NEC Article 240 — Overcurrent Protection: The Complete Exam Guide

Overcurrent protection is the safety net of every electrical system. Every conductor — whether it's a branch circuit wire, a feeder, or a service entrance cable — must be protected against the heat generated by excess current. Article 240 establishes exactly how that protection must be provided, sized, and located.

Exam questions on Article 240 come in two flavors: calculation questions (size the breaker for this load or conductor) and rule questions (where must the OCPD be located, or does this tap configuration comply?). This guide covers both.

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Article 240 Structure

Part I (240.1–240.6): Scope, definitions, and standard ampere ratings
Part II (240.10–240.15): Overcurrent device ratings and interrupting ratings
Part III (240.20–240.24): Location of overcurrent devices
Part IV (240.30–240.40): Enclosures
Part V (240.50–240.60): Plug fuses, fuseholders, and adapters
Part VI (240.60–240.61): Cartridge fuses and fuseholders
Part VII (240.80–240.92): Circuit breakers

Most exam questions draw from Parts I, II, III, and VII.

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Standard Ampere Ratings: 240.6(A)

Before you can size any overcurrent device, you need to know which ratings are available. NEC 240.6(A) lists the standard ampere ratings for fuses and inverse-time circuit breakers:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 A

Exam tip: Know the standard sizes up to 200A cold. The most common exam trap is calculating a value (say 47A) and forgetting to go to the next standard size (50A). The standard list jumps from 45A to 50A, then to 60A — there is no 55A standard breaker.

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Protecting Conductors: 240.4

The core rule of Article 240 is simple: conductors must be protected against overcurrent in accordance with their ampacity.

240.4(B) — Next Higher Standard Rating

When a conductor's calculated ampacity doesn't match a standard device size, NEC 240.4(B) permits using the next higher standard size if:

• The next higher rating does not exceed 800A
• The conductor is not part of a multi-outlet branch circuit supplying receptacles for portable cord-connected equipment

Example: A conductor with an ampacity of 47A may be protected by a 50A overcurrent device (next standard size up from 47A).

240.4(D) — Small Conductor Maximum Protection

This is one of the most tested rules in all of Article 240. NEC 240.4(D) sets hard maximum overcurrent device ratings for small conductors, regardless of what Table 310.16 shows for ampacity:

The trap: From Table 310.16 at 75°C, 14 AWG copper has an ampacity of 20A and 12 AWG has 25A. But 240.4(D) overrides that — you cannot protect 14 AWG with a 20A breaker. Maximum is 15A. Always check 240.4(D) for any question involving 14, 12, or 10 AWG conductors.

Exam tip: 240.4(D) is the override. Table 310.16 gives you the baseline ampacity, but 240.4(D) sets the ceiling on the OCPD. When in doubt, use the lower of the two.

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Overcurrent Protection: Where to Put It — 240.21

NEC 240.21 establishes the location rule: an overcurrent device must be placed at the point where the conductor receives its supply. This means at the source of every conductor run.

But the NEC also recognizes that it's not always practical to put a breaker at the exact point where a tap begins — particularly for transformers and feeders. This is where the tap rules come in.

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The Tap Rules — 240.21(B)

Tap conductors are unprotected conductors that run from a feeder to a panel, transformer, or load without their own OCPD at the source. The tap rules are among the most heavily tested provisions in Article 240.

10-Foot Tap Rule — 240.21(B)(1)

Tap conductors may run up to 10 feet without OCPD at the source if ALL of the following are met:

1. The tap conductor's ampacity is not less than the combined calculated loads it serves
2. The tap conductor's ampacity is not less than the rating of the device supplied (the breaker or fuse at the end of the tap)
3. The tap conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control device they supply
4. The tap conductors are enclosed in a raceway

Alternatively (the "1/10 rule" shortcut): the tap conductor's ampacity must be at least 1/10th of the upstream OCPD rating.

Example: A 400A feeder supplies a panel via a 10-foot tap. The tap conductor must have an ampacity of at least 400 ÷ 10 = 40A (minimum). A 6 AWG THWN copper conductor at 65A would comply.

25-Foot Tap Rule — 240.21(B)(2)

Tap conductors may run up to 25 feet without OCPD at the source if ALL of the following are met:

1. The tap conductor has an ampacity not less than 1/3 of the upstream OCPD rating
2. The tap conductor terminates in a single circuit breaker or set of fuses that limits the load to the ampacity of the tap conductor
3. The tap conductors are protected from physical damage and enclosed in a raceway
4. The tap conductors are not more than 25 feet in length

Example: A 300A feeder. A 25-foot tap must have an ampacity of at least 300 ÷ 3 = 100A. A 1/0 AWG THWN copper conductor (150A at 75°C) would comply.

Exam tip: The 10-foot rule uses 1/10; the 25-foot rule uses 1/3. These fractions are the most-tested numbers from the tap rules. Memorize both.

Transformer Secondary Conductors — 240.21(C)

Transformer secondary conductors may run up to 25 feet without OCPD at the transformer secondary if:

1. The secondary conductor ampacity is not less than the secondary current rating of the transformer
2. The secondary conductors terminate in a single OCPD that limits the load
3. The conductors are enclosed in a raceway and suitably protected from physical damage

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Comparing the Tap Rules

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Location of OCPDs: 240.24

NEC 240.24 specifies where OCPDs must (and must not) be located:

Must Be Readily Accessible: 240.24(A)

OCPDs must be installed in a readily accessible location. An OCPD above a drop ceiling or behind a wall is a code violation unless a special listed access means is provided.

Prohibited Locations: 240.24(D) and (E)

Dwelling unit bathrooms (240.24(E)): Overcurrent devices other than those serving bathroom equipment are not permitted in bathrooms of dwelling units.

Clothes closets (240.24(D)): OCPDs are not permitted in the vicinity of easily ignitable material in clothes closets. Service equipment is excepted.

Over steps (240.24(C)): OCPDs must not be located where they are likely to be exposed to physical damage — which includes above stairways where reaching up would be hazardous.

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Circuit Breaker Requirements: 240.80–240.85

Marking: 240.83

Circuit breakers must be marked with their ampere rating, which must be legible after installation.

SWD and HID Ratings: 240.85

A circuit breaker used to switch 120V or 277V fluorescent lighting must be listed and marked SWD (switching duty). A circuit breaker used to switch HID lighting must be listed and marked HID.

Exam tip: SWD and HID ratings are exam favorites. A standard breaker cannot be used to switch fluorescent or HID lighting — it must be specifically rated for that application.

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Common Exam Problem: Sizing the OCPD

Question format: What is the minimum (or maximum) standard breaker size for [conductor size] protecting a [load]?

Step-by-step:

1. Find the conductor's ampacity from Table 310.16 (using the appropriate temperature column)
2. Check whether 240.4(D) applies (14/12/10 AWG copper)
3. If 240.4(D) applies, use the maximum from that table
4. If 240.4(D) does not apply, use the conductor ampacity; if it doesn't match a standard size, go to the next higher per 240.4(B)

Example: Maximum OCPD for 12 AWG THWN copper branch circuit:

• Table 310.16 (75°C): 25A
• 240.4(D): 12 AWG copper maximum = 20A
• Answer: 20A (240.4(D) overrides the 25A ampacity)

Example: Minimum conductor for a 50A OCPD protecting a branch circuit (no derating):

• Need ampacity ≥ 50A from Table 310.16 (75°C copper)
• 8 AWG = 50A ✓
• Minimum conductor: 8 AWG copper

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Key Article 240 Numbers to Remember

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Common Exam Mistakes on Article 240

1. Using the Table 310.16 ampacity instead of 240.4(D) for 14/12/10 AWG — 240.4(D) always wins for these wire sizes
2. Confusing 1/10 and 1/3 — 10-foot tap uses 1/10; 25-foot tap uses 1/3
3. Going next higher for small conductors — 240.4(B) says you can go to the next higher standard size, but 240.4(D) exceptions prohibit this for 14, 12, and 10 AWG
4. Forgetting the SWD and HID marking requirements — a standard breaker switching fluorescent lighting is a code violation
5. Mislocating an OCPD — bathroom of a dwelling unit is always a prohibited location for OCPDs not serving bathroom equipment

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Ready to Practice Article 240 Problems?

GetLicenseReady's practice questions include overcurrent protection calculations, tap rule questions, and location requirement scenarios — exactly the types that appear on journeyman and master exams.

Related reading: NEC Table 310.16 — Wire Sizing and Ampacity | NEC Article 430 — Motors | NEC Article 220 — Load Calculations