When determining OCP ratings for motors, the full-load ampere (FLA) values given in Article 430 are used. ANSWER: Motor branch circuit conductors are protected from short-circuit and ground-fault currents by the branch circuit overcurrent devices, but these devices will not protect the conductors from overload conditions. 427 Fixed electric heating for pipelines and vessels, 430 Motors, motor circuits, and controllers, 440 Air conditioning and refrigerating equipment. The single line-to ground fault is the most common type of fault. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). These methods can detect pre-set over/under current/torque conditions to provide quicker motor/load protection than … During the period that a motor is starting it draws a high current. This allows the motor to operate at up to 15 percent above its normal rating without causing the overload protection to trip. 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. In this example, the OCP rating cannot be increased above 350 amps. The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. CxEnergy and the AABC Commissioning Group (ACG), By Steven Eich, PE, CDT, REP, LEED AP; Environmental Systems Design, Chicago, Specifying electrical distribution systems, Risk management: Commissioning electrical systems, Concrete mixture designed to cut carbon emissions, Teambuilding strategies to optimize talent. The motor-overload protector is usually a device, located in the motor starter, that responds to motor current and is set to trip open the motor controller when the motor current exceeds 125% of the nameplate current for 1.15 service factor motors or 115% of nameplate current for motors without a service factor. Since the ampacities in Table 310.15(B)(3)(a) already account for up to three current-carrying conductors, no further derating is required. For example, a 1.5-hp motor used in a hermetically sealed compressor might be able to do 2 hp of work because heat is being removed from the motor windings, allowing higher currents to flow without exceeding the winding-conductor temperature rating. QUESTION: I'm concerned about sizing motor circuit wiring and overcurrent protection. The first rule is: "The motor branch-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor." 110-14(c) Temperature Limitations Last month's In Focus introduced the topic of temperature limitation requirements. devices must be provided to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. 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. Having trouble finding time to sit down with the latest issue of ELECTRICAL CONTRACTOR? Three different manufacturer slide-rule guides were checked; they all recommend a circuit breaker OCP rating for 200 amps for the following example. Section 430-52 refers us to Table 430-152 which, depending upon the type of motor and the type of overcurrent device being used, permits a maximum percentage of full-load current that may be used to size the overcurrent device. Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Step #6: Determine the requirements for the motor control circuit overcurrent protection. The circuit is protected by a 15-amp circuit breaker. If the fault current persists, the insulation will melt and the conductors themselves will be damaged. Short-circuits and ground-faults develop current of a high magnitude and will open the overcurrent device sized according to Table 430-152 rapidly. The inverse time overcurrent protection is provided as a back-up protection against system infeed to a generator circuit fault not cleared by main protection. Next, derating for the number of conductors in the conduit must be considered. 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. This rating falls between the standard ratings of 350 amps and 400 amps. Symmetrical 3-phase faults rarely occur, but their analysis is useful in understanding a system’s response to a fault and usually results in the worst-case fault levels. 5. ANSWER: Motor branch circuit conductors are protected against two possible problems: (1) short-circuit and ground-fault and (2) overload. 15 - What provides motor running overcurrent protection... Ch. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. Overcurrent protection is generally a protection against short circuit where excessive current starts to flow in the circuit which leads to damage the connected equipment. CFE Media and Technology to Host Spring Virtual Training Week for Engineers, Three waves of disruption ahead for the A/E industry, Humidification installed for U.S. Army national museum, Weekly merger and acquisition update: February 19, 2021. Step #2: Determine the size of the motor branch circuit conductors. Code letters marked on motor nameplates show motor input with locked rotor and shall be in accordance with Table 430-7(b). This seems like an example of a noncontinuous circuit. 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. Calculation of the available short-circuit current is outside the scope of this discussion. These values are generally 115 percent or 125 percent of the motor FLC. Looking through the tables in 310.15(B), notice that some tables are based on an ambient temperature of 30°C and others are based on 40°C. The NEC allows for a single overcurrent protection device for motor overload, motor branch short circuit and ground faults. Consider a 208 V, 3-phase feeder supplying a panelboard with a noncontinuous load of 10 kVA and a continuous load of 30 kVA. What happens during an overload or fault condition? The requirement for sizing the conductors is simply 125% of the full-load current specified in Table 430.50. Moreover, VFDs have additional user defined protection methods. Step #3: Determine the fuse size (dual element) to be used as the motor branch circuit short-circuit and ground-fault protection. Overload protection of the motor circuit permits the overcurrent protection device of the circuit to function solely as a short circuit or a ground-fault device hence the term “branch-circuit short-circuit and ground-fault device.” Further, Section 240.4(D), Small Conductors, requires the OCP for #14 wire to be rated at 15 amps. Assume a 10 HP three-phase 208-volt motor with a Code letter F and a service factor of 1.15. Electrical designers face this task daily. Identifying Emergency Systems: Clear marking of emergency wiring prevents problems, Getting Started with the NEC, Stacked Neutrals and More, Temperature Limitations for Electrical Connections, Part II. An OCP rating of 300% higher than the full-load current is 3 x 124 amps = 372 amps. As soon as an overcurrent is detected, the device trips and breaks the circuit. This table, as the title states, provides for a percentage of FLC of motors to establish a "Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices." Table 240.4(G) requires Article 430 to be used for selection of motor-circuit overcurrent protection. This is enough to detect the rotor faults. Section 430-52 refers to Table 430-152 for the maximum rating or setting of motor branch circuit short-circuit and ground-fault protective devices. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment. 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 NEC requires a more conservative selection (125%) in the case of continuous loads due to increased heat dissipated by the circuit conductors as compared with noncontinuous loads. N 445.13(B) Overcurrent Protection Provided. 4. The National Electrical Code (NEC) in Section 430-32(a)(1) allows a motor with a service factor of 1.15 to use overload protection of 125 percent. 12, a member of the NECA Codes and Standards Committee, and a member of the Western Section of the International Association of Electrical Inspectors. As a result, motor branch circuits are permitted by NEC requirements to have two separate overcurrent protective devices located at physically different locations 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. As with the previous example, the wire type selected is copper THHN, which is rated for 90°C. For an ambient temperature of 120°F, the correction factor for copper THHN 90°C wire is 0.82. Ground-fault: One or more ungrounded conductors coming into contact with a grounded conductor or a grounded surface. First let's get a few definitions in order. 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. 430.32 specify that overload protection device be sized between 115% to 140% of the motor nameplate current rating. QUESTION: Are motor branch circuit conductors required to be sized at 125 percent of the motor full load current (FLC) to handle the motor starting current? Overcurrent protection devices. But there is much more to it. The National Electrical Code in Section 430-6 requires that the Tables 147 through 430-150 be used to determine the FLC of motors and not the nameplate ratings. How to protect a circuit from dangerous overloads and short circuits. QUESTION: I'm confused by the terminology "motor branch circuit short-circuit and ground-fault protection" and "overload." Referencing Table 310.15(B)(16), the minimum conductor size allowed for an OCP rating of 150 amps is #1/0. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. Also, energy codes require voltage drop to be considered. For long circuits, the conductor size may need to be increased to maintain minimum voltage-drop requirements. These are the conditions against which we must protect our motor branch circuit conductors. According to Section 240.6(a) for “Standard Ampere Ratings of overcurrent devices”, we must select a minimum 70 ampere overcurrent protection device. The requirements for motor-circuit overcurrent protection are different than branch and feeder circuits, often leading to confusion. Though at first glance the NEC requirements might not seem straightforward, there is solid reasoning behind the overcurrent-protection code rules. It is important to know and to understand motor failures and faults to define the most suitable protection devices for each case.You also must know about important terms related to motor control and protection.. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. In the case of an inverse-time circuit breaker for the motor in this example, Exception (c) allows the OCP rating to be increased from 250% to 300%. Most circuit breakers and fuses would "nuisance open" on motor startup if sized to provide motor and branch-circuit overload protection. In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). The circuit breaker does not open; the load current flows and the conductors do not overheat. For motor circuits, overload protection is provided by the motor-overload protector (refer to Article 430 Part III). 30.8 amperes x 1.15 = 35.42A (60A disconnect switch required) Electrical equipment can usually withstand some level of load current over its rating for a length of time. It is important to have a distinction continuous and noncontinuous loads because of heat. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F. Overload protection is installed in the main control circuit or power line of motor to protect from damage due to mechanical malfunction overload conditions when it is running. After applying the appropriate deratings, the calculated ampacity of the #1/0 wire is adequately protected by the 150-amp OCP selected above. In the continuous case (for example, a chiller), the load might be active for 8 hours or longer. over the full range of overcurrents between its rated currents and its interrupting rating– includes overload (OL) and short-circuit (SC) protection • Non-motor circuit Overload conditions are usually not as time-critical as short circuits and ground faults. This example requires a 50-amp OCP, therefore a #8 wire must be used. Determining whether a load is continuous or noncontinuous is not always clear. The load and cable ratings calculated above are minimum values. ANSWER: No. The fault current is approximately 5,000 amps. Where the overload relay selected is not sufficient to start the motor or carry the load, the next- higher-size overload relay shall be permitted to be used provided the trip current of the overload relay does not exceed the percentages of motor FLC shown in Section 430-34. Overload protection for the motor branch circuit conductors is provided by the motor TROUT has been an electrical contractor for many years, and is currently associated with Maron Electric Co., Skokie, Ill. Number of conductors. A fault, such as a short circuit or ground fault, is not an overload. The short-circuit current rating and interrupting rating must also be determined based on the available short-circuit current in the circuit. Short-circuit: Two or more conductors of opposite polarity coming into contact with each other with relatively low resistance between them or contact between them short of the load. In the majority of applications, motor overcurrent protection is provided by overcurrent _____ in the motor controller. Common Overcurrent Protection Devices (OCPDs) An overcurrent protection device (OCPD) is a piece of electrical equipment used to protect service, feeder, and branch circuits and equipment from excess current by interrupting the flow of current. 30.8 amperes x 1.25 = 38.5A 15 - a. Finger-safe protection is provided in the latest fuse technology for fuse holders and blocks, switches, and power distribution ... R-Rated medium voltage fuses and motor circuits 41 • Medium voltage fuses 40 • ... of Overcurrent Protection from the mid-1960s until his passing in 2015. In this case, the maximum OCP rating is then 350 amps. Short circuit protection may be provided by fuses or circuit breakers. Section 430-72(b) Exception No. Phone apps are also available that provide the same function as the slide rule. 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 conductor ampacities for a motor circuit can be determined using Article 430, Part II. [HEAD]. 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. 15 - When a metal junction box is installed as part of... Ch. Consider the case of an office storeroom lighting circuit. They are sized according to Section 430-32. An alternative way to define protection is based on the principle used in the protection scheme. 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). The requirements for overcurrent protection of equipment can be found in the NEC article that addresses that specific equipment. 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. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. Review overcurrent protection for certain types of building equipment. The settings of the stator over current relay generally of the order of 1.6 times full load current. For example, a 10 HP motor with a service factor of 1.15 can be allowed to develop current equivalent to 11.5 HP without damage to the motor winding insulation. Motor overload devices, which are generally located in the motor controller, are permitted by Section 430-32(a)(1) to be sized according to the marked "service factor" of the motor. Fortunately, the NFPA 70: National Electric Code (NEC) gives requirements for most of the applications that electrical engineers and designers encounter in their work. Symmetrical faults result in the same current flow in each phase during the fault condition.
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