Know what you’re protecting and how to protect it
Article 240 provides the requirements for selecting and installing overcurrent protection devices (OCPDs). Depending on your application, other Articles may apply (see Other Articles below).
An overcurrent exists when current exceeds the rating of conductors or equipment. It can result from overload, short circuit, or ground fault.
An overload is a condition in which equipment or conductors carry current exceeding their rated ampacity. An example is plugging two 12.5A (1,500W) hair dryers into a 20A branch circuit.
A short circuit is the unintentional electrical connection between any two normally current-carrying conductors of a circuit (line-to-line or line-to-neutral).
A ground fault is an unintentional, electrically conducting connection between an ungrounded conductor of a circuit and the equipment grounding conductor, metallic enclosures, metallic raceways, metallic equipment, or earth. During a ground fault, dangerous voltages and abnormally large currents exist.
OCPDs protect circuits and equipment, but they protect circuits in one way and equipment in another.
An OCPD protects a circuit by opening when current reaches a value that would cause an excessive temperature rise in the conductors. Using a water analogy, current rises like water in a tank — at a certain level, the OCPD shuts off the faucet. Think in terms of normal operating conditions that just get too far out of normal range. The interrupting rating must be sufficient for the maximum possible fault current available on the line-side terminals of the equipment [110.9]. You'll find the standard ratings for fuses and fixed-trip circuit breakers in 240.6.
An OCPD protects equipment by opening when it detects a short circuit or ground fault. Every piece of electrical equipment must have a short-circuit current rating that permits the OCPDs (for that equipment) to clear short circuits or ground faults without extensive damage to the electrical components of the circuit [110.10]. Short circuits and faults aren't normal operating conditions. Thus, the OCPDs for equipment have different characteristics than OCPDs for conductors.
OCPDs come in standard sizes, which are listed in 240.6. When you size conductor OCPDs, you're trying to determine which of these standard sizes to use. Begin this sizing by determining the conductor ampacity and then making ampacity adjustments specified in 310.15 [240.4].
OCPD application will vary under the following circumstances:
Conductor overload protection is not required where circuit interruption would create a hazard (e.g., a fire pump). Short-circuit protection is still required.
You can use the next higher standard rating overcurrent device (above the ampacity of the ungrounded conductors being protected) if all of the following conditions are met:
The conductors do not supply multi-outlet receptacle branch circuits.
The ampacity of a conductor after ampacity adjustment and/or correction doesn't correspond to the standard rating of a fuse or circuit breaker in 240.6(A).
The protection device rating doesn't exceed 800A.
For example, a 400A OCPD can protect 500kcmil conductors, where each conductor has an ampacity of 380A at 75°C per Table 310.16. This “next-size-up” rule doesn't apply to feeder tap conductors [240.21(B)] or secondary transformer conductors [240.21(C)].
If the OCPD exceeds 800A, the conductor ampacity (after ampacity adjustment and/or correction) must have a rating not less than the rating of the OCPD. For example, a 1,200A OCPD can protect three sets of 600kcmil conductors per phase, where each conductor has an ampacity of 420A at 75°C per Table 310.16 (Fig. 1).
Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed (after ampacity adjustment and/or correction):
A “supplementary OCPD” provides limited overcurrent protection for specific applications and utilization equipment (Fig. 2). It's usually an internal fuse. Supplementary OCPDs are often used in luminaires, appliances, and equipment for internal circuits and components.
You cannot use a supplementary OCPD as the required branch-circuit OCPD [240.10]. A supplementary OCPD doesn't have to be readily accessible [240.24(A)(2)].
Install OCPDs at the point where the branch or feeder conductors receive their power. Exceptions exist in 240.21 (A) through (G). Here's a summary of each of these, but be sure to read the details if the exception applies to your situation.
(A) Branch circuits meeting 210.19 requirements are exempted from 240.21 location requirements. Examples include multiwire and range circuits.
(B) You can't make a tap from a tap.
(C) The OCPDs for the primary side of a transformer provide protection for the secondary side, if certain conditions are met.
(D) Service conductors are covered by 230.91.
(E) Busway taps are covered by 368.17.
(F) For motors, apply 430.28 and 430.53.
(G) For generators, apply 445.12 and 445.13.
Circuit breakers and fuses must be readily accessible [240.24]. “Readily accessible” means located so a person can reach it quickly without having to climb over (or remove) obstacles or use a portable ladder. This rule does not prohibit the locking of panel doors or the placing of a padlock on a circuit breaker to restrict access [110.26].
Install OCPD enclosures such that the center of the grip of the operating handle, when in its highest position, isn't more than 6 feet 7 inches above the floor or working platform. Four exceptions exist for this rule:
Busways as provided in 368.17(C).
Supplementary OCPDs [240.10].
OCPDs as described in 225.40 and 230.92.
OCPDs located next to equipment can be mounted above 6 feet 7 inches, if accessible by portable means [404.8(A) Exception No. 2] (Fig. 3).
OCPDs must not be exposed to physical damage. Electrical equipment must be suitable for the environment. Give consideration to the presence of corrosives, which may deteriorate conductors or equipment [110.11].
Don't locate OCPDs near easily ignitable material or in locations such as clothes closets. Don't locate them in bathrooms of dwelling units or guest rooms (or guest suites) of hotels or motels. This rule also applies to the service disconnecting means, even in commercial or industrial facilities [230.70(A)(2)].
Enclosures containing OCPDs must be mounted in a vertical position unless this is impractical [240.33]. Circuit breaker enclosures can be horizontal if the circuit breaker is installed per 240.81.
The requirements of 240.81 specify that where circuit breaker handles are operated vertically, the “up” position of the handle must be in the “on” position. Therefore, an enclosure that contains one circuit breaker can be mounted horizontally, but an enclosure that contains a panelboard or load center with multiple circuit breakers on opposite sides of each other would have to be mounted vertically (Fig. 4).
Also note that these enclosures are designed for left-hand operation, under the assumption that the operator is right-handed. The intended result is that the operator is standing to one side of the enclosure, rather than in front of it (and in the blast path) when operating it. Allow space for this when installing the enclosure.
Circuit breakers must be capable of being opened and closed by hand [240.80]. Non-manual means of operating a circuit breaker, such as electrical shunt trip or pneumatic operation, are permitted only if the circuit breaker can also be manually operated.
Circuit breakers used to switch 120V or 277V fluorescent lighting circuits must be listed and marked SWD or HID. Circuit breakers used to switch high-intensity discharge lighting circuits must be listed and marked HID.
UL 489, Standard for Molded Case Circuit Breakers, permits “HID” breakers to be rated up to 50A, but an “SWD” breaker may be rated only to 20A. The tests for “HID” breakers include an endurance test at 75% power factor, but “SWD” breakers are endurance-tested at 100% power factor. The contacts and the spring of an “HID” breaker are constructed of a heavy-duty material designed to dissipate the increased heat caused by the greater current flow in the circuit that occurs because the “HID” luminaire takes a minute or two to ignite the lamp.
Before you start any OCPD calculations, first determine if you're trying to protect circuits or equipment. Next, determine if any other Articles apply for your application. Then, you can plug in the numbers and select the correct OCPD.
Your application may fall under one of these other Articles: