The new fuse amperage sizes were not interchangeable in the fuse sockets. The fuse threads are identical to a standard medium-base light bulb.Īs a result of these deficiencies, a new Type-S, plug fuse design evolved and was required by newer electrical codes (below). Type-T fuses can be interchanged regardless of amp ratings. The problem with this design was that any amperage size fuse could be interchanged with the others in the “fuse cabinet.” In addition, a copper penny fit perfectly in the bottom of this socket and could be used to bypass the protective fuse link when fuses blew due to overloading. The Type-T plug fuse was originally manufactured with screw shell threads identical to the threads used in a light bulb socket (shown below). Plug fuses must be sized correctly by the owner throughout the life of the building to prevent overheating of branch-circuit conductors. Early plug fuse designs allowed for indiscriminate interchanging of fuses regardless of proper amp ratings. The most common protected branch circuit sizes are 15 amp (AWG #14) and 20 amp (AWG #12).Īlthough plug fuses work well when properly sized, building owners usually lack the knowledge to maintain the proper fuse sizing over time. Plug fuses were commonly used for overcurrent protection prior to modern-day circuit breakers.

Proper plug fuse usage helps to avoid overheated wires and potential electrical fires. This quick review should help you to understand plug fuses and their proper application. Owners are sometimes confused about the different amp ratings and types of plug fuses that are installed in the fuse box or nearby for use as spares. Plug fuses are still found in many older homes and businesses. Before breakers were the standard, homes and facilities were built with fuse boxes that contained plug fuses.

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