As the first sentence of the National Electrical Code (NEC) states, the Code’s purpose is the practical safeguarding of people and property from hazards arising from the use of electricity [90.1(A)]. This first section continues by stating the Code is not intended as a design specification or an instruction manual for untrained people.
The second section in Article 90 pertains to the adequacy of the Code’s provisions; it states the provisions are considered necessary for safety. Installing electrical systems and equipment in accordance with the NEC’s rules and regulations along with proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient or adequate for good service or future expansion of electrical use. The last part of this sentence is sometimes overlooked. Following the rules and regulations in the NEC does not inherently mean the electrical installation will be efficient, convenient or adequate for good service or future expansion of electrical use.
For example, an electrical system is needed for a new building. A small industrial shop will be the building’s only occupant. A load calculation was performed in accordance with Article 220, and the minimum required ampacity for all of the loads was 221 amperes (A) with an electrical system of 208Y/120 volts (V), three-phase. In accordance with 240.6(A), 221 is not a standard ampere rating for fuses and inverse time circuit breakers. The next standard ampere rating above that is 225. Since the calculation was performed in accordance with the provisions in the Code, the service installed for this industrial shop could be 225A at 208Y/120V, three-phase. If a 225A service were to be installed, there would be no room for future expansion of electrical use. If any new equipment were to be installed, hazards would occur because of overloading the wiring system. As stated in the Informational Note below 90.1(B), an initial adequate installation and reasonable provisions for system changes provide for future increases in the use of electricity.
Motors and machines with motors are usually a considerable part of the total load in industrial plants and factories. Chapter 4, Article 430, covers motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers. It is important to know what information is required on a motor nameplate. A motor shall be marked with the information listed in 430.7(A)(1) through (A)(15). The first two items on this list are the manufacturer’s name and the rated volts and full-load current (FLC). If the motor is a multispeed motor, the FLC for each speed is required to be marked on the motor unless the motor is a shaded-pole and permanent- split capacitor motor where amperes are required only for maximum speed.
The next items required are the rated frequency and number of phases if an alternating current (AC) motor, the rated full-load speed, the rated temperature rise or the insulation system class and rated ambient temperature, and the time rating. The time rating shall be 5, 15, 30 or 60 minutes, or continuous. If the horsepower (hp) is 1∕8 hp or more, the rated horsepower must be marked on the motor. For a multispeed motor 1∕8 hp or more, rated horsepower for each speed shall be marked on the motor, except for shaded-pole and permanent-split capacitor motors 1∕8 hp or more where rated horsepower is required only for maximum speed.
Motors of arc welders are not required to be marked with the horsepower rating. If the motor is AC rated ½ hp or more, the code letter or locked-rotor amperes shall be marked on the motor. On polyphase wound-rotor motors, the code letter shall be omitted. For motors that are design B, C or D motors, the design letter shall be marked on the motor. If the motor is a wound-rotor induction motor, secondary volts and FLC are required to be on the motor. If the motor is a direct-current (DC) excited synchronous motor, field current shall be on the motor. If it is a DC motor, the motor nameplate shall show the winding—straight shunt, stabilized shunt, compound or series. Fractional horsepower DC motors 175 millimeters (7 inches) or less in diameter are not required to be marked.
If the motor is provided with a thermal protector complying with 430.32(A)(2) or (B)(2), the motor shall be marked “Thermally Protected.” Thermally protected motors rated 100 watts (W) or less and complying with 430.32(B)(2) shall be permitted to use the abbreviated marking “T.P.”
A motor complying with 430.32(B)(4) shall be marked “Impedance Protected.” Impedance-protected motors rated 100W or less and complying with 430.32(B)(4) shall be permitted to use the abbreviated marking “Z.P.” The last item in this list pertains to motors equipped with electrically powered condensation prevention heaters; these motors shall be marked with the rated heater voltage, number of phases and the rated power in watts (see Figure 1).
Section 430.7(B) pertains to the locked- rotor current of motors. Out in the field, the rule of thumb for locked-rotor current is six times the full-load amperes marked on the motor’s nameplate. The result of multiplying the full-load amperes of a motor by six might be close to the actual locked-rotor current or the result might not be anywhere near the actual locked-rotor current. If the locked-rotor amperes are not on the nameplate, locked-rotor current can be calculated, but certain information is needed. One of the pieces of information required to be marked on a motor is the code letter, or locked-rotor amperes if the motor is an AC motor rated ½ hp or more. In accordance with 430.7(B), the code letter indicating motor input with locked rotor shall be in an individual block on the nameplate, properly designated (see Figure 2).
Code letters marked on motor nameplates to show motor input with locked rotor shall be in accordance with Table 430.7(B). For example, what is the locked-rotor current for a 2-hp motor with a marked code letter J? The nameplate for this motor is shown below. The nameplate amperes when the motor is supplied by 208V, three-phase power is 5.7A. The first step is to look in Table 430.7(B) to find the kilovolt-amperes (kVA) per horsepower for the code letter J. The numbers across from J show the kilovolt- amperes per horsepower can range anywhere from 7.1 to 7.99. Use the higher number to calculate the highest locked-rotor current.
The next step is to find the total kilovolt- amperes. Because the motor in this example is a 2-hp motor, multiply 7.99 by 2. The total kilovolt-amperes for this motor is 15.98 (7.99 × 2 = 15.98). Because this number is kilovolt-amperes, multiply it by 1,000 to find volt-amperes (VA). The total volt-amperes for this motor is 15,980 (15.98 × 1,000 = 15,980). Now, divide the total volt-amperes by the total voltage to find the locked-rotor current. Because this is a three-phase motor, the total voltage is not 208. To find the total voltage in this three-phase system, multiply the line-to-line voltage by the square root of three (1.732). The total voltage in a 208V, three-phase system is 360V (208 × 1.732 = 360.256 = 360). When dividing the total volt-amperes by the total voltage in this problem, the locked-rotor current is 44.4A (15,980 ÷ 360 = 44.4). Although it is not necessary, the multiplication factor can be found by dividing the locked rotor current by the nameplate amperes. The locked-rotor current is 7.8 (44.4 ÷ 5.7 = 7.8) times the nameplate amperes when this 2-hp motor is connected to a source voltage of 208V, three-phase.
Although the rule of thumb out in the field is six times the nameplate amperes, the locked- rotor current depends on the code letter marked on the nameplate (see Figure 3).
Next month’s column continues the discussion of requirements for motors, motor circuits and controllers.
About The Author
Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.