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
Note: Graphics not part of this newsletter, they are contained in the textbook, Understanding the National Electrical Code.
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.
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)?
Answer: (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 x 2.5 = 55A, next size up = 60A.
Answer: (d) 8 AWG
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.
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:
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
Answer: (b) 8 AWG/80A
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.]
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.
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).
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.
Copyright © 2002 Mike Holt Enterprises,Inc.