7.4 Fuses, Circuit Breakers, Series Ratings & Article 242 Surge Protection
Key Takeaways
- NEC 240.53 limits Type S plug fuses to 125 volts maximum and 30 amperes maximum, classified at not over 0 to 15, 16 to 20, and 21 to 30 amperes, with non-interchangeable adapters that prevent over-fusing.
- Class RK1 fuses provide greater current limitation and lower peak let-through current than Class RK5 fuses of identical voltage and ampere rating.
- NEC 240.86 permits series ratings only where selected under engineering supervision in existing installations, or where the combination is tested and marked on the end-use equipment, and 240.86(C) prohibits series ratings where motor contribution exceeds 1 percent of the interrupting rating.
- Article 242 in the 2020 NEC consolidates the former Articles 280 and 285 into a single article covering overvoltage protection and surge-protective devices.
- NEC 242.14 classifies surge-protective devices as Type 1 for line-side or load-side connection, Type 2 for load-side connection only, Type 3 for point of utilization at least 10 meters from the service, and Type 4 for components.
Plug Fuses — 240.50 through 240.54
240.50(A) General. Plug fuses of the Edison-base type shall be classified at not over 125 volts and 30 amperes and below.
240.50(B) Admissible. Plug fuses shall be permitted in circuits not exceeding 125 volts between conductors, or in circuits supplied by a system having a grounded neutral where the voltage to ground does not exceed 150 volts.
240.50(C) Screw Shell. The screw shell of a plug-type fuseholder shall be connected to the load side of the circuit.
240.50(D) Marking. Each fuse, fuseholder, and adapter shall be marked with its ampere rating.
240.51 Edison-Base Fuses. Edison-base fuses shall be classified at not over 125 volts and 30 amperes and below. 240.51(B) Replacement Only — plug fuses of the Edison-base type shall be used only for replacements in existing installations where there is no evidence of over-fusing or tampering.
240.52 Edison-Base Fuseholders. Fuseholders of the Edison-base type shall be installed only where they are made to accept Type S fuses by the use of adapters.
240.53 Type S Fuses. Type S fuses shall be of the plug type and shall comply with (A) and (B):
- (A) Classification. Type S fuses shall be classified at not over 125 volts and 0 to 15 amperes, 16 to 20 amperes, and 21 to 30 amperes.
- (B) Noninterchangeable. Type S fuses of an ampere classification specified in (A) shall not be interchangeable with a lower ampere classification. They shall be designed so that they cannot be used in any fuseholder other than a Type S fuseholder or a fuseholder with a Type S adapter inserted.
240.54(C) Nonremovable. Type S adapters shall be designed so that once inserted in a fuseholder, they cannot be removed.
240.54(D) Nontamperable. Type S fuses, fuseholders, and adapters shall be designed so that tampering or shunting (bridging) would be difficult.
The entire Type S system exists to prevent over-fusing: once a 15-ampere adapter is installed, only 15-ampere fuses fit, permanently.
Cartridge Fuses — 240.60 and Fuse Classes
240.60(A) 300-Volt Type Cartridge Fuses. Permitted in single-phase line-to-neutral circuits supplied from a three-phase, 4-wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 volts.
240.60(B) Noninterchangeable. Fuseholders shall be designed so it will be difficult to put a fuse of any given class into a fuseholder that is designed for a current lower, or voltage higher, than that of the class to which the fuse belongs.
240.60(C) Marking. Cartridge fuses and fuseholders shall be marked with ampere rating, voltage rating, interrupting rating where other than 10,000 amperes, current limiting where applicable, and the name or trademark of the manufacturer.
Common Fuse Classes
| Class | Ratings | Characteristics |
|---|---|---|
| H | 250 V / 600 V, up to 600 A | Non-current-limiting, 10,000 A interrupting rating |
| K5, K1 | 250 V / 600 V | Current limiting, dimensionally interchangeable with Class H |
| RK5 | 250 V / 600 V, up to 600 A | Current limiting, 200,000 A interrupting, time-delay, rejection feature |
| RK1 | 250 V / 600 V, up to 600 A | Current limiting, 200,000 A interrupting, greater current limitation and lower peak let-through than RK5 |
| J | 600 V, up to 600 A | Current limiting, 200,000 A interrupting, smaller physical size, own rejection dimensions |
| L | 600 V, 601 to 6000 A | Current limiting, bolt-in |
| T | 300 V / 600 V | Very compact, current limiting |
| CC | 600 V, up to 30 A | Rejection type for control circuits and motor branch circuits |
| G | 480 V, up to 60 A | Smaller size, rejection type |
The RK1 versus RK5 comparison is a standard exam item. Both are Class R, both are rejection type, both carry a 200,000-ampere interrupting rating — but RK1 limits let-through current more aggressively, producing lower peak current and lower I-squared-t energy, which protects downstream equipment better. RK5 is the more common general-purpose time-delay fuse; RK1 is specified where component protection matters.
Circuit Breaker Basics
A thermal-magnetic molded-case circuit breaker has two trip mechanisms:
- A thermal (bimetallic) element that responds to sustained overload with an inverse time delay — the higher the overload, the faster it trips;
- A magnetic element that responds instantaneously to a high-magnitude short circuit.
Inverse time in NEC language means a breaker with an intentional delay that decreases as current increases. An instantaneous trip breaker has no intentional time delay and is used in motor circuits only as part of a listed combination controller (430.52(C)(3)).
240.80 requires circuit breakers to be trip free and capable of being closed and opened by hand. Nonautomatic operation by other means is permitted if the breaker can also be operated manually.
240.82 Nontamperable. A circuit breaker shall be of such design that any alteration of its trip point or the time required for its operation requires dismantling of the device or breaking of a seal for other than intended adjustments.
Series Ratings — 240.86
A series-rated combination allows a downstream breaker with a low interrupting rating to be used where the available fault current exceeds that rating, because an upstream device is tested to clear the fault first.
240.86(A) Selected Under Engineering Supervision in Existing Installations. The series-rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped. The engineer must ensure that the downstream circuit breakers used remain passive during the interruption period of the line-side fully rated, current-limiting device. That documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system.
240.86(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breakers is tested and marked on the end-use equipment, such as switchboards and panelboards.
240.86(C) Motor Contribution. Series ratings shall not be used where the sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker. Running motors feed current into a fault and can defeat the coordination the series rating relies on.
110.22(C) requires equipment enclosures for series-rated combinations to be legibly marked in the field to indicate the equipment has been applied with a series combination rating, with the marking readily visible and stating "Caution — Series Combination System Rated _____ Amperes. Identified Replacement Components Required."
Series rating is not the same as selective coordination, which requires the downstream device alone to clear a fault so upstream devices stay closed. Selective coordination is required for emergency systems (700.32), legally required standby (701.32), critical operations power systems (708.54), and elevator feeders (620.62).
Article 242 — Overvoltage Protection
The 2020 NEC consolidated the former Article 280 (Surge Arresters, Over 1000 Volts) and Article 285 (Surge-Protective Devices, 1000 Volts or Less) into a single Article 242, Overvoltage Protection. Part I covers general requirements, Part II covers surge-protective devices 1000 volts or less, and Part III covers surge arresters over 1000 volts.
242.6 / 242.14 SPD Types. Surge-protective devices are designated:
| Type | Description |
|---|---|
| Type 1 | Permanently connected SPD intended for installation between the secondary of the service transformer and the line side of the service disconnect overcurrent device, as well as the load side, including watt-hour meter socket enclosures |
| Type 2 | Permanently connected SPD intended for installation on the load side of the service disconnect overcurrent device, including SPDs located at the branch panel |
| Type 3 | Point of utilization SPD, installed at a minimum conductor length of 10 m (30 ft) from the electrical service panel to the point of utilization |
| Type 4 | Component SPD, including discrete components as well as assemblies |
242.12 Uses Not Permitted. An SPD shall not be installed where the rating of the SPD is less than the maximum continuous phase-to-ground power frequency voltage available at the point of application; on ungrounded, impedance-grounded, or corner-grounded delta systems unless listed specifically for use on those systems; or on a circuit exceeding the SPD's rating.
242.16 Number Required. Where used at a point on a circuit, the SPD shall be connected to each ungrounded conductor.
242.24 Routing of Conductors. The conductors used to connect the SPD to the line or bus and to ground shall not be any longer than necessary and shall avoid unnecessary bends. Lead length matters enormously to SPD performance — inductance in the leads adds voltage during a surge, so a long, looping SPD lead can defeat the device.
242.22 Short-Circuit Current Rating. The SPD shall be marked with a short-circuit current rating and shall not be installed at a point on the system where the available fault current is in excess of that rating.
Where SPDs Are Now Required
The 2020 NEC added several mandatory SPD requirements:
- 230.67 Surge Protection. All services supplying dwelling units shall be provided with a surge-protective device. It shall be an integral part of the service equipment or located immediately adjacent thereto, and it shall be Type 1 or Type 2. Where service equipment is replaced, all requirements of this section apply.
- 242.14 parallels this for the general case.
- Article 645 (information technology equipment), Article 646 (modular data centers), Article 670 (industrial machinery, via 670.6), Article 695 (fire pumps, via 695.15), and Article 708 (critical operations power systems, via 708.20) each contain SPD requirements.
The 230.67 dwelling requirement is a frequent exam item and its details matter: all dwelling services, Type 1 or Type 2, at or immediately adjacent to the service equipment, and triggered again on service equipment replacement.
What are the maximum voltage and ampere classifications for Type S plug fuses under NEC 240.53?
Comparing Class RK1 and Class RK5 cartridge fuses of the same voltage and ampere rating, which statement is correct?
Under NEC 240.86(C), when may a series-rated combination NOT be used?
What type of surge-protective device does NEC 230.67 require for a dwelling unit service?