For motor circuits of 600 volts or less, the controller manual disconnecting means must be within sight and not more than 50 feet away from the location of the motor controller. There are two exceptions in the Code rule requiring a disconnect switch to be in sight from the controller:
1. For motor circuits over 600 volts, the controller disconnecting means is permitted to be out of sight from the controller, provided the controller is marked with a warning label giving the location and identification of the disconnecting means, and the disconnecting means can be locked in the OPEN position.
2. On complex machinery using a number of motors, a single common disconnect for a number of controllers may be used. This disconnect may be out of sight from one or all of the controllers if it is adjacent to them.
The Code also stipulates that a manual disconnecting means must be within sight and not more than 50 feet from the motor location and the driven machinery. The exception to this rule is that the disconnecting means may be out of sight if it can be locked in the OPEN position. See figure 7-5 for other exceptions and basic rules.
The NEC ® rules allow a single switch to be the disconnecting means of a group of motors under 600 volts. Also, manual switches or circuit breakers rated in horsepower can be used as a disconnecting means and the controller for many motor circuits.
Each continuous-duty motor must be protected against excessive overloads under running conditions by some approved protective device. This protective device? except for motors rated at more than 600 volts, may consist of fuses, circuit breakers, or specific overload devices. Overload protection will protect the branch circuit, the motor, and the motor control apparatus against excessive heating caused by motor overloads. Overload protection does not include faults caused by shorts or grounds.
Each continuous-duty motor rated at more than 1 horsepower must be protected against overload by one of the following means:
1. A separate overload device that is responsive to motor current. This device is required to be rated or selected to trip at no more than the following percentage of the motor nameplate full-load current rating:
|Motors with a marked service factor not less than 1.15
|Motor with a marked temperature rise not over 40°C
|All other motors
For a multispeed motor, each winding connection must be considered separately. Modification of these values is permitted. See section 430-34.
2. A thermal, protector, integral with the motor, is approved for use with the motor that it protects on the basis that it will prevent dangerous overheating of the motor caused by overload and failure to start. The percentages of motor full-load trip current are given in section 430-32 (a-2).
3. A protective device. integral with the motor, that will protect the motor against damage caused by failure to start is permitted if the motor is part of an approved assembly that does not normally subject the motor to overloads.
Nonportable, automatically started motors of 1 horsepower or less must be protected against running overload current in the same manner as motors of over 1 horsepower, as noted in section 430-32 (c).
Motors of 1 horsepower or less that are manually started, within sight of the controller location and not permanently installed are considered protected by the branch-circuit protective device.
If regular fuses are used for the overload protec- tion of a motor, they must be shunted during the starting period since a regular fuse having a rating of 125 percent of the motor full-load current would be blown by the starting current. Many dc-motor and some wound-rotor-induction-motor installations are exceptions to this rule. Aside from these exceptions, it is not common practice to use regular fuses for the overload protection of motors. Time-delay fuses sometimes can be used satisfactorily for overload pro- tection since those rated at 125 percent of the motor full-load current will not be blown by the startingContinue Reading