conductors, it must be rigid metal conduit, intermediate metal conduit, electrical metallic tubing, flexible metal conduit, type AC cable, or the combined metallic sheath and grounding conductors of type MC cable.
Flexible metal conduit is permitted as an equip- ment grounding conductor if the following conditions are met: the length of the flex does not exceed 6 feet, the circuit conductors within are rated at 20 amperes or less, and the connectors are fittings listed for ground- ing. If the 6 feet of flex is exceeded, a bonding jumper wire, run inside the flex, must be used.
The subject of electric control circuits is quite broad. The following text will cover a few of the basic control circuit requirements and controls. For more information, refer to special books devoted to this important phase of motor circuitry. Two such books are Electric Motor Control by Walter N. Alerich and Electric Motor Repair by Robert Rosenberg and August Hand. These textbooks provide an excellent insight on how to understand, select, and design con- trol circuits.
(NEC ® 430, PART F and ARTICLE 725) A control circuit is a circuit that exercises control over one or more other circuits. These other circuits controlled by the control circuit may themselves be control circuits, or they may be "load" circuits that carry utilization current to a lighting, heating, power, or signal device. Figure 7-10 clarifies the distinction between control circuits and load circuits.
The elements of a control circuit include all the equipment and devices concerned with the function of the circuit: conductors, raceway, contactor-operating coil, source of energy supply to the circuit, overcurrent protective devices, and all switching devices that govern energization of the operating coil.
Typical control circuits include the operating-coil circuit of magnetic motor starters, magnetic contactors, and relays. Control circuits include wiring between solid-state control devices as well as between magnetically actuated components. Low-voltage relay switching of lighting and power loads also are classified as remote-control wiring.
A control circuit is divided into three classes:
Class 1 system may operate at any voltage that does not exceed 600 volts. They are, in many cases, merely extensions of light and power systems, and, with a few exceptions, are subject to all the installation rules for light and power systems.
Class 2 and Class 3 systems are those systems in which the current is limited to certain specified low values. This limiting may be accomplished by fuses or circuit breakers, by transformers that deliver only very small currents, or by other voltages at which the system operates from 5 milliamps or less. All Class 2 and Class 3 circuits must have a power source with the power- limiting characteristics described in NEC®, table 725-31(a). These requirements are in addition to the overcurrent device.
Conductors for any Class 1 control circuit must be protected against overcurrent. Number 14 and larger wires must generally be protected at their ampacities. (Review NEC®, table 310-16.) Number 18 and Number 16 control wires must always be protected at 7 and 10 amperes, respectively.
Figure 7-10. - Defining a control circuit.Continue Reading