As a Construction Electrician, you will encounter many pieces of electrical equipment and many appliances. A solid background in electrical theory and standards and a working knowledge of the components and of the machines themselves will allow you to install, maintain, troubleshoot, and repair a wide variety of equipment and appliances.
In one way or another, all machines use the same technologies. The differences are in the complexity of their operation and the tasks they perform. This chapter will not cover specific pieces of equipment or appliances but will concentrate on electrical components, motors, controllers, and circuitry that are common to most equipment and appliances.
In this chapter you will find many references to articles, parts, and sections of the National Electrical Code® (NEC® ). You should have an NEC® book on hand while reading this chapter. The chapter text is written in general terms. Many of the exceptions given in the NEC® are not included. Look up the sections when they are referenced. The NEC® can be quite confusing, so read the articles closely and pay special attention to the notes and exceptions.
A motor-branch circuit is a wiring system extending beyond the final automatic overload protective device. Thermal cutouts or motor overload devices are not branch-circuit protection. These are supplementary overcurrent protection. The branch circuit represents the last step in the transfer of power from the service or source of energy to utilization devices.
The Code requires that branch-circuit protection for motor circuits must protect the circuit conductors, the control apparatus, and the motor itself against overcurrent caused by short circuits or grounds (sections 430-51 through 430-58). Fuses or circuit breakers are the most common protectors used as branch-circuit protective devices. These protective devices must be able to carry the starting current of the motor. To carry this current, they may be rated 300 or 400 percent of the running current of the motor, depending on the size and type of motor.
Motor controllers provide motor protection against all ordinary overloads but are not intended to open during short circuits.
Motor-branch circuits are commonly laid out in a number of ways. Figures 7-1 through 7-3 show three motor-branch circuits and how the circuit protection is used in various types of layouts.
As mentioned before, the motor-branch-circuit short-circuit and ground-fault protective device must be capable of carrying the starting current of the motor. For motor circuits of 600 volts or less, a protective device is permitted that has a rating or setting that does not exceed the values given in table 430-152 of the Code. An instantaneous-trip circuit breaker (without time delay) may be used ONLY if it is adjustable and is part of a listed combination controller, having motor overload and also short-circuit and ground-fault protection in each conductor.
When values for branch-circuit protective devices, as shown in the NEC® , table 430-152, do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, or thermal protective devices, you may use the next higher size, rating, or setting.
The National Electrical Manufacturer's Association (NEMA) has adopted a standard of identifying code letters that may be marked by the manufacturers on motor nameplates to indicate the motor kilovoltampere input with a locked rotor. These code letters, with their classification, are given in the NEC®, table 430-7(b). In determining the starting current to use for circuit calculations, use values from table 430-7(b). Exceptions to the above are given in table 430-52.
When maximum branch-circuit protective device ratings are shown in the manufacturer's overload-relay table for use with a motor controller or are marked onContinue Reading