THREE-PHASE MOTOR INSTALLATIONS [AC/DC Motors, Controls, and Maintenance]

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GOALS:

• determine, for several types of three-phase AC induction motors, the

• size of the conductors required for three-phase, three-wire branch circuits.

• sizes of fuses used to provide starting protection.

• disconnecting means required for the motor type.

• size of the thermal overload units required for running overcurrent protection.

• size of the main feeder to a motor installation.

• overcurrent protection required for the main feeder.

• main disconnecting means for the motor installation.

• use the National Electrical Code (NEC).


FIG. 1 Line diagram of motor control system.

The work of the modern electrician requires a knowledge of National Electrical Code requirements that govern three-phase motor installations and the ability to apply these requirements to installations. The elements of a motor circuit are shown in FIG. 1.

This unit outlines the procedure for determining the wire size and the proper overload and starting protection for a typical three-phase motor installation. The motor installation example consists of a feeder circuit feeding three branch circuits. Each of the three branch circuits is connected to a three-phase motor of a specified horsepower rating.

The feeder circuit and the branch circuits have the necessary overcurrent protection required by the NEC.

THREE-PHASE MOTOR LOAD

The industrial motor installation described in this example is connected to a 230-volt, three-phase, three-wire service ( FIG. 2). The load of this system consists of the following branch circuits.

• One branch circuit that feeds a three-phase AC induction motor rated at 230 volts, 28 amperes, 10 hp, with a code letter F marking.

• One branch circuit that feeds a three-phase AC induction motor rated at 230 volts, 64 amperes, 25 hp, with a code letter B marking.

• One branch circuit that feeds a wound-rotor induction motor rated at 230 volts, 54 amperes, and 20 hp. The full-load rotor current is 60 amperes.

BRANCH CIRCUIT FOR EACH MOTOR

The values given in NEC Table 310.15(B)(16) shall be used with the Code book current for motors in determining ampere capacity (ampacity) of the conductor according to Article 430.6.


FIG. 2 Branch circuit for each motor.

Three specific facts must be determined for each of the three branch circuits constituting the load of the installation:

• The size of the conductors for each three-phase, three-wire branch circuit.

• The fuse size to be used for short-circuit protection. The fuses protect the wiring and the motor from any faults or short circuits in the wiring or motor windings.

• The size of the thermal overload units to be used for running protection. The overload units protect the motor from potential damage due to a continued overload on the motor.

Note: The full-load amperes shall be taken from the motor's nameplate only for calculating thermal overload units. [See NEC Article 430.6(A)(2).] Other calculations are based on Code-rated values from Articles 430.248, 430.249, and 430.250. Where motor system disconnecting means and controllers are determined from horsepower, voltage rating, and design letter, then Table 430.251(B) is used.

BRANCH CIRCUIT 1

The first branch circuit feeds a three-phase AC induction motor. The nameplate data of this motor follow:

Squirrel-Cage Induction Motor

Volts 230 Amperes 28 3 Phase Speed 1735 RPM Code Letter F Frequency 60 Hertz 10 Horsepower Temperature Rating 40° (104°) Celsius

Conductor Size

NEC 430.22 states that branch-circuit conductors supplying a single motor shall have a carrying capacity equal to not less than 125% of the full-load current rating of a motor. This general rule may be modified according to 430.22(A-G).

The following procedure is used to determine the size of the conductors of the branch circuit feeding the 10 hp motor:

1. The 10 hp motor has a full-load current rating of 28 amperes. According to Table 430.250:

28 × 125% = 35 amperes

2. Using 35 amperes and referring to Table 310.15(B)(16), a proper size of conductor is selected. This process requires the electrician to determine the temperature ratings of each termination used and the ampere rating of the equipment circuit. According to NEC Article 110.14(C), the temperature rating of the conductor used to determine the ampacity must not exceed the temperature rating of any of the connections. Unless all the terminations are marked for a higher temperature, the column in 310.15(B)(16) marked 60°C is selected to determine the conductor ampacity. Even if using a standard building wire type THHN, the conductor size is 8 AWG in the 60°C column.

If all the terminations in the branch circuit are rated for 75°C, then the second column in 310.15(B)(16) can be used for all wire ampacities. Article 110.14(C)(1) (a)(4) states that motors with design letters B, C, or D may use the 75°C rating for terminations and wire ampacity. If all the other terminations in that circuit are rated at 75°C, then 10 AWG-75°C or 10 AWG-90°C wire may be used.

3. Table C1 in NEC Annex C indicates that three 8 AWG THHN conductors will fit in a trade size 1/2 EMT conduit (metric designator 16).

The squirrel-cage induction motor is to be connected directly across the rated line voltage through an ATL motor starter. For this example, the branch-circuit, short-circuit, and ground fault protection for this motor consists of three standard nontime-delay fuses enclosed in a safety switch located on the line side of the magnetic starter. According to 430.109(A) or as specified in 430.109(B-G) of the Code, this switch shall be a motor-circuit switch with a horse power rating, a circuit breaker, or a molded case switch, or shall be a listed device.

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

The branch-circuit, short-circuit, and ground-fault protection for a three-phase AC induction motor is provided in Table 430.52. For the branch circuit 1 motor being considered, the motor circuit overcurrent device shall not exceed 300% of the full-load current of the motor (nontime delay fuses). Article 430.52 with exceptions applies to Table 430.52.

The branch-circuit fuse protection for the branch circuit feeding the squirrel-cage motor is a squirrel cage-other than Design B energy efficient.

Because the 10 hp motor has a full-load current rating of 28 amperes, and given the appropriate value from Table 430.52 for a nontime-delay fuse, then 28 × 300% = 84 amperes / 90 amperes NEC Article 430.52 Exception 1 states that if the values for branch-circuit protective devices as determined using the percentages in Table 430.52 do not correspond to the standard device sizes or ratings, then the next larger size rating or setting should be used.

NEC 240.6 indicates that the next larger, standard-size fuse above 84 amperes is 90 amperes. Standard nontime-delay cartridge fuses rated at 90 amperes may be used as the branch-circuit protection for this motor circuit.

The branch-circuit, short-circuit, and ground-fault protection may also be calculated using a time-delay fuse. From Table 430.52, the second column is selected and 175% of 28 amperes is calculated (1.75 × 28 = 49 amps). The next larger size is used. In this example, 50-ampere fuses would be the choice. The Code allows the electrician to increase the size of the fuse according to the exceptions in 430.52(C)(1).

Disconnecting Means

According to the table for safety switches ( FIG. 3), the disconnecting means for this 10 hp motor is a 15 hp, 100-ampere-rated safety switch in which the 90-ampere fuses are installed.

Because these safety switches are dual rated, it is permissible to install a 60-ampere safety switch with a maximum rating of 15 hp if the time-delay fuses are appropriate for the starting characteristics of the motor. The size of the time-delay fuses installed in the safety switch depends on the degree of protection desired and the type of service required of the motor.

Time-delay fuses ranging in size from 35 amperes to 60 amperes may be installed in the safety switch. Refer to Article 430, Part IX, for requirements of motor and controller disconnecting means.

Running Overload Protection

Running overload protection consists of three current monitors, housed in the ATL motor starter. (See the Exception following NEC Table 430.37 for an exception to this statement.) NEC 430.32(A)(1) states that the running overload protection (motor and branch-circuit overload protection) for a motor shall trip at not more than 125% of the full-load current (as shown on the nameplate) for motors with a marked temperature rise 40°C (104°) or less. In this example, the nameplate is the same as the code value, which is uncommon.

The trip current of the thermal units used as running overcurrent protection is 28 × 125% = 35 amperes When the selected overload setting of the relay is not sufficient to start the motor or to carry the load, 430.32(C) permits the use of the next higher size or rating, but must trip at no more than 140% of the full-load motor current.


FIG. 3 Table for safety switches.

Three-Pole, Three-Fuse, 230-Volt AC Safety Switches

* The Electrical Construction Materials List by Underwriters Laboratory, Inc. states that "some enclosed switches have dual horsepower ratings, the larger of which is based on the use of fuses with time delay appropriate for the starting characteristics of the motor. Switches with such horsepower ratings are marked to indicate this limitation and are tested at the larger of the two ratings."

BRANCH CIRCUIT 2

A second branch circuit feeds a three-phase AC induction motor. The nameplate data for this motor follow.

--------------

Squirrel-Cage Induction Motor

Volts 230 Amperes 64

3 Phase Speed 1740 RPM

Code Letter B Frequency 60 Hertz

25 Horsepower Temperature Rating 40° Celsius

---------------

Conductor Size

The following procedure is used to determine the size of the conductors of the branch circuit feeding the 25 hp motor:

1. The 25 hp motor has a full-load current rating of 68 amperes (see NEC, Table 430.250). (According to the Code, 430.22, 125% is needed for ampacity.) 68 × 125% = 85 amperes

2. Table 310.15(B)(16) indicates a 3 AWG Type TW or THHN copper conductor or a 3 Type THW conductor. (Assume 60°C terminations.)

3. Table C1 of Annex C shows that three 3 TW, THW, or THHN or THW conductors may be installed in a trade size 1-in. conduit.

Note: NEC 300.4G requires that where conductors of 4 size or larger enter an enclosure, an insulating bushing or equivalent must be installed on the conduit.

Motor Branch-Circuit Protection

The 25 hp, squirrel-cage induction motor is to be started using an autotransformer. The branch-circuit overcurrent protection for this motor circuit consists of three nontime delay fuses located in a safety switch mounted on the line side of the starting compensator (autotransformer).

For a squirrel-cage induction motor that is not Design B, NEC Table 430.52 requires that the branch-circuit overcurrent protection not exceed 300% of the full-load current of the motor.

Because the 25 hp motor has a full-load current rating of 68 amperes (NEC Table 430.250), the branch-circuit overcurrent protection for the branch circuit feeding this motor is

68 × 300% = 204 amperes

NEC 240.6 does not show 204 amperes as a standard size for a fuse. However, 430.52 permits the use of a fuse of the next higher size if the calculated size is not a standard size.

In this case, 200 amperes should be attempted. Therefore, three 200-ampere, nontime-delay fuses can be used as the branch-circuit protection for this motor. Article 430.52, Exception 2, allows the fuse to be increased, but not over 400%.

Disconnecting Means

According to the table for safety switches in FIG. 3, the disconnecting means for the 25 hp motor is a 25 hp, 200-ampere safety switch in which the 200-ampere fuses are installed.

Time-delay fuses may be installed in safety switches. In this example, 175% × 68 amperes = 119 amperes. The next largest size is 125 fuses, and these may be used according to exceptions in 430.52. The safety switch would be the same size.

Running Overcurrent Protection (Motor Overload Protection)

The running overcurrent protection consists of three magnetic overloads located in the starting compensator. According to the nameplate, the motor has a full-load current rating of 64 amperes. The current setting of the magnetic overload units is set to trip at 64 × 125% = 80 amperes (trip current)

BRANCH CIRCUIT 3

A third branch circuit feeds a wound-rotor induction motor. The nameplate data for this motor follow.

-----------

Wound-Rotor Induction Motor

Volts 230 Stator Amperes 54 3 Phase Rotor Amperes 60 20 Horsepower Frequency 60 Hertz Temperature Rating 40° Celsius

-------------

Conductor Size (Stator) The following procedure is used to determine the size of the conductors of the branch circuit feeding the 20 hp motor.

1. The 20 hp motor has a full-load current rating of 54 amperes. According to the NEC 430.22 and Table 430.250.

54 × 125% = 67.5 amperes

2. Table 310.15(B)(16) indicates an AWG 4 Type TW, THW, THHN conductor (70 amperes).

3. Table C1 of Annex C shows three AWG 4 TW, THW, or THHN conductors may be installed in a trade size 1 in. conduit (metric designator 27).

Note: Article 300.4(G) requires that where conductors of 4 size or larger enter an enclosure, an insulating bushing or equivalent must be installed on the conduit.

Motor Branch-Circuit Protection

The 20 hp, wound-rotor induction motor is to be started by means of an ATL magnetic motor switch. This motor starter applies the rated three-phase voltage to the stator winding. Speed control is provided by a controller used in the rotor or secondary circuit. All resistance of the controller is inserted in the rotor circuit when the motor is started. As a result, the inrush starting current to the motor is less than if the motor were started at full voltage.

The branch-circuit, short-circuit protection of a wound-rotor induction motor is required by Table 430.52 of the Code not to exceed 150% of the full-load running current of the motor.

This is for nontime-delay and time-delay fuses.

The full-load current equals 54 amperes for a 20 hp wound-rotor motor. The branch circuit overcurrent protection for the branch circuit feeding this motor is 54 × 150% = 81 amperes NEC 240.6 does not show 81 amperes as a standard fuse size. Article 430.52 allows the next larger size. A 90 ampere fuse should be chosen.

Disconnecting Means

According to the table for safety switches in FIG. 3, the disconnecting means for the 20 hp motor is a standard 25 hp, 200-ampere safety switch. Reducers must be installed in this switch to accommodate the 90-ampere fuses required for the motor branch-circuit protection. Because of the dual rating of these safety switches, it is permissible to use a 100-ampere switch with a maximum rating of 30 hp. In this case, standard 90-ampere, nontime-delay fuses or 90-ampere, time-delay fuses may be installed.

Running Overcurrent Protection (Motor Overload Protection)

The running overcurrent protection consists of three thermal overload units located in the ATL magnetic motor starter (except as indicated in the note following Table 430.37). According to the nameplate, the motor has a full-load current rating of 54 amperes. The rated trip current of each thermal unit is 54 × 125% = 67.5 amperes

Conductor Size (Rotor)

The rotor winding of the 20 hp, wound-rotor induction motor is rated at 60 amperes. The following procedure is used to determine the size of the conductors for the secondary circuit from the rotor slip rings to the drum controller:

1. NEC 430.23(A) requires that the conductors connecting the secondary of a wound rotor induction motor to its controller have a current-carrying capacity not less than 125% of the full-load secondary current of the motor for continuous duty.

60 × 125% = 75 amperes

2. Table 310.15(B)(16) indicates that several types of copper conductors can be used: AWG 3 Type TW, Type THW, or Type THHN, assuming 60° terminations.

3. Table C1 of Annex C shows that A trade size 1 in. conduit is required for three 3 THHN, TW or THW wires.

Note: Article 300.4(G) requires the use of insulating bushings or the equivalent on all conduits containing conductors of 4 size or larger entering enclosures.

If the resistors are mounted outside the speed controller, the current capacity of the conductors between the controller and the resistors shall be not less than the values given in Table 430.23(C).

For example, the manual speed controller used with the 20 hp, wound-rotor induction motor is to be used for heavy intermittent duty. NEC 430.23(C) requires that the conductors connecting the resistors to the speed controller have an ampacity not less than indicated in Table 430.23(C). This is between 35% and 110%, based on the duty of the resistors. For example, if we use heavy intermittent duty, then 85% is used.

60 × 85% = 51 amperes

Table 310.15(B)(16) indicates that 51 amperes can be carried safely by AWG 6 wire. As a result, the temperatures generated at the resistor location are an important consideration.

NEC 430.32(E) states that the secondary circuits of wound-rotor induction motors, including the conductors, controllers, and resistors, shall be considered as protected against overload by the motor running overcurrent protection in the primary or stator circuits. Therefore, no further overcurrent protection is necessary in the secondary rotor circuit.

MAIN FEEDER

When the conductors of a feeder supply two or more motors, the required wire size is deter mined using Code rules. Article 430, Part II, 430.24 states that the feeder shall have an ampacity of not less than 125 percent of the full-load current of the highest rated motor of the group plus the sum of the full-load current ratings of the remaining motors in the group. The full-load current of the motor is taken from the NEC, Table 430.250.

The motor with the largest full-load running current is the 25 hp motor. This motor has a full-load current rating of 68 amperes. The main feeder size for the highest rated motor, in compliance with 430.24, is 68 × 125% = 85 amperes

Then 85 + 54 + 28 = 167 amperes

Table 310.15(B)(16) indicates that AWG 4/0 Type TW or Type THHN copper conductors can be used when using 60° terminations.

If all the terminations in the circuits are rated for 75°C, then the second column in Table 310.15(B)(16) can be used for all wire ampacities. Article 110.14(C)(1) states that motors with design letters B, C, or D may use the 75°C rating for terminations and wire ampacity. Article 110.14(C)(1)(b) states that circuits over 100 amperes or conductors larger than 1 AWG may use the 75°C rating. If all the other terminations in that circuit are rated at 75°C, then 2/0 THWN or THHN wire can be used.

Table C1 of Annex C shows that three 4/0 TW conductors can be installed in trade size 2 conduit. Three 4/0 THHN conductors can be installed in a trade size 2 conduit.

Annex C, Table C.1 lists that 3-2/0 THHN conductors can fit in a trade size 1-1/2 in.- (metric designator 41) EMT conduit. Many newly installed systems are rated for 75°C terminations, so this will enable the electrical installation to use smaller size wire and pipe than would be allowed with 60°C terminations.

Main Feeder Short-Circuit Protection

Article 430, Part IV, 430.62(A), states that a feeder that supplies motors shall be provided with overcurrent protection. The feeder overcurrent protection shall not be greater than the largest current rating of the branch-circuit protective device for any motor of the group, based on Table 430.52, plus the sum of the full-load currents of the other motors of the group.

The branch circuit for the 25 hp motor has the largest value of overcurrent protection.

This value, as determined from Table 430.52, is a 119-ampere (68 × 1.75 or 125 ampere) fuse.

The full-load current rating of the 20 hp motor is 54 amperes, and the full-load current rating of the 10 hp motor is 28 amperes. The size of the fuses to be installed in the main feeder circuit shall not be greater than the sum of 125 + 54 + 28 = 207 amperes.

Therefore, three 200-ampere, nontime-delay fuses are used for the feeder circuit. This procedure should be in conformance with Example D8, Annex D of the Code. Exceptions may be made if the fuses do not allow the motor to start or run.

Main Disconnecting Means

Article 430, Part IX, 430.109, lists several types of disconnecting means. The disconnecting means shall have a carrying capacity of at least 115% of the sum of the current ratings of the motors. See 430.110 (C1 and 2). Therefore, the 200-ampere fuses required as the overcurrent protection for the main feeder are installed in a 200-ampere safety switch.

Wire types and sizes are selected by the ambient temperatures of the place of installation and the economics of the total installation, such as the minimum size of conduits, cost of the wire sizes, and labor cost for installing the various selections.

SUMMARY

Motor installation is one of the hardest calculations to perform and get all the components correct, in the proper location, and at the correct size. The NEC guides you through the main components of the calculation, but you must know where to look and how to apply the proper codes. There are many facets to the correct installation, including feeder and feeder protection, branch-circuit and branch protection, conductor sizes and overcurrent protection, running overcurrent protection, and secondary circuit protection.

QUIZ

Refer to the following feeder circuit description for the calculations in items 1 through 5.

A feeder circuit feeds three branch motor circuits. Branch motor circuit No. 1 has a load consisting of an induction motor with the following nameplate data:

No. 1

Squirrel-Cage Induction Motor

230 Volts 15 Amperes 3 Phase 60 Hertz Code Letter D 5 Horsepower Temperature Rating 40° Celsius

Branch motor circuit No. 2 has a load consisting of an induction motor with the following nameplate data. (This motor is equipped with an autotransformer starting compensator.)

No. 2

Squirrel-Cage Induction Motor

230 Volts 40 Amperes 3 Phase 60 Hertz Code letter F 15 Horsepower Temperature Rating 40° Celsius

Branch motor circuit No. 3 has a load consisting of a wound-rotor induction motor with the following nameplate data.

No. 3

Wound-Rotor Induction Motor

230 Volts 22 Stator Amperes

3 Phase 26 Rotor Amperes

7 1/2 Horsepower 60 Hertz

Continuous Duty Temperature Rating 40° Celsius

1. Refer to FIG. 4. Insert the answers on the diagram.

a. Determine the running overload protection in amperes required for the motor in branch circuit No. 1.

b. Determine the appropriate wire size (75° wire).


FIG. 4 Magnetic ATL motor starter switch for question 1. ASSUME CONNECTIONS FOR CONTROL CIRCUIT HAVE BEEN MADE; RUNNING OVERLOAD PROTECTION


FIG. 5 Manual autotransformer starting compensator for question 2.


FIG. 6 Magnetic ATL motor starter switch for question 3. ASSUME CONNECTIONS FOR CONTROL CIRCUITS HAVE BEEN MADE

2. Refer to FIG. 5. Insert the answers on the diagram.

a. Determine the running overload protection in amperes required for the motor in branch circuit No. 2.

b. Determine the appropriate wire size of the copper THW conductors. Note that the 15 hp squirrel-cage induction motor in this circuit is started by means of a starting compensator.

3. Refer to FIG. 6. Insert the answers of (a) and (b) on the diagram.

a. Determine the running overload protection in amperes required for the motor in branch circuit No. 3.

b. Determine the appropriate wire size of the copper conductors.

c. Determine the size of the conductors required for the secondary circuit of the wound-rotor induction motor in branch circuit No. 3. The secondary or rotor circuit feeds between the slip rings of the wound rotor and the speed controller. Indicate the size of the conduit. Use THW conductors.

4. Refer to FIG. 7. Insert the answers on the diagram.

a. Determine the current rating in amperes of the fuses (nontime-delay) used as over load protection for the main feeder circuit shown in the diagram.

b. Determine the THHN conductor size for the main feeder switch.

5. Refer to FIG. 8. Insert the answers on the diagram.

a. Using THHN-type copper conductors, determine the size of the conductors and conduit required for the main feeder circuit that feeds the three branch motor circuits. Indicate the sizes on the diagram.


FIG. 7 Fused disconnect for question 4.

b. Determine the size of fuses in amperes required for the starting overload protection for each of the branch circuits.

Motor Circuit No. 1

Motor Circuit No. 2

Motor Circuit No. 3

c. Using THHN-type copper conductors, determine the size of rigid conduit required for each of the three branch circuits.

Motor Circuit No. 1

Motor Circuit No. 2

Motor Circuit No. 3


FIG. 8 Feeder panel for question 5.

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