Maintaining the safety of your old fuse box

Before breakers were the standard, homes and facilities were built with fuse boxes that contained plug fuses. Plug fuses are still found in many older homes and businesses.

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. This quick review should help you to understand plug fuses and their proper application. Proper plug fuse usage helps to avoid overheated wires and potential electrical fires.

Plug fuses were commonly used for overcurrent protection prior to modern-day circuit breakers. The most common protected branch circuit sizes are 15 amp (AWG #14) and 20 amp (AWG #12).

Although plug fuses work well when properly sized, building owners usually lack the knowledge to maintain the proper fuse sizing over time. Early plug fuse designs allowed for indiscriminate interchanging of fuses regardless of proper amp ratings.

15 20 amp fuse
Plug fuses must be sized correctly by the owner throughout the life of the building to prevent overheating of branch-circuit conductors.

The Type-T plug fuse was originally manufactured with screw shell threads identical to the threads used in a light bulb socket (shown below).

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.

type t plug fuse
Type-T fuses can be interchanged regardless of amp ratings. The fuse threads are identical to a standard medium-base light bulb.

As a result of these deficiencies, a new Type-S, plug fuse design evolved and was required by newer electrical codes (below). The new fuse amperage sizes were not interchangeable in the fuse sockets. The use of porcelain threads prevented the use of conductive objects like pennies from bypassing the fuse link.

type s plug fuse
Newer, Type-S plug fuses have non-interchangeable porcelain threads. Non-removable adapters are shown to convert Type-T sockets to Type-S plug fuse sockets.

Fuse manufacturers developed plug fuses with time-delay characteristics to compete with new circuit breakers. Type-TL and SL fuses are considered to be regular duty fuses. Type T and Type-S are heavy duty fuses for high inrush current applications and typically use a dual-element design.

From a technical sense, plug fuse and circuit breakers are both capable of performing equivalent overcurrent protection. As a practical matter, circuit breakers are permanently installed, easily resettable and easier for the owner to understand and use. This prevents the owner from needing to know the correct size fuse to use and to have spare fuses on-hand for when fuses blow.

Installing Type-T to Type-S adapters should be performed by a licensed electrician because special knowledge of the existing wire sizes is required for proper and safe application. Plug fuse adapters are non-removable, locking devices.


© 2017 The Hartford Steam Boiler Inspection and Insurance Company. All rights reserved. This article is for informational purposes only. Under no circumstances shall HSB or any party involved in creating or delivering this article be liable for any loss or damage that results from the use of the information or images contained in or linked to in this article.

John A. Weber

John Weber is principal electrical engineer for HSB. He has over 25 years’ experience in solving facilities and electrical engineering challenges.


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