Article 427: Fixed Electric Heating Equipment for Pipelines and Vessels

II. Installation

III. Resistance Heating Elements

IV. Impedance Heating

V. Induction Heating

VI. Skin Effect Heating

427.45	Conductor Ampacity
427.46	Pull Boxes
427.47	Single Conductor Enclosure
427.48	Grounding

VII. Control and Protection

427.55	Disconnecting Means
427.56	Controls
427.57	Overcurrent Protection

There are numerous other articles with which motor and controllers must comply for special cases. These are noted in Table 430.5.

Generally, the conductor size is determined by using the allowable ampacity tables as noted in Section 310.15(B) or calculated as noted in Section 310.15(C). Use Section 400.5 to determine flexible cord conductor size.

(A) General Motor Applications. Tables 430.147 through 430.150 are used to determine ampere rating for sizes of wire, switches, circuit protection, and the like (not the nameplate ratings). If the motor has a marking in amperes but not horsepower, it is assumed that the horsepower ratings correspond to those in Tables 430.247 through 250. Motors with speeds less than 1200RPM and high-torque motors can have higher full load currents. Multispeed motors have full load currents that vary with speed. In these cases, use the nameplate current ratings. There are three exceptions. Separate motor overload protection is based on the nameplate reading.

(B) Torque Motors. The rated current to be used for these motors is the locked-rotor current. This nameplate value is used to determine the size of the branch circuit conductors as noted in Sections 430.22 and 430.24, the rating of the overload protection, and the rating of the motor overload protection and the rating of the branch circuit short circuit and ground fault protection.

(C) Alternating Current Adjustable Voltage Motors. For these types of motors use the maximum operating current marked on the motor. If there is no marking, then use 150% of the values in Tables 430.149 and 430.150.

Table 430.10(B) indicates the minimum wire bending space at the terminals of enclosed motors.

Tables 430.10(B), 430.12(B), 430.12(C)(1), and 430.12(C)(2) give various dimension and space requirements. There is also information on connections.

If this is to be determined for compliance with Sections 430.24, 430.53(B), and 430.53(C), use the rated full-load current from Tables 430.147, 430.148, 430.149, and 430.150.

II. Motor Circuit Conductors

The rules of this part apply to circuits supplying motors at 600 V or less.

Generally size conductors at 125% of the motor full-load rated current as noted in Section 430.6(A)(1) for the supply to a single motor rated for continuous duty. There is an exception to this rule. It is for direct-current motors that operate from a single-phase power supply which is rectified. There are regulations for multispeed motors and wye-start delta-run motors and part winding motors. When the junction box is allowed to be separated from the motor, a special exception is made for wire size on motors of 1 hp or less. There are provisions for other than continuous-duty motors.

The secondary leads for a continuous-duty wound-rotor motor must be rated at least 125% of the full-load secondary current. If the motor is not continuous, Table 430.22(E) must be used. Table 430.23(C) is used when the secondary resistor is separate from the controller.

In this case the conductors are rated at 125% of the full load current of the largest motor (the same as a single motor) plus the full-load current of all additional motors plus the ampere rating of the other loads which are calculated in accordance with Article 430.6(A) and other sections of the NEC . There are three exceptions: continuous-duty, motor-operated fixed space heating, and interlocked motors.

The conductors for these loads are based, as a minimum, on the circuit ampacity marked on the equipment in accordance with Section 430.7(D). If the equipment is not factory wired, and the nameplates visible as per Section 430.7(D)(2), the conductors are calculated in accordance with Section 430.24.

If all motors will not operate simultaneously for any number of reasons, the authority having jurisdiction can allow the use of a demand factor.

Feeder tap conductors must be sized at least according to Part II of this section and either be enclosed and 3.0 m (10 ft.) or less in length or be one-third or more of the rating of the feeder, 7.5 m (25 ft.) or less in length, and protected or have an ampacity not less than the feeder conductors. There is an exception for feeder taps over 7.5 m (25 ft.) in high bay manufacturing buildings . There are additional rules.

III. Motor and Branch-Circuit Overload Protection

The overload protection is usually the thermal overloads in the controller or starter. It should not be confused with the short-circuit protection, which is the fuse or circuit breaker in the panelboard. Diagram 430.1 in the Code should be referred to, as well as Annex D, Example No. D8.

There are four methods acceptable:

(A) More than 1 Horsepower

1. A separate overload device must be used. It must be rated at not more than 125% of the motor nameplate rating if the motor is rated with a temperature rise of not more than 40°C or a service factor of 1.15 or more. The rating is reduced to 115% for all other motors. Exceptions are provided in Section 430.32(C).

2. Thermal protectors as part of the motor are listed in this paragraph.

3. If the motor is part of an approved assembly, will not normally have overloads, and has some protection against failure to start, additional overload protection is not necessary.

4. Motors larger than 1500 hp are discussed in this paragraph.

(B) One Horsepower or Less, Automatically Started. Here again there are four acceptable methods; the first three are similar to the first three for motors rated more than 1 hp. These include separate devices (ratings are indicated), thermal protectors, and cases where no additional protection other than branch-circuit protection is required (where protection is integral with the motor). A fourth case is that where the impedance of the motor protects against overheating when the motor does not start.

(C) Selection of Overload Relay. In the event the ratings chosen as listed in Section 430.32(a)(1) and (c)(1) will not allow the motor to start, this section gives larger permissible ratings.

(D) One Horsepower or Less, Nonautomatically Started. If not permanently installed then if it is within sight of the controller location, the ground-fault protection and branch-circuit short-circuit protection is sufficient. The sizings noted in Part IV are to be used. It can be on a 120-V circuit rated 20 A or less. If not within sight of the controller or permanently installed, it must comply with Section 430.32(B). If permanently installed comply with 430.32(B).

(E) Wound Rotor Secondaries. These can be protected by the motor-overload device.

You can omit the separate overload protection for these types of motors and use only the branch-circuit, short-circuit and ground- fault protective device.

There are cases where the overload protection can be shunted during starting.

When used as overload protection, put one in each ungrounded conductor and also the grounded conductor if the supply is three-phase, three-wire ac with one phase grounded.

Table 430.37 gives the requirements.

The number of conductors to be opened must prevent current to the motor for devices other than fuses or thermal protection.

This is permitted if it is in accordance with Table 430.37 and operator in the running position and also in the starting position for an ac motor.

If the device used for motor overload protection cannot clear short circuits, they must be protected by either fuses or circuit breakers or a motor short-circuit protector sized as noted in Section 430.52. There is one exception to the sizing rulegroup installations and where an overload relay is used.

This section indicates what overload protection must be used when motors are connected to general-purpose branch circuits. It must also comply with Article 210.

(A) Not over 1 Horsepower. Can be connected without overload protection if the rules of Section 430.32(B) and (D) and Section 430.53(A)(1) and (A)(2) are followed.

(B) Over 1 Horsepower. Each motor must have overload protection as noted in Section 430.32.

(C) Cord- and Plug-Connected. If no overload protection is provided, as in (a) above, the ratings of the plug and receptacle cannot be larger than 15 A 125 V or 10 A 250 V. If protection is required as in (b) above, the overload device must be part of the motor or appliance.

(D) Time Delay. Time delay must be provided in the branch-circuit and ground-fault devices to let the motor start.

If an orderly shutdown is necessary to protect people rather than a quick opening of the circuit, the overload device can be connected to an alarm rather than open the circuit under the provisions of this section.

IV. Motor Branch-Circuit, Short-Circuit, and Ground-Fault Protection

The device must carry the starting current. Table 430.152 lists the maximum settings of the protective devices. If the values in the tables are not a standard-size device, the rating can be increased to the next highest rating. If the rating in the table will not permit the motor to start, the rating can be increased as follows :

1. Nontime-delay fuse less than 601 A up to 400% of full-load current

2. Time-delay fuses up to 225% of full-load current

3. Circuit breakers (inverse time) up to 400% for full-load currents of up to 100 A and 300% for larger currents

4. Fuses between 601 and 6000 A up to 300% of full-load current

Also listed in this section are the rules for when instantaneous trip circuit breakers and motor short-circuit protectors can be used; rules for multispeed motors; multispeed motor; power electronic devices; rules for torque motors; and rules for phase converters. Where a maximum rating is indicated by the manufacturer, it cannot be exceeded. The reader is referred to this section of the Code for complete information.

This section indicates under what conditions two or more motors or combined loads are permitted on one branch circuit.

(A) If none of the motors is greater than 1 hp, the circuit is protected at 20 A at 120 V or not more than 15 A at 600 V, the full-load current of each motor is not more than 6 A, individual overload protection meets the requirements of Section 430.32, and the rating marked on any controller for branch-circuit short circuit and ground-fault protection is not exceeded, more than one motor or combination loads are permitted on one branch circuit.

(B) When the branch-circuit short-circuit and ground-fault protective devices rating is not larger than required for the smallest motor and all motors have individual overload protection and the combination load will operate under these conditions, more than one motor or combination loads are permitted.

(C) Group installations of more than one motor or combination loads are permitted on one branch circuit if the overload devices and motor controllers are in a listed factory assembly, and motor short circuit and ground-fault protection are in the assembly or noted on the assembly; or if the controllers, overload devices, branch circuit, short circuit, and ground-fault devices are field-installed separately with manufacturer's instructions for use with each other and the following conditions are met:

1. The overload device must be approved for group installation and have a specified maximum fuse or inverse time circuit breaker rating, or both.

2. The motor controller for each motor must comply with the same rules as in (1) above.

3. The circuit breakers must be listed and have inverse time characteristics.

4. The rating of the fuse or circuit breaker for the branch circuit must comply with rules of Sections 430.52 and 240.3(B). It must be at least that of Section 430.52 for the largest motor and the sum of full-load ratings of other motors and loads.

5. The rating allowed in Section 430.40 for the smallest motor will also be the maximum for the branch-circuit fuses or inverse time circuit breakers.

6. For loads other than motor loads, the overcurrent protection complies with Parts I through VII of Article 240.

(D) In a group installation as described in (c) above, if the conductors to any single motor are not larger than the branch-circuit conductors or are not less than one-third the rating of the branch-circuit conductors in accordance with Section 430.22, and not farther than 7.5 m (25 ft.) from the controller and physically protected, then installation will not require individual branch-circuit short-circuit and ground-fault protection.

The branch-circuit short-circuit and ground-fault device rating must not be larger than the rating marked on the equipment and comply with Section 430.7(D).

As long as the rating is not larger than that required for overload protection in Section 430.32, the branch-circuit short-circuit and ground-fault protection and overload protection can be in one device.

V. Motor Feeder Short-Circuit and Ground-Fault Protection

The rating must be that of the largest branch-circuit protective device plus the full-load currents of all other motors. If larger feeders have been installed for future expansion, the rating can be that of the feeder. Reference is made to Sections 430.52 and 440.22(A). There are two exceptions and additional requirements.

When combined loads are installed in one feeder, the rating can be sufficient to carry the lighting and appliance load plus that permitted by Section 430.52 for a single motor and Section 430.62 for more than one motor and 440.22 for a single hermetic refrigerant motor-compressor. There is an excpetion for a motor control center.

VI. Motor Control Circuits

This part applies to particular conditions and contains modifications of general requirements of motor control circuits.

If the circuit is tapped from the load side of a branch-circuit protective device, it must have protection as specified in Section 430.72. If not, it must be protected as specified in Section 725.23. There are two exceptions to this rule.

There are also requirements for a control circuit transformer and additional requirements for conductor protection.

Motor control circuits must have a disconnecting means. One device can disconnect both the motor and control circuits from power, or separate ones can be used. If a transformer is used to reduce the voltage for the control circuit and it is located in the controller enclosure, the disconnecting means must be in the supply side of the transformer. There are a number of exceptions.

VII. Motor Controllers

The intention of this part is to require suitable controllers for all motors. When the motor is stationary and of not more than 1/8 hp, the branch-circuit protective device can be used if the motor is normally left running and cannot be damaged by not starting or by overloads. The plug and receptacle can be the controller if the motor is portable and not more than 1/3 hp.

Generally, it must have a horsepower rating at least equal to that of the motor. There are a number of additional rules.

Unless it serves as a disconnecting device, it does not have to open all conductors.

This is permitted if it also simultaneously opens all other conductors.

Each motor must normally have its own controller. There are exceptions to this rule if the motors are rated at not more than 600 V and the single controller is rated at not less than the sum of all motors it will control. Under these conditions it is allowed if more than one motor is used in one device or one overcurrent device is used for a group of motors [Section 430.53(A)] or the motors are in one room in sight of the controller.

VIII. Motor Control Centers

A motor control center is an assembly of enclosed sections which principally contain motor control units and have a common power bus.

They must have overcurrent protection, either an overcurrent device upstream or a main device in the motor control center. It must be in accordance with Parts A, B and I of Article 240 and sized in accordance with the common power bus.

All sections must be bonded with an equipment grounding conductor or grounding bus in accordance with Table 250.95. All equipment grounding conductors must terminate here. In the case of a single section a grounding termination point must be provided.

(A) Support and Arrangement. They must be protected from damage and held firmly. Only the conductors connected or used for control are allowed in a section.

(B) Phase Arrangement. Three phase buses are arranged A, B, C, left to right, top to bottom, or front to back when standing in the front.

(C) Minimum Wire Bending Space. As noted in Article 373.

(D) Spacings. As noted in Table 430.97.

(E) Barriers. Used in service-entrance motor control centers.

IX. Disconnecting Means

It must be provided for and in sight from the controller location. An exception is made for motors over 600 V if it can be locked out and a warning sign is posted. Also, one disconnect can be used for a group of controllers for a multimotor device. A disconnecting means must also be located within sight of the motor location except in accordance with Section 430.102(a). There are some additional exceptions.

If all conductors are opened simultaneously, one pole can be in a grounded conductor.

The disconnecting device must be noted in (A) below unless permitted in (B) through (G) meeting the specified conditions.

(A) General. It must be a horsepower-rated motor circuit switch. It can also be a molded case circuit breaker, molded case switch, an instantaneous trip circuit breaker which is part of a combination motor controller, a self-protected combination controller, or a manual motor controller if it is marked as such, if it is installed between the motor and the final motor branch circuit and ground-fault protective device, all of the items being listed. There are additional requirements for a manual motor controller. System isolation equipment must be listed for disconnection purposes and must be installed on the load side of the overcurrent protection and its disconnecting means. The disconnecting means must be one of those noted in 430.109(A)(1) through (A)(3).

(B) Stationary Motors of 1/8 Horsepower or Less. The branch circuit overcurrent device can be used.

(C) Stationary Motors of 2 Horsepower or Less. Stationary motors 2 hp or less and 300 V or less can have a general-use switch rated twice that of the motor. On ac circuits a general-use snap switch listed for ac use only can be used if the motor is not more than 80% of the rating of the switch. A listed manual motor controller can also be used if it is marked as being suitable as a motor disconnect and it is horsepower rated not less than the rating of the motor.

(D) Autotransformer-Type Controlled Motors. Motors over 2 hp up to 100 hp can use a general-use switch for motors with autotransformer controllers if the motor drives a generator with overload protection; the controller can interrupt locked rotor current, has no voltage release, and has running overload protection not larger than 125% of full-load current; separate fused or inverse time circuit breakers are provided and rated not more than 150% of full load current.

(E) Isolating Switches. Stationary motors more than 40 hp dc or 100 hp ac can have a general use or isolating switch if marked "Do Not Operate Under Load."

(F) Cord- and Plug-Connected Motors. A horsepower rated attachment plug and receptacle rated at least the rating of the motor can be used. This is not required for cord and plug appliances (Section 422.32), room air conditioners (Section 440.63) or a portable motor 1/3 hp or less.

(G) Torque Motors. A general-use switch can be used for a torque motor.

(A) The disconnecting means must have a rating of at least 115% of the full-load current rating of the motor. There is an exception.

(B) It must have a rating of at least 115% of the nameplate current for torque motors.

(C) For combination loads of motors or motors and other types of loads, the ratings of the disconnect must not be less than 115% of the total of the full-load current of all the loads. The Code makes reference to the method of determining the combined load.

A switch or circuit breaker can be used as the disconnect and controller if it complies with Section 430.83: if it opens all ungrounded conductors to the motor, has an overcurrent protective device which opens all ungrounded conductors to the switch, and is either an air-break switch, an inverse time circuit breaker, or an oil switch.

Generally, a motor must have its own disconnecting means. There is an exception.

Generally, each source must have a disconnect. There are two exceptions.

X. Adjustable-Speed Drive Systems

XI. Over 600 Volts, Nominal

XII. Protection of Live PartsAll Voltages

If operating at 50 V or more, the exposed live parts must be guarded against accidental contact by enclosure or location. This can be done in a special room or enclosure, on a balcony or by installation at least 2.5 m (8 ft.) above the floor. There is an exception.

If the only guards are by location, as noted in Section 430.132, then insulating mats or platforms are required.

XIII. GroundingAll Voltages