By Mike Holt for EC&M magazine
Protecting circuits and equipment from overcurrent involves more than simply selecting a fuse or breaker from a table.
Article 240, divided into seven parts, provides the requirements for selecting and installing Overcurrent Protection Devices (OCPDs) (Figure 240-1). Before addressing those requirements, let's review the basic concept. When current exceeds the rating of conductors or equipment-due to overload, short circuit, or ground fault-you have overcurrent [Article 100] (Figure 240-2). To protect conductors and equipment, you use OCPDs.
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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 (for that equipment) to clear short circuits or ground faults without extensive damage to the electrical components of the circuit. [110.10]. You must also apply the other Articles referenced in 240.3, depending on the equipment you are 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 (Figure 240-10). The ampere rating of an adjustable circuit breaker is the maximum possible long-time pickup current setting that is 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 does not pertain to flexible cords, flexible cables, or fixture wires. Second, there's an extensive list of "otherwise permitted" items in 240.4, Parts A through G-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 Article 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
Table 310.16, 430.22, and Table 430.150. To size the branch-circuit protection for a motor,
refer to 240.6(A), 430.52(C), and Table 430.150.
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):
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 (Figure 240-22). Supplementary OCPDs (often used for luminaires, appliances, or for internal circuits and components of equipment) do not need to be readily accessible [240.10] (Figure 240-23). OCPDs located next to equipment they supply can be accessible by portable means (See 404.8(A) Ex. 2 and Figure 240-24).
Each occupant must have ready access to all OCPDs protecting the conductors supplying that occupancy.
Exception No. 1: Service and feeder OCPDs are not required to be accessible to occupants of multiple-occupancy buildings r guest rooms of hotels and motels, if electric maintenance is provided under continuous building management.
Exception No. 2: Branch-circuit OCPDs are not required to be accessible to occupants of guest rooms of hotels and motels, if electric maintenance is provided in a facility under continuous building management.
Be aware of two location restrictions:
Don't locate OCPDs near easily ignitable material. For example, keep them out of clothes closets (Figure 240-26).
Don't locate OCPDs in the bathrooms of dwelling units or guest rooms of hotels or motels (Figure 240-27). Note that 230.70(A)(2) prohibits locating the service disconnect in any bathroom.
Circuit breaker particulars
Fuse requirements are in 240.50 through 240.61. Here, we want to look at circuit breakers. All circuit breakers must meet the following criteria:
Ensure the breaker has sufficient interrupting circuit rating for the available fault current. A breaker with inadequate interrupting current rating could allow equipment destruction from a line-to-line or line-to-ground fault-plus serious injury or even death. See 110.9 and Figure 240-33.
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 up to 50A, whereas an "SWD" breaker is rated up to 20A. The tests for "HID" breakers include an endurance test at 75 percent power factor, whereas "SWD" breakers are endurance-tested at 100 percent 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 (e.g., 240V) on a circuit where the nominal voltage between any two conductors does not exceed the voltage rating of the breaker. You can use a breaker with a slash rating (e.g., 480Y/277V) on a solidly grounded circuit where the nominal voltage of any one conductor to ground does not exceed the lower of the two values and the nominal voltage between any two conductors does not exceed the higher value [240.85]. You cannot 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 120 line-to-ground voltage rating (Figure 240-36).
Some key things to remember about OCPDs:
Article 240 is full of exceptions and references to other Articles. So, be sure to start with the basic concepts in mind and then read each of the seven sections as it applies to your application.
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