Article 430: Motors, Motor Circuits, and Controllers

Charting a course through the largest chapter of the NEC

Mike Holt for EC&M Magazine

Article 430, with its tough subject and thirteen parts, presents a challenge. At first glance, correctly applying Article 430 may seem impossible. But, a closer look shows reveals usability features-such as its tables-that make Article 430 more user-friendly than many people realize. One especially notable feature is Figure 430.1, which provides a graphical representation of how to apply Article 430. Figure 430.1 allows you to proceed through Article 430 methodically and not miss a key requirement. Let's see what some of those requirements are.

Part I. General Requirements

  • Full load currents. Use the motor full load current rating (see 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 62], and the ampere rating of switches [430.110]. Do not use the current rating on the motor nameplate for this purpose (Figure 430-2).

Note: Graphics not part of this newsletter, they are contained in the textbook, Understanding the National Electrical Code.

  • Motor nameplate current rating. Use the motor nameplate current rating when selecting devices intended to protect motors, motor control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start [430.31].
  • Motor controller terminals. Connect motor controllers and terminals of control circuit devices 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. See [430.9], [110.3(B)], and [110.14 FPN].
  • Motor locations. Locate motors to facilitate maintenance and provide adequate ventilation [430.14].

Part II. Conductor Size

Some people misapply Chapter 3 ampacity tables when sizing motor conductors, and therefore undersize the motor conductors. Motors have inrush current, which conductor sizing must allow for.

  • Single motors. Per [430.22], size motor branch-circuit conductors no smaller than 125 percent of the motor FLC rating listed in Table 430.147 or 430.148 (Figure 430-4). Size the branch-circuit short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1.
  • Multiple motors. Per [430.24], size multiple motor conductors as follows. First, multiply the full-load current rating of the highest-load motor by 1.25. Then, add up the full-load current ratings of all the other motors in the group. Add these two numbers. That's your motor load for calculating ampacity. Add any other loads on that conductor, to calculate total conductor ampacity.

Test your knowledge, with the following question. What size branch-circuit conductor does a 71/2-hp, 230V, 3Ø motor need if the conductor terminals are rated 60°C (Figure 430-5)?

(a) 14 AWG
(b) 12 AWG
(c) 10 AWG
(d) none of these

Answer: (c) 10 AWG
22A x 1.25 = 27.5A, 10 AWG, rated 30A at 60°C.

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 x 2.5 = 55A, next size up = 60A.
Let's try sizing a motor feeder conductor. Here's your question. What size 75°C feeder conductor is required for two 71/2-hp, 230V, 3Ø motors (Figure 430-7)?

(a) 14 AWG
(b) 12 AWG
(c) 10 AWG
(d) 8 AWG

Answer: (d) 8 AWG
(22A x 1.25) + 22A = 49.5A, 8 AWG rated 50A at 75°C

Size the feeder protection device [430.62] as follows:

Step 1. Determine the branch-circuit protection rating. Inverse-Time Breaker: 22A x 2.5 = 55A, next size up 60A.
Step 2. Size the feeder protection device. Inverse-Time Breaker: 60A + 22A = 82A, next size down, 80A.

Part III. Overload Protection

An overload is not a short-circuit or ground fault. It's an operating current that is just too high. Overload protection devices will interrupt a current that is too high, when it persists for too long (typically on the order of seconds). The time factor allows for the starting current of the motor, which is higher than the operating current but only momentary. 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 (Figure 430-11).

You must protect each motor branch circuit against short circuit and ground fault 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 percent of motor nameplate current rating [430.32].

Part VI. 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 the control circuit conductors are not tapped from the controller disconnect, provide a separate disconnect for the control circuit conductors and locate it adjacent to the controller disconnect (Figure 430-17). The control circuit disconnect cannot not be higher than 6 ft 7 in. above the floor or working platform, unless located adjacent to the equipment it supplies [404.8(A)].

Part V. 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 protective device for any motor
  • Plus the sum of the full-load currents 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 (Figure 430-15)?

Motor 1: 20-hp, 460V, 3Ø = 27A
Motor 2: 10-hp, 460V, 3Ø = 14A

(a) 8 AWG/70A
(b) 8 AWG/80A
(c) 8 AWG/90A
(d) none of these

Answer: (b) 8 AWG/80A
(27A x 1.25) + 14A = 48A
8 AWG rated 50A at 75°C [110.14(C) and Table 310.16]

Feeder Protection [430.63(A)] not greater than largest branch-circuit protection device plus other motor FLC.

Step 1. Determine largest branch-circuit protection device [430-52(C)(1)]

20-hp Motor = 27A x 2.5 = 68, next size up = 70A [430.52(C)(1) Ex.]
10-hp Motor = 14A x 2.5 = 35A

Step 2. Size Feeder Protection = 70A + 14A, = 84A, next size down = 80A

Part VII. Motor Controllers

Each motor requires its own controller [430.87]. Select an enclosure suitable for the environment that 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 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 having an ampere rating not less than twice the full-load current rating of the motor.
  • A general-use snap switch where the motor full-load current rating is not more than 80 percent 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 2-wire, 1Ø motor; two conductors must open to control a 3-wire, 3Ø motor (see Figure 430-19). The controller starts and stops the motor; it is not a disconnecting means [430.103].

Part IX. Disconnecting Means

You need a disconnect for each motor controller. You must locate it within sight of the controller (see Figure 430-20). "Within sight" means visible and not more than 50 ft from each other [Article 100]. Under certain circumstances, [430.102(B)] allows exceptions to this requirement.

The controller disconnect must open all circuit conductors simultaneously [430.103] (see Figure 430-21). The controller disconnect can serve as the disconnect for 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] (see Figure 430-24).

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 (Figure 430-25).

Additional Tables

Table 430.148 lists the full-load current 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 full-load current 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 151(A) contains locked-rotor current for single-phase motors, and Table 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 is not marked on the motor nameplate.

Article 430 is an amazing document, 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 will correct apply Article 430 every time.


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