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

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)].

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