Protecting circuits and equipment from overcurrent from overcurrent involves more than simply selecting a fuse or breaker from a table
Art. 240, which is divided into seven parts, provides the requirements for selecting and installing overcurrent protection devices (OCPDs) (Fig. 1). Before addressing those requirements, let's review the basic concept of overcurrent protection. When current exceeds the rating of conductors or equipment — due to overload, short circuit, or ground fault — you have overcurrent (Fig. 2 below). To protect conductors and equipment, you use OCPDs.
An OCPD protects equipment by opening when it detects an overload, short circuit, or ground fault. Every piece of electrical equipment must have a short-circuit current rating that permits the OCPD to clear short circuits or ground faults for that equipment without extensive damage to the circuit's electrical components (110.10). You must also apply the other Articles referenced in 240.3, depending on the equipment you're protecting.
An OCPD protects a circuit by opening when current reaches a value that will cause an excessive temperature rise in conductors. The OCPD 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. The ampere rating of an adjustable circuit breaker is the maximum possible long-time pickup current setting permitted. A circuit breaker with restricted access to the adjusting means can have an ampere rating equal to the long-time pickup current setting.
You must protect conductors against overcurrent, based on the ampacities in 310.15. As you might expect, this rule has exceptions. First, it doesn't pertain to flexible cords, flexible cables, or fixture wires. Second, 240.4, Parts A through G, provides an extensive list of “otherwise permitted” items. These include fire pumps (A), tap conductors (F), and an extensive table of specific conductor applications (G).
Don't confuse motor circuit protection with motor overload protection. You'll find the requirements for both kinds of motor-related protection in Art. 430. You must protect motor circuit conductors against short circuits and ground faults per 430.52 and 430.62 (430.51).
To size the branch-circuit conductor for a motor, refer to the requirements in 430.22 and Tables 310.16 and 430.150. To size the branch-circuit protection for a motor, refer to 240.6(A), 430.52(C), and Table 430.150.
If you're using motor controls, size and protect the motor control circuit conductors per 430.72. If you're using remote controls, protect your remote-control, signaling, and power-limited circuit conductors against overcurrent per 725.23 and 725.41.
You must install an OCPD in series with each ungrounded conductor (240.20). Circuit breakers must open all ungrounded conductors of the circuit, unless the circuit meets one of the three exception criteria detailed in 240.20(B):
Multiwire branch circuits
Single-phase, line-to-line loads
3-phase, line-to-line loads
OCPD location. Locate OCPDs so you prevent exposure to physical damage [110.27(B)]. But also make the OCPDs readily accessible (240.24). “Readily accessible” means located so a person can reach the OCPD quickly without having to climb over or remove obstacles (Fig. 3). Supplementary OCPDs, which are often used for luminaires, appliances, or for internal circuits and components of equipment, don't need to be readily accessible (240.10). OCPDs located next to the equipment they supply can be accessible by portable means [404.8(A) Ex. 2].
Each occupant must have ready access to all OCPDs that protect the conductors that supply that occupancy, with a few exceptions.
Service and feeder OCPDs need not be accessible to occupants of multiple-occupancy buildings or guest rooms of hotels and motels if electric maintenance is provided under continuous building management, and branch-circuit OCPDs need not be accessible to occupants of guest rooms of hotels and motels if electric maintenance is provided in a facility under continuous building management.
However, there are two location restrictions. Don't locate OCPDs near easily ignitable material. For example, keep them out of clothes closets. Don't locate them in the bathrooms of dwelling units or guest rooms of hotels or motels. Note that 230.70(A)(2) prohibits locating the service disconnect in any bathroom.
Circuit breaker particulars.
Fuse requirements are noted in 240.50 through 240.61. All circuit breakers must meet several criteria. First, it must be possible to open and close them by hand. Nonmanual means of operating a circuit breaker, such as electrical (shunt trip) or pneumatic operation — are permitted if the breaker can also be operated manually (240.80). They must also clearly indicate whether they're in the open/off or closed/on position. When the handle of a circuit breaker is operated vertically, the up position of the handle is the on position [240.33 and 404.6(C)]. They must require an interrupting rating of 5,000A, unless marked otherwise and be marked with a voltage rating that corresponds with their interrupting rating (240.83 and 240.85).
Ensure the breaker has a sufficient interrupting circuit rating for the available fault current. A breaker with an inadequate interrupting current rating could allow equipment destruction from a line-to-line or line-to-ground fault — plus serious injury or even death (110.9 and Fig. 4).
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 on Molded Case Circuit Breakers, permits HID breakers to be rated as high as 50A, whereas an SWD breaker is rated to 20A. The tests for HID breakers include an endurance test at 75% power factor, whereas SWD breakers are endurance-tested at 100% power factor. The contacts and the spring of an HID breaker are heavier duty to dissipate the increased heat from greater current flow during the HID ignition period.
You can use a breaker with a straight voltage rating, such as 240V, on a circuit where the nominal voltage between any two conductors doesn't exceed the voltage rating of the breaker. You can use a breaker with a slash rating, such as 480Y/277V, on a solidly grounded circuit where the nominal voltage of any one conductor to ground doesn't exceed the lower of the two values and the nominal voltage between any two conductors doesn't exceed the higher value (240.85). You can't use a 120/240V slash circuit breaker on the high leg of a solidly grounded 120/240V delta system because the phase-to-ground voltage of the high-leg is 208V. This exceeds the 120V line-to-ground voltage rating (Fig. 5).
Remember the following:
OCPDs protect conductors and equipment by opening the circuit.
OCPDs protect against excess heat generated by overcurrent sources like overload, short circuit, or ground fault.
Size OCPDs to keep conductors from exceeding their ampacities.
Art. 240 contains a listing of other Articles you must apply, depending on the equipment you're protecting.
Art. 240 is full of exceptions and references to other articles. Start with the basic concepts in mind and then read each of the seven sections as it applies to your application.