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newSchoolWorks.com › VoTechWorks › Building & Construction › Electrical › National Electrical Code (NEC) Application for Building & Construction › Lesson 8
BLD-ELEC05-03 · Lesson 8 · 22 clock hours · theory

Overcurrent Protection — Articles 240 and 230

Objective. Students will select the correct overcurrent protective device (fuse or circuit breaker) type, ampere rating, and interrupting rating for a given circuit, and identify NEC requirements for service entrance overcurrent protection.

Overcurrent Protection — NEC Articles 240 and 230

Why Overcurrent Protection Matters

Every electrical circuit carries a maximum safe current. When current exceeds that limit — because of an overload, a short circuit, or a ground fault — heat builds up in conductors and equipment. Without a protective device to interrupt the circuit, insulation melts, fires start, and people get hurt. The National Electrical Code (NEC) assigns the rules for selecting and installing overcurrent protective devices (OCPDs) primarily in Article 240 (overcurrent protection in general) and Article 230 (services, including service-entrance overcurrent protection).

Before handling any OCPD — replacing a fuse, resetting a breaker, or working inside a panel — apply lockout/tagout (LOTO) procedures and wear appropriate PPE: safety glasses, insulated gloves rated for the voltage present, and arc-flash PPE if the panel is energized. Never bypass or upsize an OCPD without a code basis.

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Types of Overcurrent Protective Devices

Fuses are single-use devices. When excess current flows through the fusible element, it melts and opens the circuit. Common types include:

Circuit breakers interrupt current automatically and can be reset after tripping. Common types include:

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Article 240 — Key Rules

Conductor protection (240.4). In general, the OCPD rating must not exceed the ampacity of the conductor it protects. For example, a 12 AWG copper conductor with 90 °C insulation has a table ampacity, but you apply correction and adjustment factors, then limit the breaker to the conductor's corrected ampacity.

The next-higher standard rating rule (240.4(B)). When the calculated load current does not match a standard OCPD size listed in 240.6(A), you may go up to the next-higher standard rating — but only if all three conditions are met:

1. The conductor is not part of a multi-outlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

2. The ampacity of the conductor (after all correction and adjustment factors) does not correspond to a standard rating.

3. The next-higher standard size does not exceed 800 amperes.

Standard ratings listed in 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 amperes, and larger sizes.

Interrupting rating (240.83, 240.85, and related sections). Every OCPD must have an interrupting rating equal to or greater than the available fault current at the point of installation. Most household breakers carry a standard interrupting rating (commonly 10,000 A); high-interrupting-rated (HIR) fuses or breakers are required where fault current is higher. Undersized interrupting ratings are a serious safety hazard — the device can explode during a fault.

Location of OCPDs (240.21). An OCPD must be placed at the point where the conductor receives its supply, unless a tap rule exception applies (e.g., the 10-foot tap rule, 25-foot tap rule, or transformer secondary tap rules). Know these tap rules by name and condition; they appear frequently in field work and on exams.

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Article 230 — Service-Entrance Overcurrent Protection

The service entrance is where the utility supply enters the building. Article 230 governs service conductors, service equipment, and the main disconnect.

Number of disconnects (230.71). A service may have no more than six disconnects grouped at one location to serve as the service disconnecting means.

Service-entrance conductor sizing. Service conductors must be sized to carry the calculated load from the load calculation (Article 220). They are protected by the service overcurrent device.

Main service OCPD. The overcurrent device protecting the service conductors must be located at or near the service equipment. Its rating must not exceed the ampacity of the service-entrance conductors as determined by Article 310 tables and any applicable correction factors.

Residential service example. A single-family dwelling with a calculated demand load of 150 amperes requires service conductors ampacity ≥ 150 A and a main breaker rated to protect those conductors. Select a standard rating from 240.6(A) that is ≥ the calculated load but ≤ the conductor ampacity (or use the next-higher rule where permitted).

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Safety Summary

Key terms8

Overcurrent Protective Device (OCPD)A fuse or circuit breaker designed to open a circuit automatically when current exceeds a safe level, protecting conductors and equipment from heat damage.
Interrupting RatingThe maximum fault current an OCPD is listed to safely interrupt without rupturing; it must equal or exceed the available fault current at the installation point.
Time-Delay (Dual-Element) FuseA fuse with two separate elements: one that tolerates brief high-inrush currents (such as motor starting) and one that opens quickly on a short circuit.
Next-Higher Standard Rating RuleNEC 240.4(B) provision allowing an OCPD to be sized one step above the conductor's corrected ampacity when specific conditions are met and the rating does not exceed 800 amperes.
HACR BreakerA circuit breaker listed for Heating, Air-Conditioning, and Refrigeration equipment; designed to withstand high motor-starting inrush without nuisance tripping.
AFCI BreakerArc-Fault Circuit Interrupter breaker that detects dangerous arcing conditions in wiring and opens the circuit to prevent fires; required by NEC in specified residential areas.
Service Disconnecting MeansThe main switch or breaker(s) — no more than six per 230.71 — that disconnects all service conductors from the building's wiring system.
Tap RuleNEC Article 240.21 provisions that permit a smaller conductor to be connected to a larger feeder without an OCPD at the tap point, subject to strict length and ampacity conditions.

Worked examples4

1. A 20 A, 120 V branch circuit is protected by a 20 A breaker and wired with 12 AWG copper (ampacity 20 A after all adjustments). The calculated continuous load is 16 A. Is the 20 A breaker correct, and does the next-higher rule apply?
  1. Step 1 — Identify conductor ampacity: 12 AWG copper, after all correction and adjustment factors, has an ampacity of 20 A.
  2. Step 2 — Check 240.4: The OCPD must not exceed the conductor ampacity of 20 A. A 20 A breaker equals the ampacity exactly — this is permitted.
  3. Step 3 — Check whether next-higher rule is needed: The conductor ampacity (20 A) matches a standard rating in 240.6(A), so the next-higher rule is not needed and would not apply here.
  4. Step 4 — Verify continuous load: The breaker must carry 125% of the 16 A continuous load = 20 A. The 20 A breaker meets this requirement (per Article 210.20).
Answer: A 20 A breaker is correct. The conductor ampacity equals a standard rating, so no next-higher adjustment is needed or permitted.
2. After correction and adjustment factors, a feeder conductor has an ampacity of 55 A. No standard OCPD matches 55 A exactly. May the next-higher standard rating be used, and if so, what size?
  1. Step 1 — Check 240.6(A) standard ratings around 55 A: the two bracketing sizes are 50 A and 60 A.
  2. Step 2 — Apply 240.4(B): The conductor ampacity (55 A) does not correspond to a standard rating, so the next-higher rule may be considered.
  3. Step 3 — Confirm conditions: This is not a multi-outlet branch circuit for cord-and-plug portable loads, and the next-higher size (60 A) is well below 800 A. Both conditions are satisfied.
  4. Step 4 — Select the OCPD: The next-higher standard rating above 55 A is 60 A. A 60 A OCPD is permitted.
Answer: Yes, the next-higher standard rating is permitted. Select a 60 A OCPD per NEC 240.4(B) and 240.6(A).
3. A residential load calculation (Article 220) yields a calculated demand load of 175 A. Service-entrance conductors are selected with a corrected ampacity of 200 A. What is the correct main breaker rating?
  1. Step 1 — Confirm conductor ampacity: 200 A after all corrections.
  2. Step 2 — Identify the minimum OCPD rating: The OCPD must protect the conductors, so it cannot exceed 200 A, and it must be at least large enough to carry the 175 A load.
  3. Step 3 — Consult 240.6(A): Standard ratings near this range include 175 A and 200 A.
  4. Step 4 — Select: A 200 A main breaker is ≤ the conductor ampacity (200 A) and ≥ the calculated load (175 A). A 175 A breaker would also be within the conductor rating but is undersized relative to the conductor chosen; the common industry practice and most economical match is a 200 A main breaker, which matches the conductor selection.
Answer: Select a 200 A main breaker. It does not exceed the service-entrance conductor ampacity and accommodates the 175 A calculated load with headroom.
4. A panelboard is installed where the utility company's available fault current is 22,000 A. A contractor proposes standard 10,000 A interrupting-rated breakers. Is this acceptable?
  1. Step 1 — Identify the available fault current at the panel: 22,000 A.
  2. Step 2 — Identify the proposed breaker interrupting rating: 10,000 A.
  3. Step 3 — Apply NEC 240.83 / 240.85: The OCPD interrupting rating must equal or exceed the available fault current at the point of installation.
  4. Step 4 — Compare: 10,000 A < 22,000 A. The standard breakers are NOT acceptable. High-interrupting-rated (HIR) breakers or current-limiting fuses rated at least 22,000 A interrupting capacity must be specified.
Answer: Not acceptable. The OCPD interrupting rating (10,000 A) is less than the available fault current (22,000 A). HIR devices rated ≥ 22,000 A are required.
In class: Device show-and-tell: instructor displays standard cartridge fuses, plug fuses, time-delay fuses, standard breakers, HACR breakers, and GFCI/AFCI breakers; students match each device to its Article 240 application using a provided reference sheet. · Next-higher rating rule practice: for six circuits where the calculated load does not correspond to a standard fuse or breaker size, students apply Article 240.4 to determine whether the next-higher standard rating is permitted or not and justify their answer in writing. · Service entrance sizing problem: using a previously completed residential load calculation, students work through Article 230 to select the service-entrance conductor, the main disconnect, and the service overcurrent device, documenting each step with a code citation.
Materials: NEC Articles 230 and 240; physical samples of fuses and circuit breakers (various types); load calculation from Lesson 4; NCCER Electrical Level 2 overcurrent protection section
Check: Written quiz: 10 OCPd selection problems; student must choose the correct device type, standard ampere rating, and cite the applicable NEC section; 80% correct (8/10) to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NEC Application
1. Which NEC article is the primary source of rules for overcurrent protection of branch circuits and feeders?
NEC Application
2. A 10-foot feeder tap is being considered without an OCPD at the tap point. Which NEC section permits this arrangement under specified conditions?
NEC Application
3. After correction and adjustment factors, a conductor has an ampacity of 38 A. No standard fuse matches 38 A. Which of the following OCPD ratings is correct if the next-higher standard rating rule of 240.4(B) applies?
NCCER Electrical Knowledge
4. Which type of circuit breaker is specifically listed for use with heating, air-conditioning, and refrigeration equipment to handle motor-starting inrush current?
NEC Application
5. According to NEC 230.71, the maximum number of disconnects permitted to serve as the service disconnecting means at one location is:
Workplace Safety / NEC Application
6. An OCPD has a standard interrupting rating of 10,000 A. The available fault current at the panelboard is 18,000 A. What is the correct action?
NCCER Electrical Knowledge
7. A time-delay (dual-element) fuse is most appropriate for which of the following applications?
Workplace Safety / OSHA 10
8. Before replacing a blown fuse in a 240 V residential panel, which safety step is most critical?
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