NEC Article 240 Fuses and Circuit Breakers for the Journeyman Exam: Plug Fuses, Type S, Interrupting Ratings and Slash-Rated Breakers

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The devices, not the sizing

240.4, the standard sizes in 240.6 and where protection goes are on the services and overcurrent protection page. Here we look at the fuses and breakers themselves: fuse types, breaker markings, and how to tell whether a breaker is rated for the system in front of you.

Plug fuses (240.50 to 240.54)

Item	Rule
Voltage	125 V or less between conductors, or a grounded system where no conductor exceeds 150 V to ground
Shape	Plug fuses of 15 A and lower have a hexagonal window, cap or other prominent part, so they can be told apart by sight
Screw shell	The screw shell of a plug fuseholder is connected to the load side of the circuit
Edison base	Replacements only, in existing installations with no evidence of overfusing or tampering
Type S classes	0 to 15 A, 16 to 20 A, and 21 to 30 A. A fuse from a higher class cannot fit a holder for a lower class
Type S adapters	Fit Edison-base fuseholders, are designed to be nonremovable once installed, and are made so they cannot be easily altered

Why Type S exists: an Edison-base holder takes a 30 A fuse just as easily as a 15 A one, even on 14 AWG copper that generally needs 15 A protection. A Type S adapter screws in once and then accepts only its own size class.

Cartridge fuses (240.60, 240.61)

• 300 V type. A 300 V cartridge fuse or fuseholder is used only on circuits of 300 V or less 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 V.
• Noninterchangeable. Fuseholders are designed so it is difficult to put a fuse of a given class into a holder designed for a lower current or a higher voltage than that class. Fuseholders for current-limiting fuses do not accept fuses that are not current-limiting.
• Marking. Cartridge fuses are marked with ampere rating, voltage rating, interrupting rating when it is other than 10,000 A, current limiting where applicable, and the manufacturer.
• Classification. Fuses are classified by the listing, such as Class H, K, R, J, T, G, CC and L. Class R holders reject fuses that are not Class R.

Circuit breakers (240.80 to 240.87)

• Trip free and manual operation. A breaker can be closed and opened by hand, and it trips even if the handle is held closed.
• ON is up. Where breaker handles are operated vertically, the up position is ON.
• Not easily changed. The trip setting or time needed to operate cannot be changed without dismantling the breaker or breaking a seal, except where adjustable by design and accessible only to qualified persons.
• Markings. The ampere rating is durably marked and visible after installation. The interrupting rating is marked when it is other than 5,000 A. Breakers used as switches in 120 V and 277 V fluorescent lighting circuits are listed and marked SWD or HID; breakers used as switches in HID lighting circuits are marked HID.
• Supplementary protection. Supplementary overcurrent devices, such as fuses inside a luminaire, cannot replace the required branch-circuit overcurrent protection (240.10). They do not have to be readily accessible, under the supplementary-device provision in 240.24.

Interrupting rating versus ampere rating

The ampere rating is the device’s rated current. A breaker does not trip the instant current goes past that number; how long it takes depends on how far over the rating the current is and on the breaker’s trip curve. The interrupting rating is the most fault current it can safely open. A 20 A breaker with a 10,000 A interrupting rating installed where 22,000 A of fault current is available can fail violently during a short circuit. An overcurrent device needs an interrupting rating at least equal to the available fault current at its line terminals, unless it is part of a series-rated combination allowed by 240.86.

Series ratings (240.86). A downstream breaker can be used where the available fault current is above its own interrupting rating when the combination meets 240.86. Normally that means a tested series-rated combination, identified and marked on the equipment, using only the identified devices. The section also limits motor contribution between the two devices. For existing installations, the section also allows an engineered series rating selected under the supervision of a licensed professional engineer, with its own documentation and field marking.

Slash ratings (240.85)

Breaker rating	System	OK?
120/240 V	120/240 V single-phase, 3-wire	Yes
120/240 V	208Y/120 V, 3-phase, 4-wire	Yes, on a solidly grounded 208Y/120 V system. 120 V to ground and 208 V between conductors are both within the ratings
120/240 V	240 V high-leg delta, circuit that uses the high leg	No. The high leg is about 208 V to ground, above the 120 V slash
120/240 V	240 V high-leg delta, circuit on the two 120 V legs only	Yes. Neither conductor is more than 120 V to ground
480Y/277 V	480 V ungrounded or corner-grounded delta	No. Use a breaker with a straight 480 V rating
Straight 240 V	System with no more than 240 V between conductors	Yes, when the breaker’s listing and pole configuration fit the application

Arc energy reduction (240.87)

If a breaker’s highest continuous current trip setting, including the highest setting it can be adjusted to, is 1,200 A or more, 240.87 requires documentation, an approved way to reduce clearing time, and performance testing of that method. Examples include zone-selective interlocking, differential relaying, and an energy-reducing maintenance switch with a local status indicator. Check 240.87 in your NEC for the full list.

Worked example: an old fuse box

A fuse panel in a 1950s house has a 30 A Edison-base fuse in a socket feeding 14 AWG copper. You find no other signs of tampering.

1. Allowed wire protection: 14 AWG copper is generally limited to 15 A overcurrent protection.
2. Problem: a 30 A fuse on 14 AWG is overfusing. Edison-base fuses are allowed only as replacements where there is no evidence of overfusing or tampering, so putting another Edison-base fuse in this socket would not meet 240.51.
3. Correction: first have the fuseholder, wiring and connected loads checked by a qualified electrician. If they are suitable, a 15 A Type S adapter with a matching 15 A Type S fuse keeps a larger fuse out. After the adapter is in, only a 0 to 15 A Type S fuse fits.

Five quick questions

1. Maximum voltage between conductors for plug fuses on a circuit without a grounded neutral point? 125 V.
2. Which way is ON for a vertically operated breaker handle? Up.
3. A 120/240 V slash-rated breaker on a circuit that uses the high leg of a 240 V high-leg delta. OK? No.
4. Breaker interrupting rating that does not need to be marked? 5,000 A.
5. Cartridge fuse interrupting rating that does not need to be marked? 10,000 A.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 110 and 240