695), its primary OCPD shall be size… Feeder circuits. The primary purpose of a circuit overcurrent protection device (a fuse, a circuit breaker, or some other type of current-limiting device) is to limit the temperature of the circuit conductors to a value that will not damage the conductors or their insulation. Referring to Table 310.15(B)(2)(a), the correction factor for 90°C-rated copper THHN wire with a maximum ambient temperature of 120°F is 0.82. For an ambient temperature of 85°F, the correction factor for copper THHN 90°C wire is 1.0, so no ampacity adjustment is required. It is common practice to add 20% of the minimum cable rating to be used for future load additions. For this example, the wires are routed through an office environment where the highest temperature is expected to be 85°F during times when the cooling systems are turned off. Once the ampacity is determined, the voltage drop should also be considered. An OCP rating of 300% higher than the full-load current is 3 x 124 amps = 372 amps. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. What if the sensor failed or the room was temporarily repurposed as an office? There are some conditions under which the derating factors do not apply, as seen in 310.15(B)(3)(a)(2) through (4). Overcurrent relays are the typical protection for conventional distribution systems. max be the upper limit on the speed of primary protection. If the load is debatable, be conservative and design for a continuous load. These simplifying assumptions make it straightforward to write equations for voltage, current and power, and integrate the power equation to determine the energy dissipated in the … The three major categories or types of overcurrent are overload, short-circuit, and ground-fault. The full-load current rating of the connected load dictates the size (by rated ampacity) of the supply conductors and the rating or setting of the OCPD. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. Overcurrent protection Overcurrent refers to the operating state of electric motor or electrical apparatus element surpassing the rated current. Determining whether a load is continuous or noncontinuous is not always clear. As with branch circuits, the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the noncontinuous load. The fuses or circuit breakers located downstream provide full overcurrent protection for any circuits or equipment located on their load side while providing only overload protection for their line- or supply-side circuit. Unfortunately, overloads and short circuits do occur. Consider a 208 V, 3-phase feeder supplying a panelboard with a noncontinuous load of 10 kVA and a continuous load of 30 kVA. Some examples of continuous loads include office lighting, exterior lighting, data center equipment, fixed storage-type water heaters with capacities of less than 120 gal (450 l; as per NEC 422.13), and chilled/hot-water circulating pumps. Overcurrent protection is one of the essential elements of a proper electrical installation. This rating falls between the standard ratings of 350 amps and 400 amps. The OCP used to provide power to the motor controller and motor has to then provide short-circuit and ground-fault protection for the motor circuit. In this case the relay R 1 would have to backup the fuse. Some permit reviewers might require this to be considered a continuous load. The operation of the consecutive time-inverse overcurrent relays are coordinated according to the discrepancy between the fault currents measured by these relays. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). Overload is a controlled overcurrent situation, normally of low magnitude. This is achieved by limiting the amount (value) of current the conductors are required to convey. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition. If a transformer serves a fire pump (Art. Sections for articles pertaining to equipment typically found in commercial buildings include: The general requirement for overcurrent protection of conductors is provided in Section 240.4, Protection of Conductors. The air conditioning and refrigeration equipment manufacturer must also indicate whether a fuse or circuit breaker can be used to supply the equipment. After applying the appropriate deratings, the calculated ampacity of the #1/0 wire is adequately protected by the 150-amp OCP selected above. Figure 1 is the drawing of a single-phase AC, 3-wire, 120/240-volt supply to a building such as a home or small industrial facility). By the same token, the rating or setting of the OCPD and the rated ampacity of the circuit conductors dictate the maximum full-load current that can be supplied from the service, feeder, or branch circuit. Overcurrent Protection Circuit Working A: Overcurrent protection for the transformer is covered by 450.3(B) and Table 450-3(B). Starting from the load end, we have a dual-element or slow-blow fuse at the input of the power supply to the personal computer. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. The OCPD must be installed in accordance with the requirements of the National Electric Code. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. Phone apps are also available that provide the same function as the slide rule. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Thus, the calculated ampacity for the #1/0 copper THHN wire used in this example is: #1/0 copper THHN ampacity = 170 amps x 0.82 = 139.4 amps. 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. The requirements for branch-circuit overcurrent protection are found in Section 210.20. For example, a 20-amp branch circuit is modified with an additional lamp, which increases the load current to 22 amps: this would be a circuit overload. It owes its application from the fact that in the event of fault the current would increase to a value several times greater than maximum load current. NFPA 70: National Electric Code (NEC) gives little guidance regarding continuous and noncontinuous loads and why this is important. This fuse will open the 120 volt circuit for any large fault within the computer. In this case, the maximum OCP rating is then 350 amps. Consider a 460 V, 3-phase motor branch circuit supplying power to a 100-hp, squirrel-cage motor protected by an inverse-time molded-case circuit breaker. Which type is intended for all types of fault current conditions? Secondary conductor overcurrent protection must comply with 240.21. In this example, the wires are routed through an environment where the highest temperature is expected to be no higher than 104°F. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. By referencing Table 310.15(B)(16), the minimum conductor size allowed is #14. A fault, such as a short circuit or ground fault, is not an overload. To use this table, you must know the type of motor used in the circuit and the type of OCP used to protect the circuit. This should not be mixed with ‘overload’ relay protection, which typically utilizes relays that function in a time-related in some degree to the thermal capacity of the equipment to be protected. Ground fault: A ground fault is a specific type of short circuit involving at least one of the phase conductors encountering a grounded conductor or surface. The paper goes on to discuss the rules for the coordination of negative-sequence overcurrent characteristics for sensitive phase-to-phase fault protection in feeders, as well as for unbalanced current protection of induction motors. The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. In this article, we will cover types of overcurrent, what overcurrent protection devices are, and their place in an electrical circuit. The short circuit can be across two or more transformer single-phase AC windings. But there is much more to it. Don't have an AAC account? If designed to current energy code requirements, it should have a vacancy sensor to automatically shut off the lights when there are no occupants detected. The derated temperature rating is higher than the MCA value of 31.0 amps for the #8 conductor, which is acceptable for this example. Consider a 25-amp load flowing in a circuit. Overcurrent Protection in AC Power Systems, Historical Engineers: George Westinghouse and the AC Power Grid, Improving Home Appliance Safety with IEC Standards, Common Analog, Digital, and Mixed-Signal Integrated Circuits (ICs). The circuit breaker will allow the short circuit current to flow for only a short time. Protection 2 •Zones of Protection •Ground Fault Protection •Phase Fault Protection ... 51V Voltage Controlled/Restrained Overcurrent 24 Volts per Hertz 32 Reverse Power 40 Loss of Field ... this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable. A utilization-equipment load will operate at 240 volts when connected between the two ends of the two series-connected 120-volt windings. Working Principle of Over Current Relay. Fortunately, the NFPA 70: National Electric Code (NEC) gives requirements for most of the applications that electrical engineers and designers encounter in their work. Transformer primary protection— Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. Also, minimum circuit amperes (MCA) must be used to determine the minimum conductor rating. Figure 4 shows the result of a short circuit condition. In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). In this example, the wires are routed through a boiler room where the highest temperature is expected to be no higher than 120°F. This places a short-circuit across the supply-transformer winding. In general, the conductors of all of these circuits must be provided with a means of overcurrent protection at the point where they receive their electrical supply. In this example, the OCP is simply equal to the manufacturer-supplied MOP value of 50 amps, since 50 amps is a standard OCP rating per Table 240.6(A). [HEAD]. Section 210.19(A) requires the conductor to be sized in the same manner as the OCP-no less than 125% of the continuous load and 100% of the noncontinuous load. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. And common method is to connect fuse protector or low voltage circuit breaker. Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. Information regarding equipment-overload capability often comes from the manufacturer. The load and cable ratings calculated above are minimum values. Therefore, the minimum conductor size is 10 AWG, and the overcurrent protection or … Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Ambient-temperature correction factors for 40°C tables are found in Table 310.15(B)(2)(b). In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. The short-circuit current rating and interrupting rating must also be determined based on the available short-circuit current in the circuit. 408 IV. Then . Search Products And Discover New Innovations In Your Industry!! Thus overcurrent relays should be used over a limited length in the 3 feeder sections. The circuit is protected by a 15-amp circuit breaker. Article 310 provides the general requirements for conductors, insulation, markings, mechanical strength, and ampacity rating. Figure 5 shows a ground-fault condition. The motor-circuit OCP is providing only short-circuit and ground-fault protection. Any current magnitude that is greater than the rated ampacity of the conveying wires or the rated-load current of the electrical utilization equipment—such as light fixtures, motors, or transformers—is described as an overcurrent. Where possible, it is preferred that instantaneous methods of detecting overcurrent be used as the primary protection method on all of the major equipment associated with the power system. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. Overcurrent Protection Circuit. Review overcurrent protection for certain types of building equipment. How to Size an Overcurrent Device for a Transformer. In the case of hermitically sealed motor compressors, the motor FLA values found in Article 430 will not be higher than actual motor values due to the cooling effect the refrigerant has on the motor windings. Table 310.15(B)(2)(a) provides the ambient-temperature correction factors, which must be applied to the ampacities given in Table 310.15(B)(16). Several articles applicable to commercial buildings modify the general NEC rule for overcurrent protection, as summarized below: In the following examples, the rating of the OCP trip value will be determined along with the ampacity of the conductors used in the circuit. The general requirement is to size the OCP for no less than 125% of the continuous load and 100% of the noncontinuous load. Since the ampacities in Table 310.15(B)(3)(a) already account for up to three current-carrying conductors, no further derating is required. In the example above, there are three current-carrying conductors routed in the conduit. Fig. In the overloaded circuit, the load current is nearly 20 amps. Next, derating for the number of conductors in the conduit must be considered. This example requires a 50-amp OCP, therefore a #8 wire must be used. The requirements for motor-circuit overcurrent protection are different than branch and feeder circuits, often leading to confusion. Page 3 Introduction The use of overcurrent protection (OCP) is a common practice in designing electrical circuits, and several common methods currently exist, … As shown in Figure 5, the electrical power distribution system within a building or other structure has three major classifications: the service, the feeder circuits, and the branch circuits. However, many authorities having jurisdiction have made voltage drop a code requirement. The fault current is approximately 5,000 amps. The circuit load for the purpose of sizing the OCP is: The next highest standard OCP (see table 240.6(A)) is 150 amps. The requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. The motor full-load current used in this calculation is not the nameplate current, but the current value found in Table 430.250. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). Branch circuits. As with branch circuits, the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the noncontinuous load. Although Table 310.16 shows a 12 AWG conductor as having an ampacity of 25A, the asterisk directs us to Section 240.3(D) where we find that a 12 AWG conductor is limited to 20A for our purposes. The wire size will be based on the manufacturer-supplied MCA value, which in this case is 31.0 amps. The different types of faults are shown in Figure 1 to illustrate the concept of overcurrent protection. Section 430.52(C)(1) has an exception that allows the next higher standard rating to be used. However, the next higher rating allowance does not apply to Exception (c). As discussed above, the 60°C ampacity of 15 amps must be used for the #14 wire in this example despite the higher ampacity calculated. It protects the equipment of the power system from the fault current. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F. Overcurrent protection seems like a simple concept: Limit the current flow in a circuit to a safe value. Understand the three types of overcurrent conditions to consider in typical NFPA 70: National Electrical Code applications. For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. Consideration should be given to load growth. The general rule for overcurrent protection is to select an overcurrent protection device (OCPD) that protects conductors based on their ampacities (after ampacity correction and adjustment per 310.15). Without some form of overcurrent protection device installed in series with the circuit conductors, the only limit of the fault overcurrent is the conductor resistance and the amount of power available from the transformer. 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