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
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-
CONTROL CIRCUITS GENERAL (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
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.