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Article 240 Overcurrent Protection
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Parts I through VII of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal. Part VIII covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 1000 volts, nominal. Part IX covers overcurrent protection over 1000 volts, nominal.
Informational Note: Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation. See also 110.9 for requirements for interrupting ratings and 110.10 for requirements for protection against fault currents.
The definitions in this section shall apply only within this article.
Current-Limiting Overcurrent Protective Device. A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.
Supervised Industrial Installation. For the purposes of Part VIII, the industrial portions of a facility where all of the following conditions are met:
- Conditions of maintenance and engineering supervision ensure that only qualified persons monitor and service the system.
- The premises wiring system has 2500 kVA or greater of load used in industrial process(es), manufacturing activities, or both, as calculated in accordance with Article 220.
- The premises has at least one service or feeder that is more than 150 volts to ground and more than 300 volts phase-to-phase.
This definition excludes installations in buildings used by the industrial facility for offices, warehouses, garages, machine shops, and recreational facilities that are not an integral part of the industrial plant, substation, or control center.
Tap Conductor. A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.
Equipment shall be protected against overcurrent in accordance with the article in this Code that covers the type of equipment specified in Table 240.3.
Table 240.3 Other Articles
| Equipment | Article |
|---|---|
| Air-conditioning and refrigerating equipment | 440 |
| Appliances | 422 |
| Assembly occupancies | 518 |
| Audio signal processing, amplification, and reproduction equipment | 640 |
| Branch circuits | 210 |
| Busways | 368 |
| Capacitors | 460 |
| Class 1, Class 2, and Class 3 remote-control, signaling, and power-limited circuits | 725 |
| Cranes and hoists | 610 |
| Electric signs and outline lighting | 600 |
| Electric welders | 630 |
| Electrolytic cells | 668 |
| Elevators, dumbwaiters, escalators, moving walks, wheelchair lifts, and stairway chairlifts | 620 |
| Emergency systems | 700 |
| Fire alarm systems | 760 |
| Fire pumps | 695 |
| Fixed electric heating equipment for pipelines and vessels | 427 |
| Fixed electric space-heating equipment | 424 |
| Fixed outdoor electric deicing and snow-melting equipment | 426 |
| Generators | 445 |
| Health care facilities | 517 |
| Induction and dielectric heating equipment | 665 |
| Industrial machinery | 670 |
| Luminaires, lampholders, and lamps | 410 |
| Motion picture and television studios and similar locations | 530 |
| Motors, motor circuits, and controllers | 430 |
| Phase converters | 455 |
| Pipe organs | 650 |
| Receptacles | 406 |
| Services | 230 |
| Solar photovoltaic systems | 690 |
| Switchboards and panelboards | 408 |
| Theaters, audience areas of motion picture and television studios, and similar locations | 520 |
| Transformers and transformer vaults | 450 |
| X-ray equipment | 660 |
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.14, unless otherwise permitted or required in 240.4(A) through (G).
Informational Note: See ICEA P-32-382-2007 (R2013), Short Circuit Characteristics of Insulated Cables, for information on allowable short-circuit currents for insulated copper and aluminum conductors.
Conductor overload protection shall not be required where the interruption of the circuit would create a hazard, such as in a material-handling magnet circuit or fire pump circuit. Short-circuit protection shall be provided.
Informational Note: See NFPA 20-2013, Standard for the Installation of Stationary Pumps for Fire Protection.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
- The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
- The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
- The next higher standard rating selected does not exceed 800 amperes.
Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6.
Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
7 amperes, provided all the following conditions are met:
- Continuous loads do not exceed 5.6 amperes.
- Overcurrent protection is provided by one of the following:
- Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire
- Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire
- Class CC, Class J, or Class T fuses
10 amperes, provided all the following conditions are met:
- Continuous loads do not exceed 8 amperes.
- Overcurrent protection is provided by one of the following:
- Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire
- Branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire
- Class CC, Class J, or Class T fuses
15 amperes
15 amperes
20 amperes
25 amperes
30 amperes
Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:
- 210.19(A)(3) and (A)(4), Household Ranges and Cooking Appliances and Other Loads
- 240.5(B)(2), Fixture Wire
- 240.21, Location in Circuit
- 368.17(B), Reduction in Ampacity Size of Busway
- 368.17(C), Feeder or Branch Circuits (busway taps)
- 430.53(D), Single Motor Taps
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).
Table 240.4(G) Specific Conductor Applications
| Conductor | Article | Section |
|---|---|---|
| Air-conditioning and refrigeration equipment circuit conductors | 440, Parts III, VI | |
| Capacitor circuit conductors | 460 | 460.8(B) and 460.25 |
| Control and instrumentation circuit conductors (Type ITC) | 727 | 727.9 |
| Electric welder circuit conductors | 630 | 630.12 and 630.32 |
| Fire alarm system circuit conductors | 760 | 760.43, 760.45, 760.121, and Chapter 9, Tables 12(A) and 12(B) |
| Motor-operated appliance circuit conductors | 422, Part II | |
| Motor and motor-control circuit conductors | 430, Parts II, III, IV, V, VI, VII | |
| Phase converter supply conductors | 455 | 455.7 |
| Remote-control, signaling, and power-limited circuit conductors | 725 | 725.43, 725.45, 725.121, and Chapter 9, Tables 11(A) and 11(B) |
| Secondary tie conductors | 450 | 450.6 |
Flexible cord and flexible cable, including tinsel cord and extension cords, and fixture wires shall be protected against overcurrent by either 240.5(A) or (B).
Flexible cord and flexible cable shall be protected by an overcurrent device in accordance with their ampacity as specified in Table 400.5(A)(1) and Table 400.5(A)(2). Fixture wire shall be protected against overcurrent in accordance with its ampacity as specified in Table 402.5. Supplementary overcurrent protection, as covered in 240.10, shall be permitted to be an acceptable means for providing this protection.
Flexible cord shall be protected, where supplied by a branch circuit, in accordance with one of the methods described in 240.5(B)(1), (B)(3), or (B)(4). Fixture wire shall be protected, where supplied by a branch circuit, in accordance with 240.5(B)(2).
Fixture wire shall be permitted to be tapped to the branch-circuit conductor of a branch circuit in accordance with the following:
- 15- or 20-ampere circuits — 18 AWG, up to 15 m (50 ft) of run length
- 15- or 20-ampere circuits — 16 AWG, up to 30 m (100 ft) of run length
- 20-ampere circuits — 14 AWG and larger
- 30-ampere circuits — 14 AWG and larger
- 40-ampere circuits — 12 AWG and larger
- 50-ampere circuits — 12 AWG and larger
Flexible cord used in listed extension cord sets shall be considered to be protected when applied within the extension cord listing requirements
Flexible cord used in extension cords made with separately listed and installed components shall be permitted to be supplied by a branch circuit in accordance with the following:
20-ampere circuits — 16 AWG and larger
The standard ampere ratings for fuses and inverse time circuit breakers shall be considered as shown in Table 240.6(A). Additional standard ampere ratings for fuses shall be 1,3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.
Table 240.6(A) Standard Ampere Ratings for Fuses and Inverse Time Circuit Breakers
| Standard Ampere Ratings | ||||
|---|---|---|---|---|
| 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 | — | — | — |
The rating of adjustable-trip circuit breakers having external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of 240.6(C), shall be the maximum setting possible.
A circuit breaker(s) that has restricted access to the adjusting means shall be permitted to have an ampere rating(s) that is equal to the adjusted current setting (long-time pickup setting). Restricted access shall be achieved by one of the following methods:
- Located behind removable and sealable covers over the adjusting means
- Located behind bolted equipment enclosure doors
- Located behind locked doors accessible only to qualified personnel
- Password protected, with password accessible only to qualified personnel
Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.
Thermal relays and other devices not designed to open short circuits or ground faults shall not be used for the protection of conductors against overcurrent due to short circuits or ground faults, but the use of such devices shall be permitted to protect motor branch-circuit conductors from overload if protected in accordance with 430.40.
Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection. Supplementary overcurrent devices shall not be required to be readily accessible.
Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted:
Informational Note: The monitoring system may cause the condition to go to alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazard and equipment damage.
Ground-fault protection of equipment shall be provided in accordance with 230.95 for solidly grounded wye electrical systems of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more.
This section shall not apply to the disconnecting means for the following:
- Continuous industrial processes where a nonorderly shutdown will introduce additional or increased hazards
- Installations where ground-fault protection is provided by other requirements for services or feeders
- Fire pumps
A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit.
Informational Note: For motor circuits, see Parts III, IV, V, and XI of Article 430.
Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).
Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiwire branch circuits that serve only single-phase line-to-neutral loads.
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