Code Q&A: Edison-Base Fuses and Type S Adapters

Feb. 7, 2013

Your most pressing National Electrical Code (NEC) questions answered

Q. What are the NEC requirements for the installation of Edison-base fuses and Type S adapters?

A. Edison-base fuses are classified to operate at not more than 125V and have an ampere rating of not more than 30A [240.51(A)]. In addition, the fuses are permitted only for replacement in an existing installation where there's no evidence of tampering or overfusing [240.51(B)]. Edison-base fuseholders must be used only if they're made to accept Type S fuses by the use of adapters [240.52].

Type S adapters are designed to fit Edison-base fuseholders [240.54(A)], and must be designed for Type S fuses only [240.54(B)]. Type S adapters are designed so they can't be removed once installed [240.54(C)]. Type S fuses, fuseholders and adapters must be designed so that tampering or shunting would be difficult [240.54(D)]. Dimensions of Type S fuses, fuseholders, and adapters shall be standardized to permit interchangeability regardless of the manufacturer [240.54(E)].

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.