Don’t be intimidated by the largest chapter of the NEC
With 13 parts and a focus on challenging subject matter, Art. 430 can seem overwhelming. After a quick scan, it may seem impossible to correctly apply its requirements, but a closer look reveals usability features, including its tables, that make Art. 430 more user-friendly than you may think. One especially notable feature is Figure 430.1, which provides a graphical representation of how to apply the rules of Art. 430. It allows you to proceed through Art. 430 methodically and not miss any of the following key requirements.
General requirements. For general motor applications, current ratings shall be based on 430.6(A)(1) and (2). You should also be familiar with the difference between full load currents and nameplate current ratings of motors.
Full load currents [430.6(A)(1)]. Use the motor full load current (FLC) rating (Tables 430.147, 430.148, or 430.150) when determining conductor ampacity (430.22), the branch-circuit short-circuit and ground-fault protection device (430.52 and 430.62), and the ampere rating of switches (430.110). Don't use the current rating on the motor nameplate for this purpose (Fig. 1).
It's also important to know how to properly connect motor terminals and where to locate motors within the system.
Motor nameplate current rating [430.6(A)(2)]. Use motor nameplate current ratings when selecting devices for protecting motors and their control apparatus and branch-circuit conductors against excessive heat caused by motor overloads and failure to start (430.31).
Motor controller terminals. Connect control circuit devices' motor controllers and terminals with copper conductors, unless identified otherwise. Torque motor control conductors 14 AWG and smaller at a minimum of 7 lb-in. for screw-type pressure terminals, unless identified otherwise [430.9, 110.3(B), and 110.14 FPN].
Motor locations. Locate motors to facilitate maintenance and provide adequate ventilation (430.14).
Conductor size. Be careful. If you misapply Chapter 3 ampacity tables when sizing motor conductors, you can undersize them. Motors have inrush current, which conductor sizing must allow for.
Single motors. Per 430.22, size motor branch-circuit conductors no smaller than 125% of the motor FLC rating listed in Tables 430.147, 430.148, or 430.150 (Fig. 2). Size the branch-circuit short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1.
Note: The Code doesn't require the wire size to address voltage drop or the resulting inability to start the motor when the conductor length is excessively long.
Test your knowledge with the following question. What size branch-circuit conductor does a 7.5-hp, 230V, 3-phase motor need if the conductor terminals are rated 60°C?
(a) 14 AWG
(b) 12 AWG
(c) 10 AWG
(d) none of the above
Per Table 430.150, a 7.5-hp 230V, 3-phase motor has an FLC of 22A. Size the branch-circuit conductors no less than 125% of FLC: 22A×1.25 = 27.5A. Referring to Table 310.16, a 10 AWG conductor is rated 30A at 60°C. Therefore, the correct answer is (c), 10 AWG.
Size the branch-circuit short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1. For an inverse-time breaker: 22A×2.5=55A (Next size up is 60A.)
Multiple motors (feeder). Per 430.24, size multiple motor conductors as follows. Multiply the FLC rating of the highest-load motor by 1.25 and add that to the sum of the FLC ratings of all the other motors in the group. That's your motor load for calculating ampacity. Add any other loads on that conductor to calculate total conductor ampacity.
Let's try sizing a motor feeder conductor. What size 75°C feeder conductor is required to serve two 7.5-hp, 230V, 3-phase motors?
(a) 14 AWG
(b) 12 AWG
(c) 10 AWG
(d) 8 AWG
Solution: (22A×1.25)+22A=49.5A. Per Table 310.16, an 8 AWG conductor is rated 50A at 75°C. Therefore, the correct answer is (d), 8 AWG.
Overload protection. An overload isn't a short circuit or ground fault; it's an operating current that's too high. Overload protection devices will interrupt a current that's too high when it persists for too long. An overload protection device allows time for the starting current of the motor, which is higher than the operating current, but only momentarily. A branch-circuit short-circuit and ground-fault protection device protects the motor, the motor control apparatus, and the conductors against short circuits or ground faults, but not against overload (Fig. 3 above).
You must protect each motor branch circuit against short circuits and ground faults by a protection device sized no greater than the percentages listed in Table 430.52. Motor branch-circuit conductors are protected against overcurrent by overloads sized at 115% to 125% of motor nameplate current rating (430.32).
Feeder short-circuit and ground-fault protection. Per 430.62, protect feeder conductors against short circuits and ground faults by a protection device sized not greater than the largest rating of the branch-circuit short-circuit and ground-fault protection device for any motor, plus the sum of the FLCs of the other motors in the group.
What size feeder protection (inverse-time breakers with 75°C terminals) and conductor do you need for the following two motors (Fig. 4):
Motor 1 - 20-hp, 460V, 3-phase=27A
Motor 2 - 10-hp, 460V, 3-phase=14A?
(a) 70A breaker and
8 AWG conductor
(b) 80A breaker and
8 AWG conductor
(c) 90A breaker and
8 AWG conductor
(d) none of the above
The size of the conductor is determined by taking the largest FLC of the motors and multiplying it by 1.25 (430.24), and then adding the FLC of all other motors in the group: (27A×1.25)+14A=48A. Per 110.14(C) and Table 310.16, you would use an 8 AWG conductor rated 50A at 75°C.
Size the feeder protection [430.63(A)] no greater than the largest branch-circuit protection device plus other motor FLCs.
Step 1. Determine the largest branch-circuit protection device [430-52(C)(1)].
20-hp motor=27A×2.5=68A (Next size up is 70A.) [430.52(C)(1) Ex.]
10-hp motor=14A×2.5=35A
Step 2. Size feeder protection: 70A+14A=84A. Select the next size down protection device, which would be an 80A breaker.
Therefore, the correct answer is (b), 80A breaker and 8 AWG conductor.
Motor control circuits. You must provide motor control circuit conductors with a disconnecting means that opens all conductors of the motor control circuit (430.74). The controller disconnect can serve as the disconnecting means for control circuit conductors if the control circuit conductors are tapped from the controller disconnect [430.102(A)]. If they aren't, provide a separate disconnect for the control circuit conductors and locate it adjacent to the controller disconnect. The control circuit disconnect can't be located higher than 6 feet 7 inches above the floor or working platform, unless located adjacent to the equipment it supplies [404.8(A)].
Motor controllers. Each motor requires its own controller (430.87). Select an enclosure suitable for the environment that the controller occupies, per Table 430.91.
Controllers other than circuit breakers and molded case switches must have a horsepower rating no less than that of the motor. A circuit breaker can serve as a motor controller (430.111). A molded case switch, rated in amperes, can also serve as a motor controller.
For stationary motors rated at 2 hp or less and 300V or less, the controller can be either of the following:
A general-use switch with an ampere rating not less than twice the full-load current rating of the motor.
A general-use snap switch with a motor full-load current rating not more than 80% of the ampere rating of the switch.
The motor controller is required to open only as many conductors of the circuit as necessary to start and stop the motor (430.84). For example, one conductor must open to control a two-wire, single-phase motor; two conductors must open to control a three-wire, 3-phase motor.
Note: The controller starts and stops the motor; it isn't a disconnecting means (430.103).
Disconnecting means. You need a disconnect for each motor controller. You must locate it within sight of the controller (Fig. 5). “Within sight” means visible and not more than 50 feet from each other (Art. 100). Under certain circumstances, the requirements of 430.102(B) allow exceptions to this requirement.
The controller disconnect must open all circuit conductors simultaneously (430.103). The controller disconnect can serve as the disconnecting means for the motor control circuit conductors (430.74) and the motor [430.102(B) Ex.].
The disconnecting means for the motor controller and the motor must open all ungrounded supply conductors simultaneously [430.103].
The disconnecting means must be legibly marked to identify its intended purpose (110.22 and 408.4). When operated vertically, the “up” position corresponds to the “on” state [240.81 and 404.6(C)]. The controller disconnect or motor disconnect required by 430.102 must be readily accessible.
Additional tables. Table 430.148 lists the FLC for single-phase alternating-current motors. Use these values to determine motor conductor sizing, ampere ratings of disconnects, controller rating, and branch-circuit and feeder protection — but not overload protection [430.6(A)(1)].
Table 430.150 lists the FLC for 3-phase alternating-current motors. Use these values to determine motor conductor sizing, ampere ratings of disconnects, controller rating, and branch-circuit and feeder protection — but not overload protection [430.6(A)(1)].
Table 430.151(A) contains locked-rotor current for single-phase motors, and Table 430.151(B) contains the locked-rotor current for 3-phase motors. Use these values in the selection of controllers and disconnecting means when the horsepower rating isn't marked on the motor nameplate.
Art. 430 is an interesting section of the Code, given the complexity of the subject it covers and the ease with which you can apply it. That ease of application is possible because of usability features, such as the various tables and Figure 430.1. If you proceed methodically, you'll correctly apply Art. 430 every time.