4.2 Overcurrent Protection & Surge Protection (Articles 240 and 242)

Overcurrent Protection & Surge Protection (Articles 240 and 242)

Article 240 decides how big an overcurrent device may be, where it goes, and when a conductor may run unprotected for a short distance as a tap. Article 242 covers surge-protective devices (SPDs). Tap rules and the small-conductor limits are favorite calculation questions, and 240.4 is one of the most-cited sections in the code.

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1. Protecting Conductors (240.4)

The basic rule: conductors must be protected at their ampacity (Article 310). Then come the modifiers:

Worked check: Four 3/0 copper THWN conductors in a raceway have an ampacity of 200 A at 75°C.

• A 200-A breaker matches exactly.
• If adjustment reduced the ampacity to 176 A, 240.4(B) would allow the next standard size up, 200 A.
• At 900 A of conductor ampacity, 240.4(C) would forbid a 1000-A device.

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2. Standard Ratings and Device Rules (240.6–240.8, 240.15)

• Standard ratings (240.6(A)) include 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, and 6000 A. There are additional fuse ratings below 15 A.
• Adjustable-trip breakers (240.6(B), (C)): The rating is the maximum possible setting. The lower setting counts only if access to the adjustment is restricted (a sealed cover, locked room, or password).
• Parallel devices (240.8): Fuses or breakers may not be paralleled unless they are factory-assembled and listed as a unit.
• Ungrounded conductors (240.15): Each ungrounded conductor gets an OCPD. Individual single-pole breakers with identified handle ties may protect each ungrounded conductor of a multiwire branch circuit serving only line-to-neutral loads, or of a 240-V single-phase circuit.

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3. Location in the Circuit: The Tap Rules (240.21)

The OCPD normally goes where the conductor receives its supply. Feeder taps (240.21(B)) are the main exceptions:

Transformer secondary conductors (240.21(C)) have parallel rules:

• a 10-ft rule;
• two 25-ft rules, one general and one for industrial installations;
• an outside rule; and
• the case where a 2-wire single-phase or delta-delta 3-wire secondary is protected by the primary OCPD.

Worked example: 25-ft tap. A 400-A feeder is tapped to supply a 150-A panel 20 feet away.

• Minimum tap ampacity = 400 ÷ 3 = 133.3 A.
• The tap must also carry the 150-A device it terminates in. 1/0 copper at 75°C has an ampacity of 150 A, so it meets both requirements.
• The tap ends in a 150-A main breaker.

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4. Location in the Premises (240.24)

• (A) Accessibility: OCPDs must be readily accessible. The center of the operating-handle grip at its highest position may be no more than 6 feet 7 inches (2.0 m) above the floor or working platform. Exceptions include busway plug-ins, supplementary devices, and devices next to the motors or appliances they serve.
• (B) Occupancy: Each occupant must have ready access to the OCPDs protecting that occupant's conductors. Service and feeder devices may be excepted under building-management supervision.
• (C) Physical damage: Not where exposed to damage.
• (D) Ignitible material: Not near easily ignitible material, such as in clothes closets.
• (E) Bathrooms: Not in bathrooms of dwelling units, dormitory units, or hotel and motel guest rooms and suites.
• (F) Stairways: Not over the steps of a stairway.

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5. Interrupting Ratings, Fault Current, and Arc Energy

• 110.9: Every device intended to interrupt fault current must have an interrupting rating at least equal to the available fault current at its line terminals.
• 110.10: Equipment short-circuit current ratings must also be adequate.
• Unmarked ratings: A breaker without a marked interrupting rating is rated 5,000 A (240.83(C)). Class H cartridge fuses are rated 10,000 A. Current-limiting fuses such as Class J and RK1 are commonly 200,000 A.
• 110.24: In other than dwelling units, service equipment must be field-marked with the available fault current and the date it was calculated. The marking is updated when conditions change.
• Arc energy reduction (240.87 and 240.67): Breakers and fuses rated 1200 A or more need an arc energy reduction method, such as zone-selective interlocking, a differential relay, an energy-reducing maintenance switch, or an active arc-flash mitigation system.
• Plug fuses (240.50–240.54): Allowed only where the circuit does not exceed 125 V between conductors, or 150 V to ground on a grounded neutral system. Edison-base fuses are for replacement only. New work uses Type S fuses with nontamperable adapters, in size classes 0–15, 16–20, and 21–30 A.

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6. Surge-Protective Devices (Article 242, Part II)

Article 242 combined the former surge arrester and SPD articles in the 2020 edition. For systems of 1000 V or less:

• Listing and ratings: SPDs must be listed. Their voltage rating may not be less than the system's maximum continuous phase-to-ground voltage.
• Ungrounded systems: SPDs are not used on ungrounded or impedance-grounded systems unless listed for them.
• Conductors: Keep SPD conductors as short as practicable, avoiding unnecessary bends, because lead length adds let-through voltage.
• Where SPDs are required: 230.67 (dwelling-type services), 215.18 (feeders to those occupancies), 695.15 (fire pump controllers), 700.8 (emergency system switchboards and panelboards), and 708.20 (critical operations power).

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7. Common Exam Traps

• Rounding up over 800 A: The next-size-up rule of 240.4(B) stops at 800 A.
• 25-ft tap ratio: The 25-ft tap needs 1/3 of the feeder OCPD rating. The 1/10 ratio belongs to the field-installed 10-ft tap.
• Handle height: The limit is 6 feet 7 inches to the center of the handle grip at its highest position.
• Small conductors: 12 AWG copper is limited to a 20-A OCPD even though Table 310.16 shows 25 A at 75°C.