Chapter 4 of the National Electrical Code (NEC), Equipment for General Use, contains 22 articles. Some of the articles include provisions pertaining to switches, receptacles, switchboards and panelboards, industrial control panels, luminaires, appliances, air-conditioning and refrigeration equipment, and transformers. One of the most referenced articles in Chapter 4 is Article 430, Motors, Motor Circuits, and Controllers. This article covers motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers and motor control centers (MCCs).
Most of the articles in the NEC are divided into parts; most of those are divided into three to five parts. Only two articles are divided into more than 10 parts: Article 410 (15 parts) and Article 430 (14 parts). Because of the larger number of parts, finding rules and regulations in Article 430 can be challenging, but there is a helpful tool located at the beginning of the article.
Figure 430.1 makes it easier to find specific rules pertaining to motors, motor circuits and motor controllers because it is like a table of contents to the whole article. This figure is divided into two segments. The top half of Figure 430.1 shows all 14 parts in the numerical order as they appear in Article 430. For example, provisions pertaining to motor branch-circuit short-circuit and ground-fault protection are in Part IV.
Figure 430.1 even shows the section numbers in each part. The section numbers in Part IV are 430.51 through 430.58. The bottom half of Figure 430.1 shows the components or elements in relationship to the motor. For example, the vertical line below the second rectangle or box from the top represents motor branch-circuit conductors. The right side of this drawing shows the requirements are in Part II.
As stated in 430.21, Part II specifies ampacities of conductors that are capable of carrying the motor current without overheating under the conditions specified. Another component in the bottom half of Figure 430.1 is the rectangle directly above the drawing of the motor; this rectangle represents motor overload protection. Overload devices are intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. Requirements pertaining to motor overload protection are in Part III.
The bottom half of Figure 430.1 does not show the section numbers, but they are easy to find because they are in the top half of this figure. The section numbers in Part III are 430.31 through 430.44 (see Figure 1).
The bottom half of Figure 430.1 only shows eight of the 14 parts in Article 430. This does not mean the parts not shown are not important and do not pertain to motors; all of the parts in Article 430 pertain to motors either directly or indirectly. The parts not shown contain requirements for MCCs, adjustable-speed drive systems, motors rated over 1,000 volts (V), protection of live parts, grounding for motors and the tables in Part XIV.
One of the parts not shown in the bottom half of Figure 430.1 is Part VIII, which covers MCCs installed for the control of motors, lighting and power circuits. The requirements in Part VIII pertain to MCCs. Article 100 defines an MCC as an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. Conductors and equipment installed inside MCCs must meet specific requirements; these requirements might not be found in Part VIII.
For example, one of the compartments (usually referred to as buckets) in an MCC provides power to a motor. This bucket also contains branch circuit conductors and motor circuit components for this motor. While the MCC itself must be installed in accordance with Article 430, Part VIII, the motor branch circuit conductors and motor circuit components must be sized and installed in accordance with provisions located in other parts of Article 430. Requirements for motor branch circuit conductors are in Part II.
A few of the components that might be in a MCC bucket are motor overload devices (Part III), motor control circuits (Part VI), and motor controllers (Part VII). Motor controllers are usually referred to as starters. The definition of a controller is at the beginning of Article 430 in 430.2. For the purpose of Article 430, controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current. A few more components that might be in an MCC bucket are motor branch-circuit short-circuit and ground-fault protection (Part IV), motor disconnecting means (Part IX), and even adjustable-speed drives (Part X) (see Figure 2).
Article 430 contains a number of provisions for sizing motor circuits and their components. Some of the motor circuit provisions and calculations include finding the full-load current of the motor, sizing motor overload protection, determining the locked rotor amperes, determining the minimum ampacity required for branch circuit conductors, deciding which type and size of motor controllers to install, determining the maximum size branch-circuit short-circuit and ground-fault protection, finding the minimum rating required for motor disconnecting means, determining the minimum ampacity required for feeder conductors, and determining the maximum size feeder short-circuit and ground-fault protection (see Figure 3).
Some, but not all, motor conductor and component sizing is to find the minimum size. Some provisions in Article 430 are there to ensure the conductor or circuit component meets or exceeds a minimum size. For example, there are provisions requiring a minimum size for branch-circuit and feeder conductors, and there are provisions requiring a minimum rating for the motor disconnecting means.
Some provisions in Article 430 are there to ensure certain motor circuit components do not exceed a maximum size. For example, there are stipulations in Article 430 for determining the maximum size branch-circuit short-circuit and ground-fault protection as well as determining the maximum size feeder short-circuit and ground-fault protection.
The provisions in Part III pertain to overload devices intended to protect motors, motor-control apparatuses, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start.
As defined in Article 100, an overload is operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. The definition continues by saying a fault, such as a short circuit or ground fault, is not an overload. Motor overload devices are usually referred to as overloads or heaters.
Sometimes on electrical journeyman and master exams, there are questions asking for the minimum or maximum size motor overload device ratings or settings. For these questions, the minimum sizes are determined by using the percentages in 430.32(A)(1), and the maximum sizes are determined by using the percentages in 430.32(C). The percentages for motor overload protection in 430.32(A)(1) are not for calculating minimum sizes.
Specifications in 430.32(A)(1) are not to be exceeded unless the size of the overload device is not sufficient to start the motor or carry the load. At that point, the percentages in 430.32(C) can be used to calculate the sizes as long as the ratings do not exceed the percentages in 430.32(C).
Next month’s column continues the discussion of requirements for motors, motor circuits and controllers.