A group of apprentices asked if there is an easy way to locate equipment and wiring method requirements in the Na-tional Electrical Code (NEC). If indeed there is a procedure, they wanted me to demonstrate such by designing and installing a motor system with examples.
To understand the NEC, the requirements first must be assumed to be applicable as described in a particular section, unless the specific wording contains an exception that indicates when and how the requirements can be altered or voided. Second, requirements must never be taken as an exception to another NEC requirement, unless an exception is provided that specifically addresses that par-ticular subject. Third, without special permission from the inspector, electricians must fully satisfy each and every NEC requirement under all design and installation conditions. As asked to demonstrate, let’s consider the components for a motor system for our example.
Motor circuit conductors
The subject is branch-circuit conductors supplying a motor, so first reference Article 430, “Motors, Motor Circuits, and Control-lers.” Next, look up Part II and the title “Branch-Circuit Conductors,” then fast forward to page 304 and apply 430.22(A).
Motor and branch-circuit overload protection
Motor and branch-circuit overload protection is the subject. Therefore, select Part III of Article 430 because it covers our subject. Next, the electrician must advance to page 306 and follow 430.32(A)(1). These requirements cover the sizing of the overloads used to protect the branch-circuit conductors and motor windings.
Motor and branch-circuit short-circuit and ground-fault protection
Part IV of Article 430 on the table of contents takes precedence and informs the electrician to make this design and installation. The specific references are to the provisions in 430.52 and Table 430.52 on page 310. First of all, 430.52(C)(1) implies that the size of the overcurrent protection device (OCPD) must be rounded down, if the percentages in Table 430.52 times the full-load current (FLC) of the motor fails to correspond to a standard size OCPD listed in 240.6(A). Secondly, Exception 1 of 430.52(C)(1) permits, under cer-tain conditions, the next higher size above this value can be used where the OCPD does not correspond to a standard device. And fi-nally, if the motor fails to start and drive the load, the maximum size device is permitted per Exception 2 of 430.52(C)(I).
Motor control circuits
When designing and installing motor control circuits, select Part VI of Article 430 that will in turn send the electrician to page 313 to use 430.72(B). However, the size of the OCPDs are determined by the requirements in Table 430.72(B) on page 314.
Part VII of Article 430 refers the electrician to page 315 and 430.83(A)(1), which states that the controller must be at least equal to the horsepower rating of the motor. For example, a 50 horsepower motor must be energized and de-energized by a controller rated at least for 50 horsepower. A motor’s controller and disconnecting means is probably the easiest com-ponents to size.
Motor control centers
When motor control centers (MCCs) are used instead of individual motor controllers, Part VIII of Article 430 sends the electrician to page 316 that contains the requirements for installing and using a self-contained MCC. [One of the main advantages for an electri-cian is the controller and disconnecting means are contained in the bucket of the MCC.] The only other time this occurs is when the combination controller is equipped with both.
For the final design and installation step, using the table of contents, reference Part IX of Article 430 on page 317. Then thumb and scan the sections until 430.109(A)(1) and 430.110(A) on pages 318 and 319 are located. Plainly stated, the disconnecting means must be at least 115 percent of the motor’s FLC rating or not less than the horsepower rating of the motor, whichever is applicable.
I believe that electricians can use this systematic approach and key procedure to locate requirements in the NEC in easily and quickly. After being sent to a particular part from the table of contents, electricians can simply scan through the article sections, un-til the requirements governing their installation are found.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.