it works just as well as an old Type-D fuse the socket was originally designed for.
With adapters installed in a panel, you can no longer use Type-D fuses, and you completely eliminate the danger of accidentally over-fusing a circuit.