Article 240 Overcurrent Protection

I. General

Parts I through VII give requirements for circuits of up to 600 V. Part VIII covers supervised industrial installations up to 600 V. Part IX covers over 600 V.

Table 240.3 refers to other articles in the Code which pertain to overcurrent profection for specific equipment and systems.

For other than flexible cords and fixture wires, use ampacities specified in Section 310.15 when calculating overcurrent protection. There are a number of cases where this rule does not necessarily hold true with special requirements. Some of them are: power loss hazard; devices rated 800 amperes or less; devices rated over 800 amperes; tap conductors; motor-operated appliance circuit conductors; motor and motor control circuit conductors; phase converter supply conductors; air conditioning and refrigeration equipment circuit conductors; transformer secondary conductors; capacitor circuit conductors; electric welder circuit conductors; remote control signalling and power limited circuit conductors; and fire protective alarm system circuit conductors. Refer to the NEC® for a complete list and for other sections and articles that may be relevant.

The standard ratings for fuses and inverse time circuit breakers are: 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. Fuses are also rated at 1, 3, 6, 10, and 601 A. There is a requirement for circuit breakers having external adjustable means and for restricted access adjustable trip circuit breakers.

If an orderly shutdown is required to reduce hazards to equipment and personnel, the following method can be used; coordinated short-circuit protection and/or overload indication.

For solidly grounded wye systems that are more than 150 V to ground and not exceeding 600 V phase to phase, ground-fault protection of equipment must be provided for each main disconnecting means rated 1000 A or more, in accordance with Section 230.95. The three exceptions are where an orderly shutdown in an industrial process is required, for fire pumps, and if the disconnect is protected by service ground-fault protection.

II. Location

An overcurrent device (fuse or circuit breaker) has to be connected in series with each ungrounded conductor. An overcurrent relay with a current transformer is allowed. Generally, a circuit breaker has to open all ungrounded conductors. Single-pole circuit breakers are permitted under certain conditions listed in this section. In a closed-loop power distribution system, listed devices can be used instead of fuses or circuit breakers.

This section lists the location of the overcurrent device in relation to the ungrounded circuit conductor.

(A) Branch-Circuit Conductors. If a branch-circuit tap meets the requirements of Section 210.19, then the overcurrent protection can be located in accordance with that Section.

(B) Feeder Taps. Taps can be made at conductors without overcurrent protection at the tap as noted in section 240.21(B)(1) through (5) below. The provisions of 240.4(B) cannot be applied for tap conductors.

(1) Taps Not over 3 m (10 Feet) Long. They may be tapped to a feeder or transformer secondary without overcurrent protection when the tap is not longer than 3 m (10 ft.); the rating of the tap conductor is not less than the loads and devices it supplies and the overcurrent device at the end of the tap; the tap ends at the device it supplies; the tap conductors are enclosed in a raceway, and for field installations where the tap leaves the enclosure of the tap the rating of the overcurrent device on the line side of the tap is not more than 10 times the ampacity of the conductors.

(2) Taps Not over 7.5 m (25 Feet) Long. When the tap is not longer than 7.5 m (25 ft.) it does not require protection if the ampacity of the tap is at least one-third the rating of the feeder or overcurrent protection it was tapped from; ends in a single overcurrent device rated for the device; and is protected from physical damage by an approved method.

(3) Taps Supplying a Transformer [Primary Plus Secondary Not over 7.5 m (25 Feet) Long]. The rule is similar to the 7.5 m (25 ft.) tap (item c above). No overcurrent protection is required for a tap for a feeder supplying a transformer if the ampacity of the primary wires is at least one-third the rating of the overcurrent device protecting the feeder; when you multiply the rating of the conductors supplying the secondary by the ratio of the secondary to primary voltage it should be at least one-third the feeder protective overcurrent device rating; if both primary and secondary wires are not longer than 7.5 m (25 ft.) combined; the primary and secondary conductors are protected in an approved way from physical damage; and the secondary conductors end in a single overcurrent device limiting the load current to that permitted by Section 310.15.

(4) Taps over 7.5 m (25 Feet) Long. No overcurrent protection is required for a tap located in manufacturing installations with bays over 11 m (35 ft.) in height if the horizontal length is not longer than 7.5 m (25 ft.) with the overall length not longer than 30 m (100 ft.). All the rules of the tap not over 7.5 m (25 ft.) tap rule (2 above) apply, and in addition, the tap conductors must be at least 6 AWG copper or 4 AWG aluminum; they can not go through floors, walls, or ceilings; the tap must be least 9 m (30 ft.) from the floor; the tap conductors have no splices and are continuous from one end to the other and only qualified persons service the system.

(5) Outside Taps of Unlimited Length

(C) Transformer Secondary Conductors. There is a complete discussion of transformer taps. Among the items discussed are protection by primary overcurrent device, transformer secondary conductors not over 3 m (10 ft.) long, secondary conductors not over 7.5 m (25 ft.) long, outside secondary conductors, industrial installation secondary conductors not over 7.5 m (25 ft.) long, and secondary conductors from a feeder tapped transformer.

(D) Service Conductors

(E) Busway Taps. Busways and their taps can be protected in accordance with Sections 364.10 through 364.14.

(F) Motor Circuit Taps. Motor branch circuit conductors can be protected as indicated in Sections 430.28 and 430.53.

(G) Conductors from Generator Terminals. They can be protected as indicated in Section 445.4 if sized as per 445.5.

No overcurrent device should be placed in series with a grounded conductor unless it simultaneously opens all other conductors of the circuit or when required by Sections 430.36 and 430.37.

Overcurrent devices must be readily accessible. The center of the grip of the handle on the overcurrent device has to be installed so that it is not more than 2.0 m (6 ft. 7 in.) above the floor or working platform at its highest position. There are four exceptions. Each occupant must have access to the ones protecting the conductors for his or her occupancy. The overcurrent devices must not be exposed to physical damage and must not be located near easily ignitible material or in bathrooms of dwelling units or guest rooms or guest suites of hotels and motels. Refer to the NEC® for certain exceptions.

III. Enclosures

IV. Disconnecting and Guarding

In general, the disconnection means must be provided on the supply side of the fuse. This does not hold true for service current-limiting devices as per Section 230.82 or single disconnecting devices for multiple sets of fuses as per Section 430.112 and Section 424.22(C).

V. Plug Fuses, Fuseholders, and Adapters

These are not permitted in circuits where the voltage between conductors is more than 125 V except with a grounded neutral and not more than 150 V to ground. There are other requirements.

VI. Cartridge Fuses and Fuseholders

These are permitted in circuits operating at 300 V between conductors if rated 300 V and for single-phase line to neutral circuits where this voltage is not larger than 300 V and the circuit is supplied from a three-phase, four-wire, solidly grounded neutral system. The holders must prevent interchangeability where a fuse can be put into a circuit of lower ampere rating or higher voltage rating. All fuses must be marked, indicating the current, voltage, and interrupting rating, if it is current limiting, and the manufacturer's name and trademark. The interrupting rating is not required when they are used for supplementary protection. Class H cartridge fuses that are renewable type can be used for replacement only in existing installations where there has been no overfusing or tampering.

VII. Circuit Breakers

Normally, they must be trip free and capable of manual operation in addition to other normal means, such as electrical or pneumatic operation.

Must indicate on or offon is up position if mounted vertically.

They must all be properly marked. When properly used as a switch, they must be approved for switching duty and marked SWD or HID. There are requirements for voltage marking, interrupting ratings, and location.

VIII. Supervised Industrial Locations

IX. Overcurrent Protection over 600 Volts, Nominal