Each controller must be capable of starting and
stopping the motor which it controls and, for an
alternating-current motor, it must be capable of
interrupting the stalled-rotor current of the motor.
The controller must have a horsepower rating not
lower than the horsepower rating of the motor.
Exceptions are indicated below.
For a stationary motor rated at 1/8 horsepower or
less, normally left running and so constructed that it
cannot be damaged by overload or failure to start (such
as clock motors), the branch-circuit overcurrent device
may serve as the controller.
For a stationary motor rated at 2 horsepower or
less and 300 volts or less, the controller may be a
general-use switch with an ampere rating of at least
twice the full-load current rating of the motor.
For a portable motor rated at 1/3 horsepower or
less, the controller may be an attachment plug connector
A branch-circuit circuit breaker, rated in amperes
only, may be used as a controller. Branch-circuit
conductors must have an amperage capacity (ampacity)
not less than 125 percent of the motor full-load current
Single Controller Serving a Group of Motors
Each motor must have an individual controller,
except for motors of 600 volts or less; a single controller
can serve a group of motors under any one of the
A number of motors drive several parts of a
single machine or piece of apparatus, such as a
metal and woodworking machine, crane, hoist,
and similar apparatus.
A group of motors is under the protection of one
A group of motors is located in a single room
within sight of the controller location.
Conductors supplying two or more motors must have an
ampacity equal to the sum of the full-load current rating
of all motors plus 25 percent of the highest rated motor
in the group.
Controllers are marked with the makers name or
identification, the voltage, the current or horsepower
rating, and other data as may be needed to properly
indicate the motors for which it is suitable. A controller
that includes motor running overcurrent protection or is
suitable for group motor application is marked with the
motor running overcurrent protection and the maximum
branch-circuit overcurrent protection for such
applications. Be extremely careful about installing
unmarked controllers into any circuit. Controllers
should be properly marked.
Before you condemn a motor, make sure that the
fault does not lie within the controller. The only way to
be sure the fault is not in the controller is to understand
the circuitry of the controller. As previously mentioned,
there are two general types of motor controllers: manual
Manual controllers (motor starters) are available up
to 7 1/2 horsepower at 600 volts (three-phase) and to 3
horsepower at 220 volts (single-phase).
TOGGLE SWITCHES OR CIRCUIT
BREAKERS. A toggle switch or circuit breaker can
serve as a controller, provided its ampere rating is at
least twice the full-load current rating of the motor and
the motor rating is 2 horsepower or less. It must be
connected in a branch circuit with an overcurrent device
that opens all ungrounded conductors to the switch or
circuit breaker. These switches or circuit breakers may
be air-brake devices operable directly by applying the
hand to a lever or handle. An oil switch can be used on a
circuit with a rating which does not exceed 600 volts or
100 amperes, or on a circuit exceeding this capacity,
under expert supervision and by permission. A single-
phase motor requires a one-element overload device,