These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. The calculated ampacity for the #2/0 copper THHN wire used in this example is: The conductor ampacity derated for ambient temperature is higher than the 75°C ampacity column, so it is acceptable for use in this example. The motor-circuit OCP is providing only short-circuit and ground-fault protection. Circuit breakers protect transformers from overcurrent situations and short circuits. A few of the symptoms of overcurrent in a motor are shorts, blown fuses and … What happens during an overload or fault condition? These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. This fuse will open the 120 volt circuit for any large fault within the computer. Section 215.2 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 fault current is approximately 5,000 amps. 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). Overload conditions are usually not as time-critical as short circuits and ground faults. In this case, the load current is 10 amps. In this example, the wires are routed through an environment where the highest temperature is expected to be no higher than 104°F. For example, the derating factors do not apply to type armored cable (AC) and metal-clad (MC) cables provided the cables do not have an overall jacket, each cable has not more than three current-carrying conductors, the conductors are #2 AWG, and not more than 20 current-carrying conductors are installed without maintaining spacing. Consideration should be given to load growth. The load and cable ratings calculated above are minimum values. In the overloaded circuit, the load current is nearly 20 amps. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). A: Overcurrent protection for the transformer is covered by 450.3(B) and Table 450-3(B). Overload is a controlled overcurrent situation, normally of low magnitude. The OCPD must be installed in accordance with the requirements of the National Electric Code. As with the previous example, the wire type selected is copper THHN, which is rated for 90°C. 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. Next, derating for the number of conductors in the conduit must be considered. Short-circuit overcurrent normally involves an accidental cross-connection of at least two circuit conductors (supply and return). The requirements for determining the maximum rating or setting for motor-branch-circuit short circuit and ground-fault protection can be found in Table 430.52. For this example, Table 430.52 allows a circuit breaker with a maximum rating of 2.5 times the full-load current of the motor. An electrical overload overcurrent also occurs when a motor is mechanically overloaded. You must provide the protection where the conductors receive their supply [240.4 and 240.21], but several exceptions to this rule exist [240.4(A) through (G)]: • Power loss hazard. Both the conductors and the connected load they supply must be protected at the correct amperage. Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). The answers depend on the application, the equipment being protected, and the strength of the source. Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. Depending on the time of operation the overcurrent relay is categorized into following types. Consider the case of an office storeroom lighting circuit. The three major categories or types of overcurrent are overload, short-circuit, and ground-fault. Coordinated protective devices provide an optimal balance between fault localization and circuit protection based on the responsible engineer’s judgment. Determining whether a load is continuous or noncontinuous is not always clear. The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time. Table 240.4(G) requires Article 430 to be used for selection of motor-circuit overcurrent protection. Next, select a conductor in accordance with sections 215.2 and 310.15. NEC Table 240.3 provides a list of the applicable sections. The wire size will be based on the manufacturer-supplied MCA value, which in this case is 31.0 amps. In our example, there are six current-carrying conductors routed in the conduit. The overcurrent protection scheme is used to protect the distribution lines of electric grids integrated with DER. Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Overcurrent protection is one of the essential elements of a proper electrical installation. If you'd like to know more about a specific topic relating to overcurrent, please share your thoughts in the comments section below. The NEC has informational notes concerning voltage drop for branch circuits and feeders, but it is not a code rule. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. In cases where a load is certain to be noncontinuous, size the circuit to 100% of the load. Thus overcurrent relays should be used over a limited length in the 3 feeder sections. Overload conditions can occur at the service, feeder, or branch-circuit level of a building's electrical-power distribution system. It is important to have a distinction continuous and noncontinuous loads because of heat. In the continuous case (for example, a chiller), the load might be active for 8 hours or longer. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. Search Products And Discover New Innovations In Your Industry. Instantaneous clearing of The air conditioning and refrigeration equipment manufacturer must also indicate whether a fuse or circuit breaker can be used to supply the equipment. Instant, definite time, direction, and non-directional, IDMT (Inverse Definite Minimum Time Relay), are some of the overcurrent characteristics that are often used in complex distribution systems. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment. Next, derating for the number of conductors in the conduit must be considered. Electrical designers face this task daily. Search Products And Discover New Innovations In Your Industry!! There are five different common overcurrent protection trip types that a circuit breaker may incorporate. If the motor starting torque and time to reach operating speed is such that the motor will not start, Section 430.52(C)(1), Exception 2, allows the OCP rating to be raised even higher. Based on the relay operating characteristics, overcurrent relays can be classified into three groups: definite current or instantaneous, definite time, and inverse time. A simple overcurrent protection circuit can be designed by using an Op-Amp to sense the overcurrent and based on the result we can drive a Mosfet to disconnect/connect the load with the power supply. 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. In the example above, the circuit conductors (copper heat-resistant thermoplastic (THHN[A1] [A2] )) are routed through the office environment in a conduit containing six current-carrying conductors. 230 II. The rated ampacity of the conductors, the full-load current rating of the connected load, and the size or load-rating of the OCPD are interrelated. Referring to Table 310.15(B)(16), using the 75°C column, the minimum size wire acceptable is #2/0 with a rating of 175 amps. The derated temperature rating is higher than the MCA value of 31.0 amps for the #8 conductor, which is acceptable for this example. The conductor ampacities for a motor circuit can be determined using Article 430, Part II. If a transformer serves a fire pump (Art. The circuit diagram for the same is simple and can be seen in the below image. His expertise includes 29 years of designing electrical systems for industrial and commercial projects including high-rise buildings, hospitals, schools, theaters, museums, hotels, convention centers, manufacturing facilities, water treatment plants, and nuclear power facilities. Using Table 310.15(B)(16), the 75°C column, the minimum wire size is #8. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. Either a circuit breaker or fuse could be used since the manufacturer listed the equipment with both types of OCP device. 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. The fault current is approximately 10,000 amps. As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. The following discussion will be limited to coordinated and selectively coordinated overcurrent protection-the most common form of circuit protection. 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). This waveform is divided into six periods based on the state of operation of the MOSFET, the drain current relative to the current limit threshold and the output state of the protection circuit. In this example, the OCP rating cannot be increased above 350 amps. Therefore, the minimum conductor size is 10 AWG, and the overcurrent protection or … Calculation of the available short-circuit current is outside the scope of this discussion. 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. As with motors, the requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. Referencing Table 310.15(B)(16), the minimum conductor size allowed for an OCP rating of 150 amps is #1/0. 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). 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. 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. Since there are only three current-carrying conductors in the conduit for this example, derating for the number of conductors is not required. The model for an overcurrent waveform is shown in Fig. Working Principle of Over Current Relay. For an ambient temperature of 104°F, the correction factor for copper THHN 90°C wire is 0.91. There are three types of overcurrent conditions to consider in typical NEC applications: Overload: NEC 2017 defines overload as operation of equipment in excess of normal, full-load rating or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. These three principal components are supplemented with a means of ON/OFF control and a means of limit control. Electrical equipment can usually withstand some level of load current over its rating for a length of time. They also protect circuits downstream from the transformer. 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). I. A single primary winding in the transformer supplies (by induction) two 120-volt windings wired in series in the secondary. 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. There are several applicable sections of the NEC that set the requirements for the selection of OCPs and conductors for commercial buildings. Overcurrent protection schemes are generally designed with a primary means of clearing a fault, as well as one or more backup methods. Section 430.22 applies to this example in that it is a single motor circuit. This section requires the use of the 75°C column in Table 310.15(B)(16), because the terminations for equipment rated 100 amps or higher are required to be rated for 75°C unless listed and labeled otherwise. The means of limit control is normally an overcurrent protective device, which at the electrical-power distribution level is a fuse or circuit breaker (as seen in Figure 4). 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. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). There are some conditions under which the derating factors do not apply, as seen in 310.15(B)(3)(a)(2) through (4). Fig. It protects the equipment of the power system from the fault current. Comparing the two cases, the chiller circuit conductors will be at a higher temperature during operation than the sump pump circuit conductors. “Reference grounding” requires the common connection of one end of one or more of the single-phase AC transformer windings (wye transformer configuration) to a grounding-electrode system, creating both grounded and ungrounded circuit/supply conductors. 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. 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. 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 cable ampacity must be corrected for an ambient temperature of 120°F. Consider an example of an air conditioning unit with a MOP (circuit breaker or fuse) of 50 amps and an MCA of 31.0 amps. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. Feeder circuits. The circuit load for the purpose of sizing the OCP is: OCP sizing load = 1.25 x 1,000 VA + 1.00 x 100 VA. Now select a conductor in accordance with sections 210.19(A) and 310.15. 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. The #10 wire has sufficient ampacity, but per Section 240.4(D), it must be protected with an OCP with a rating of 30 amps or less. Adjustments for the number of current-carrying conductors in a raceway are found in Table 310.15(B)(3)(a). Overcurrent relays are the typical protection for conventional distribution systems. The basic rule for overcurrent protection of conductors—other than using flexible cords, flexible cables, and fixture wires—is to protect the conductor in accordance with the ampacities specified in Section 310.15. An overcurrent occurs when the current exceeds the rated amperage capacity of that circuit or of the connected equipment (such as an appliance) on that circuit. Figure 5 shows a ground-fault condition. Where the primary overcurrent protective device does not exceed 125 percent of the primary full load current, secondary overcurrent protection for the transformer is not required. True. NEC 450-3 CEC Part 1, 26-256 Recommendations for Overcurrent Protection UL and CSA (North American) Standards UL and CSA (North American) Standards North American standards, including UL 508, National Electric Code 450, and the Canadian Electrical Code, Part 1, require overcurrent protection on all control circuit transformers. Overcurrent protection Overcurrent refers to the operating state of electric motor or electrical apparatus element surpassing the rated current. Section 430.52(C)(1) has an exception that allows the next higher standard rating to be used. 695), its primary OCPD shall be size… The conduit will contain three current-carrying conductors. Time Graded Over Current Protection Unsymmetrical faults have different fault currents in each phase. By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. This data will be found on the equipment nameplate and can also be obtained from the manufacturer in the form of a data sheet. Overcurrent Protection Circuit Working Typical practice is to use an OCP rating less than the maximum calculated above. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. This places a short-circuit across the supply-transformer winding. If circuit interruption will create a hazard (e.g., fire pump circu… 2.1 Overcurrent protection. An overcurrent protection device (OCPD) is a piece of equipment used in electrical systems that are at risk of experiencing overcurrent due to overloads, short circuits, or ground faults. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. Symmetrical faults result in the same current flow in each phase during the fault condition. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. In the example above, there are three current-carrying conductors routed in the conduit. 3: Trip characteristic of two-stage, non-directional, maximum-overcurrent time protection If several protective devices are connected in series across the network, this leads to a graded curve (Figure 4), the nearest protective relay being tripped in the event of a fault. 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). A three-phase AC electrical power distribution system, as shown in Figure 2, will normally have a higher value of short-circuit overcurrent because the short will normally involve more than one single-phase AC transformer winding. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition. Then . In this example, the ground-fault path adds approximately .012 ohms of resistance in parallel with the load resistance, resulting in a much lower circuit resistance. If the load is debatable, be conservative and design for a continuous load. 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. The copper THHN circuit conductors are routed in an area with an ambient temperature not exceeding 104°F, and the number of current-carrying conductors in the raceway is three. The requirement for sizing the conductors is simply 125% of the full-load current specified in Table 430.50. Table 1describes current, voltage and protection circuit assumptions for each period. Distance protection. [nextpage title=”Over Current Protection (OCP)”] There are a lot of misconceptions about the over current protection (OCP) and an explanation of why this protection exists is in order. 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. Number of conductors. The short-circuit current rating and interrupting rating must also be determined based on the available short-circuit current in the circuit. However, many authorities having jurisdiction have made voltage drop a code requirement. 11.4.3.1 Fault Detection Methods. If the fault current persists, the insulation will melt and the conductors themselves will be damaged. The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). Even though this example is using copper THHN wire, rated for 90°C, the 60°C column must be used due to the requirement of Section 110.14(C)(1)(a). 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. Consider a 208 V, 3-phase feeder supplying a panelboard with a noncontinuous load of 10 kVA and a continuous load of 30 kVA. In this case, the 75°C column must be used due to the requirement of Section 110.14(C)(1)(a). Protection based on the manufacturer-supplied MCA value, which in this case is 31.0.! 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