Plug Fuses, Type S Fuses and Cartridge Fuse Rules: An NEC Article 240 Guide

This guide from ATEK Distribution explains the NEC Article 240 rules for plug fuses and cartridge fuses in plain English. It is written for electricians, apprentices studying for licensing exams, inspectors and facility maintenance teams. For a broader overview of fuse formats, start with our types of fuses guide. For specifying industrial UL fuse classes, see the industrial fuse selection guide.

A plug fuse is a fuse with a threaded screw base, similar to a light bulb base, that screws into a fuseholder. Plug fuses were the standard overcurrent protection in older residential fuse panels, and they are still found in many existing homes and small buildings. A plug fuse is typically used to protect a branch circuit.

NEC 240.50 sets the general rules for all plug fuses:

• Voltage limit (240.50(A)): plug fuses are permitted only in circuits that do not exceed 125 volts between conductors, or in circuits supplied by a system with a grounded neutral point where the line-to-neutral voltage does not exceed 150 volts. In short, plug fuses are never permitted in circuits exceeding 150 volts to ground.
• Marking (240.50(B)): each fuse, fuseholder and adapter must be marked with its ampere rating.
• Hexagonal shape (240.50(C)): plug fuses rated 15 amperes and below must have a hexagonal window, cap or other prominent part so they can be told apart from higher ratings.
• No exposed live parts (240.50(D)): plug fuses and fuseholders must not have exposed energized parts once the fuse is installed.
• Screw shell on the load side (240.50(E)): the screw shell of the fuseholder must be connected to the load side of the circuit.

An Edison-base plug fuse uses the standard screw base, the same thread as an ordinary lamp. Because every Edison-base fuse fits every Edison-base fuseholder, a 30-ampere fuse can be screwed into a socket protecting 14 AWG wire. That is overfusing, and it is a fire risk. For that reason the NEC restricts them:

• Classification (240.51(A)): Edison-base fuses are classified at not over 125 volts and 30 amperes and below. The highest current rating of an Edison-base plug fuse is 30 amperes.
• Replacement only (240.51(B)): Edison-base fuses may be used only as replacements in existing installations where there is no evidence of overfusing or tampering.
• Fuseholders (240.52): Edison-base fuseholders may be installed only where they are made to accept Type S fuses by the use of adapters.

A Type S fuse is a plug-type fuse designed to stop overfusing. Each Type S fuse has a base sized to its ampere class, and it only fits a matching adapter. Once an adapter is screwed into an Edison-base fuseholder, it locks in place, and that socket will then accept only Type S fuses of the correct class.

• Classification (240.53(A)): Type S fuses are classified at not over 125 volts and in three ampere classes: 0 to 15 A, 16 to 20 A, and 21 to 30 A. Type S fuses are therefore available up to 30 amperes.
• Noninterchangeable (240.53(B)): a Type S fuse of one ampere class cannot be used in a fuseholder or adapter for a lower class, and Type S fuses cannot be used in any holder other than a Type S holder or an Edison-base holder fitted with a Type S adapter.
• Adapters (240.54): Type S adapters must fit Edison-base fuseholders, must be non-removable once installed, must be designed to be difficult to tamper with, and must have standardized dimensions so fuses and adapters from different manufacturers are interchangeable.

Both Edison-base and Type S plug fuses are made in two speeds. Fast-acting (non-time-delay) plug fuses open quickly on any significant overcurrent and suit resistive loads such as lighting and heating. Time-delay (dual-element) plug fuses ride through the brief inrush when a motor starts, so a refrigerator, furnace blower, well pump or window air conditioner does not blow the fuse every time it cycles on, while still opening fast on a short circuit.

If a circuit with a motor load keeps blowing a correctly sized fast-acting fuse at start-up, the fix is a time-delay fuse of the same rating, never a larger fuse.

Converting an Edison-base socket to Type S is the single most effective safety fix in an old fuse panel. It takes a few minutes per circuit:

Most plug fuses have a clear window over the fusible link, and the window tells you what happened:

Signs of overfusing or tampering, which under NEC 240.51(B) mean Edison-base fuses can no longer simply be replaced:

• A fuse rated higher than the conductor allows, such as a 25 A or 30 A fuse on 14 AWG wire
• A coin, foil or wire placed behind a fuse to bypass it
• Scorching, discoloration or melted insulation around a socket
• A fuse panel or fuses that are warm to the touch in normal use
• Mismatched or unmarked fuses and a missing panel directory

Any of these calls for Type S adapters sized to the conductors, and usually a wider inspection of the panel.

Cartridge fuses and fuseholders of the 300-volt type may be used on circuits not exceeding 300 volts between conductors, or on single-phase line-to-neutral circuits supplied from a 3-phase, 4-wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 volts.

Fuseholders must be designed so that it is difficult to insert a fuse of a given class into a fuseholder designed for a lower current or a higher voltage than the fuse. Fuseholders for current-limiting fuses must not accept fuses that are not current-limiting. This is why rejection-style fuseholders exist: a Class R holder, for example, will not accept a non-rejection Class H or K fuse.

Cartridge fuses must be plainly marked with:

1. Ampere rating
2. Voltage rating
3. Interrupting rating, where other than 10,000 amperes
4. “Current limiting,” where applicable
5. The manufacturer’s name or trademark

Because the interrupting rating only has to be marked when it is other than 10,000 amperes, an unmarked cartridge fuse should be treated as having a 10,000-ampere interrupting rating. That is also the minimum interrupting rating of a UL branch-circuit cartridge fuse. The interrupting rating marking is not required on fuses used only for supplementary protection.

Class H cartridge fuses of the renewable type, where the fusible link can be replaced, may be used only as replacements in existing installations where there is no evidence of overfusing or tampering. This is the cartridge-fuse equivalent of the Edison-base rule.

Cartridge fuses and fuseholders are classified according to their voltage and amperage ranges. Fuses rated 1,000 volts nominal or less may be used at voltages at or below their rating.

Branch-circuit cartridge fuses are listed to the UL 248 series in lettered classes. The table below is a memory aid. For selection guidance, including RK1 versus RK5 and Class J versus Class T, see our industrial fuse selection guide.

Class CF fuses combine Class J electrical performance with a finger-safe plug-in body. The class is defined by UL 248-17, an Outline of Investigation, and ampere ranges vary by manufacturer. Always confirm ratings on the manufacturer’s data sheet.

Dual-element fuses are time-delay fuses. They contain two elements in series: an overload element that tolerates short, harmless surges such as motor starting current, and a short-circuit element that opens very fast under fault current. Non-time-delay fuses have a single element and open quickly on any significant overcurrent.

This matters most for motor circuits. NEC Table 430.52 sets the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device as a percentage of motor full-load current:

Percentages apply to most AC polyphase and single-phase motors. Exceptions in 430.52(C) allow the next higher standard size in some cases. Semiconductor (high-speed) fuses are permitted in place of the devices in Table 430.52 for power electronic devices in a solid-state motor controller system, provided the replacement-fuse marking is placed next to the fuses (430.52(C)(5)).

The NEC standard ampere ratings for fuses and inverse-time breakers run from 15 A upward to 6,000 A. For fuses only, 1, 3, 6, 10 and 601 amperes are also standard ratings (highlighted below).

A properly fused panel with Type S adapters protects its conductors well. The practical limits are capacity, since many fuse panels were sized for loads far below today’s, and the lack of AFCI and GFCI breaker options that newer codes call for on many circuits. When a panel is replaced, ATEK supplies the circuit breakers and power distribution equipment for the upgrade.

These questions come up often on journeyman and master electrician exams. Try each one before you open the answer.

Always check your exam’s reference edition. Exam boards set questions against a specific NEC year.

ATEK Distribution supplies fuses, fuse blocks and fuse holders to contractors, facility maintenance teams and government buyers from Minneapolis, Minnesota, shipping nationwide. We are a certified Service-Disabled Veteran-Owned Small Business (SDVOSB), a GSA contract holder and BBB accredited, and we have supplied electrical projects since 2018, including the VA Health Care EHRM project in Minneapolis. Our supply partners include Mersen, a major fuse manufacturer.

1. NFPA 70, National Electrical Code, 2023 edition: Article 240 (Overcurrent Protection), 240.4(D), 240.6(A), 240.50 to 240.61; Table 430.52. nfpa.org
2. UL 248-1, Low-Voltage Fuses, Part 1: General Requirements, and the UL 248 series class standards.
3. UL 248-11, Low-Voltage Fuses, Part 11: Plug Fuses.
4. UL 4248 series, Fuseholders.
5. Manufacturer data sheets for fuse class ratings, time-current curves and let-through data.