SINGLE-PHASE MOTOR AND DC MOTOR INSTALLATION AND THE NATIONAL ELECTRICAL CODE [AC/DC Motors, Controls, and Maintenance]

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GOALS:

• determine the requirements for installation of AC single-phase motors.

• follow the National Electrical Code requirements and find code references.

• install typical residential applications of single-phase motors to meet the code.

• follow NEC installation requirements for DC motors.

NATIONAL ELECTRICAL CODE REGULATIONS NEC 430.32(D)(2) states that any motor of 1 hp or less that is not permanently installed, is manually started, and is within sight of the starter location shall be considered as protected against overload by the overcurrent device protecting the conductors of the branch circuit.

This branch overcurrent device shall not be larger than that specified in Article 430, Part IV (Motor Branch Circuit, Short-Circuit and Ground-Fault Protection). An exception is that any such motor may be used at 120 volts or less on a branch circuit protected at not over 20 amperes. A distance of more than 50 ft is considered to be out of sight from the starter location. NEC 430.32(B) covers motors of 1 hp or less that are automatically started. NEC 430.32(B) (1) states that any motor of 1 hp or less that is started automatically shall have a separate overcurrent device that is responsive to the motor current. This overload unit shall be set to trip at not more than 125% of the nameplate full-load current rating for motors marked to have a temperature rise 40°C (104°F) or less, or with a service factor 1.15 or higher and not more than 115% for all other types of motors. Other options are to use motors with integral thermal protection or impedance-protected motors.

Single-phase motor installation must follow the National Electrical Code. As you have seen throughout the guide, there are many different types of motors and these can be installed in residential applications as well as non-dwelling units. Article 430 governs the installation of motors in all applications. The motors include AC single-phase motors as well as three-phase motors and DC motors. Single-phase AC motors are most common in residential applications and include most household applications such as garbage disposal motors, furnace blower motors and air-conditioning compressor motors.

Residential Single Phase AC Motors

To get a good view of the motor installation requirements, refer to Figure 430.1 in the NEC.

See FIG. 1. For small residential uses, the motor circuits start at the branch circuit over current protective device (OCP). A branch circuit is generally defined as the circuit conductors that extend from the last circuit protective device for overcurrent, to the power utilization (outlet). Here we start with the motor and work back to the branch-circuit protection.

As the motor is installed, the nameplate information is needed. As noted in Article 430.7, all of the information is needed to properly install the motor in a safe and efficient matter. A few of the items are of particular importance. Item 13 in Article 430.7(A) indicates that some motors may have a thermal protector installed on the motor. If there is an internal (integral) protector, it shall be marked "thermally protected." If such a motor is less than 100 watts and complies with 420.32(B)(2) (if the protector is connected to a control circuit rather than the power circuit), they may be marked "TP" Item 14 states that a motor may be impedance protected-meaning that the internal impedance of the motor windings is high enough to protect the motor from overheating. If these motors are 100 watts or less and they comply with 430.32(B)(4), then they may be marked "ZP." As indicated in 430.32(B)(4), these motors have sufficient impedance to prevent overheating even if the motor does not start and stays connected to the branch circuit.

Typical motors of this type are clock motors less than 1/20 hp. NEC 430.32(D)(2)(a) states that for 1 hp or less, and not automatically started these "ZP" protected motors can be considered protected by the branch-circuit protection, and the Exception states that any such motor can be protected by a 120-volt branch circuit not over 20 amps. Therefore, most small clock motors need no further protection as long as they are not permanently installed. Other larger motors are protected as follows.


FIG. 1 Figure 430-1 in article 430 of National Electrical Code.

Garbage Disposal Motors

The branch-circuit protection also provides the motor branch circuit, short circuit, and ground fault protection" as required in the NEC under Part IV of Article 430. The motor circuit conductor as required in Part II of Article 430 is the wiring to the motor itself. This can include the cord connected with a plug to a receptacle under the sink (one method of installation). Other installations may have a permanent connection with flexible conduit. Motor controllers are covered in Part VI of Article 430. NEC 430.81(A) describes small motors that are stationary and 1/8 hp or less, such as a clock motor, stating the branch-circuit overcurrent device can serve as the motor controller. Article 430.82 states that a controller must be capable of starting and stopping the motor. The motor controller for a garbage disposal motor installation is a switch, usually found on the backsplash, next to the sink. It is capable of starting and stopping the motor. Article 430.83 outlines the rating of the controller. NEC 430.83(A) states that the controller must have a horsepower rating unless the motors fall under Part B, C, or D. Part C refers to the small stationary motors that are 2 hp or less and 300 volts or less. A general-use snap switch may be used as a controller if it has an ampere rating at least twice the full-load current rating of the motor. For AC motors, the general-use snap switch is rated only for AC, not AC/ DC snap switches, where the motor full-load rating is not more than 80% of the switch rating.

The controller is not required to open all conductors to the motor but in this case would open the hot conductor. These single-phase motors usually have built-in integral overload protection in the form of a current sensor that has a resettable breaker. In case the food gets jammed in the mechanism, the motor is protected. The overload protection required by the NEC to prevent overheating of the motor and its wiring is covered in Article 430, Part III. In the case of a garbage disposal motor, most are referred to in Article 430.32 either Part A for motors more than one horsepower or Part B for motors 1 hp or less. Either part refers to Section 2, which states that a thermal protector in the motor must protect it from dangerous overheating due to overloads or failure to start.

Furnace Motors

Small residential furnace installations fall under the same information as the garbage disposal motors as related to NEC Article 430, Parts I, II, III, and IV. Forced air gas or oil burner furnace installations typically have one or more motors that provide combustion air and also a fan that circulates the warm air to the inhabited space. Electricians typically connect the branch-circuit power and the local disconnect (a switch) for the furnace power, and all the rest of the controls for the fan are internal to the furnace controls.

Air Conditioning and Refrigeration Equipment

Article 440 of the NEC is pertinent to cooling systems. Article 440.2 states that equipment that has a running overcurrent protection device that allows a continuous current greater than the normal full-load current, shall be marked with the higher current and referred to as the branch-circuit selection current. This branch-circuit selection current is the current to be used in sizing branch-circuit conductors, disconnects, controllers, branch-circuit, short-circuit and ground-fault protection. This is also noted in 440.6(B). If the equipment does not have a nameplate, then the nameplate current on the hermetic refrigerant motor compressor shall be used for calculation as noted in 440.6(A). A disconnecting means is required outside next to a condenser unit that contains a fan and a compressor motor. The disconnect is rated according to Article 440.12-at least 115% of the appropriate current. Branch-circuit overcurrent protection is sized according to Part III of Article 440. Generally, a rating not exceeding 175% of the rated load current is used for protecting against short circuits and ground faults. Branch-circuit conductors are sized to 125% of the load current according to Part IV of Article 440. Motor controllers are sized according to Part V of Article 440. Generally, the controllers must be rated to carry the full-load current plus the locked rotor current of the motor. Motor compressor overload must follow Part VI of Article 440. Overload relays must trip at no more than 140% of rated current. A thermal protector integral with the motor is pre-rated to the motor. A fuse or inverse time circuit breaker is rated at 125% of full current as protection.

Other Single-phase Motor Installations

In other motor installations in residential or nonresidential applications, the NEC is the guide to proper protection for the motor, to prevent overheating and causing a fire. This protection is covered in Article 430, Part III. The wiring to the motor is also protected according to Article 430, Part IV, of the NEC under the heading Motor Branch Circuit Short Circuit and Ground Fault Protection. In general, the requirements for protections are detailed in 430.52(A) and (B). The branch-circuit protection is based on NEC Table 430.52 as a percentage of full-load cur rent. Remember that the full-load current referred to here is the Code book value as noted in 430.6. The full-load current is listed in Table 430.248 for single-phase motors. If using a typical inverse time breaker, the percentage for single-phase motors is 250% of the Code book value for the size of the motor at the connected voltage. This high percentage is allowed so that the breaker does not trip every time the motor starts, because the motor draws many times the rated current as it starts. The high percentage is considered proper to protect the wiring from short circuit, and faults to ground in the branch circuit conductors.

DC Motors

DC motor installations also fall under Article 430 of the NEC. As with all these motor installations, refer to Article 430.6 to determine which current value to use for the calculations needed.

For DC motors, the values given in Table 430.247 are used for calculations except for the separate motor overload (running overcurrent protection) calculations that do use the actual nameplate data to protect the specific motor installed. Most DC motor installations recently installed have electronic controls in which much of the overload protection is part of the solid-state control circuitry. As indicated in Article 430.22(A) for single DC motors supplied from a DC rectifier power source, the ampacity to the input of the solid-state controller must not be less than 125% of the rated input current of the controller. If the field control for the DC motor is fed by a half-wave rectifier, the field wiring must be at least 190% of the motor full-load current. If the field is supplied by a full wave rectifier configuration, the field wiring must be not less that 150% of the nameplate data.

Refer to Table 430.37 to determine where running protection is required for DC motors installed under various wiring system supplies, and how many overload units are required.

SUMMARY

Single-phase motors are often used in small motor needs for typically light loads, sometimes up to 10 hp. These installations are very common in small to moderate residential applications where single-phase power supplies are used. Where these motors are installed, the NEC is followed to prevent dangerous overheating of motors and branch-circuit faults that could ignite fires. Figure 430.1 0f the NEC details, where each item for protection is located within Article 430. Table 430.5 lists other articles that have specific requirements for motor installation with special equipment or special occupancies. DC motors are less commonly installed, but the provisions of Article 430 apply to these motor installations as well.

QUIZ

1. Describe the difference between motor short-circuit protection and motor overload protection.

2. When determining the short-circuit protection percentage for a 1 hp single-phase motor that has a name plate current of 14.7 amps, what value of current would you use?

3. If a single-phase motor supplied by a two-wire, 120-volt circuit with one conductor grounded, needed an overload unit, in which conductor would the unit be installed?

4. An impedance-protected motor would be protected against overcurrent by the __.

5. Where installing a single-phase motor that is not Z protected, the short-circuit ground fault protection is rated at ___% of ___ value when using an inverse time breaker.

6. If you have to install a constant voltage DC motor using an inverse time breaker for short-circuit protection, then use a breaker rated at ___ % of Code book value.

7. The branch-circuit conductor that supplied power to a single-phase cap start motor should be rated at ___ % of the ___ ___ value for the motor at connected voltage and rated hp.

8. The standard controller for a motor used in residential garbage disposal is a ___.

9. A single-phase air-conditioner outdoor condenser/compressor would have a ___ connected within sight of the unit.

10. If a continuous duty motor more than 1 H.P. has a thermal protector protecting it from overload and the service factor is 1.15, then the trip value of the OL would be ___% of the nameplate current of the motor.

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