Published In June 2000
This month's column concerns overcurrent protection for conductors in motor and controller circuits and overload protection for motors, subjects that many subscribers to our "Code Question of the Day" online feature have inquired about. 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? ANSWER: No. The motor starting or "inrush" current, which is also called "locked rotor current," is only present during the acceleration period at the moment the motor is started. The inrush current decreases rapidly as the motor begins to rotate. The motor branch-circuit overcurrent protection as calculated from Table 430-152 easily handles these currents within the limitations of the motor branch circuit conductors. These motor branch-circuit overcurrent protective devices are permitted to be sized much higher than the rated ampacity of the motor branch circuit conductors. They are also able to protect the motor branch circuit conductors from short-circuit or ground-fault currents because of the magnitude of the currents produced so rapidly by these types of faults. Why are the motor branch circuit conductors sized at 125 percent of the motor full load current? See the next question. QUESTION: Why are motor branch circuit conductors sized at 125 percent of the motor FLC? 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. 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. These values are generally 115 percent or 125 percent of the motor FLC. There are exceptions to this as shown in Section 430-32(a)(2). If a motor is allowed to at up to 125 percent of motor FLC, then we protect the branch circuit conductors by sizing them at 125 percent of motor FLC as well. QUESTION: What is the service factor of a motor and what do they mean by "marked service factor?" ANSWER: Service factor is a margin of safety. When the manufacturer designs a service factor into a motor, it means that motor can develop more than its rated current without damage to itself. 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. 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. This allows the motor to operate at up to 15 percent above its normal rating without causing the overload protection to trip. QUESTION: I'm concerned about sizing motor circuit wiring and overcurrent protection. Why is it that we can over-fuse the motor with Table 430-152? ANSWER: Table 430-152 does not specify overcurrent protection for motors. 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." These overcurrent devices (fuses or circuit breakers) protect the conductors feeding the motor from overcurrents caused by short-circuit or ground-faults. They are not designed to protect the motor windings. The motor branch circuit conductors are protected from overload conditions by the motor overload protection specified in Section 430-32. The key to understanding conductor and motor protection is in knowing the meaning of ground-fault, short-circuit, and overload. See the next question. QUESTION: I'm confused by the terminology "motor branch circuit short-circuit and ground-fault protection" and "overload." Can you clarify this for me? ANSWER: Motor branch circuit conductors are protected against two possible problems: (1) short-circuit and ground-fault and (2) overload. First let's get a few definitions in order. 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. Ground-fault: One or more ungrounded conductors coming into contact with a grounded conductor or a grounded surface. Overload: Operation of equipment or a conductor with a current value in excess of its rated ampacity, which would cause damage or dangerous overheating. A fault such as a short-circuit or ground-fault is not an overload. These are the conditions against which we must protect our motor branch circuit conductors. Short-circuit and ground-fault protection of conductors is covered in Section 430-52. The first rule is: "The motor branch-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor." 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. This overcurrent protective device may be either a fuse or circuit breaker. Short-circuits and ground-faults develop current of a high magnitude and will open the overcurrent device sized according to Table 430-152 rapidly. Overload protection for the motor branch circuit conductors is provided by the motor overload devices, which also protect the motor windings from an overload condition. These overload devices are located in the motor controller or are an integral part of the motor. They are sized according to Section 430-32. Because these overload devices can be sized as high as 125 percent of motor full-load current, the branch circuit conductors are required by Section 430-22 to be sized at 125 percent of the motor FLC as well. Six Steps to a Basic Motor Installation Assume a 10 HP three-phase 208-volt motor with a Code letter F and a service factor of 1.15. The motor will be fed from a distribution panel 60 feet from the motor location. The motor will be manually started by means of a start-stop button in the cover of the motor controller and will have a remote stop button located 50 feet from the motor controller. The motor controller will be located next to the motor and will contain the motor overload protective devices. Step #1: Determine the FLC of the motor. 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. Table 430-150 covers 3-phase alternating current motors and using this table we find that a 10 HP 208-volt motor has a FLC of 30.8 amperes. Step #2: Determine the size of the motor branch circuit conductors. Section 430-22 requires the branch circuit conductors supplying a single motor to have an ampacity of not less than 125 percent of the FLC rating. 30.8 amperes x 1.25 = 38.5A Step #3: Determine the fuse size (dual element) to be used as the motor branch circuit short-circuit and ground-fault protection. 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. 30.8 amperes x 1.75 = 53.9A. Sections 430-52 and Section 240-6, next higher standard-size (60A). Step #4: Determine the rating required for the motor-disconnect switch. Section 430-110 requires the motor disconnecting means to have an ampere rating of at least 125 percent of the FLC rating of the motor. 30.8 amperes x 1.15 = 35.42A (60A disconnect switch required) Section 430-102 requires that a disconnecting means be located in sight from the controller location. "In sight from" is defined as being visible and not more than 50 feet distant from the other. Step #5: Determine the motor and branch circuit overload protection required. Section 430-32 requires a separate overload device that is responsive to motor current or a thermal protector integral with the motor that will prevent overheating of the motor due to overload or failure to start. In our installation the motor overload protective devices will be in the motor controller. For motors with a service factor of not less than 1.15 Section 430-32(a)(1) allows 125 percent of the FLC of the motor for the motor overload protective device. 30.8 amperes x 1.25 = 38.5A 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. For a motor with a service factor of not less than 1.15 a percentage of 140 percent may be used. 30.8 amperes x 1.40 = 43.12A Step #6: Determine the requirements for the motor control circuit overcurrent protection. Section 430-72 outlines these requirements. The motor control circuit extends beyond the motor controller to a remote Stop Button. Section 430-72(b) Exception No. 2 permits the motor control circuit to be protected by the branch circuit protective device if it does not exceed the value specified in Column C of Table 430-72(b). The branch circuit overcurrent protective device used in this installation is 60 ampere and if No. 12 copper conductors are used for the motor control circuit conductors then they shall be considered as protected by the branch circuit overcurrent protective device and no supplemental overcurrent protection shall be required. Code letters marked on motor nameplates show motor input with locked rotor and shall be in accordance with Table 430-7(b). Most motors have a continuous duty rating and can operate indefinitely at their rated load. During the period that a motor is starting it draws a high current. This "inrush" current could be as high as four to 10 times the motor full-load current. Table 430-152 allows a percentage increase of FLC so that the motor may be successfully started while maintaining full overcurrent protection. TROUT has been an electrical contractor for many years, and is currently associated with Maron Electric Co., Skokie, Ill. He is chairman of the National Electrical Code-making panel No. 12, a member of the NECA Codes and Standards Committee, and a member of the Western Section of the International Association of Electrical Inspectors.