Goals of this Discussion:
- Determine the full load current rating of different types of motors using the National Electrical Code (NEC)
- Determine the conductor size for installing motors.
- Determine the overload size for different types of motors.
- Determine the size of the short circuit protective device for individual motors and multi-motor connections.
- Select the proper size starter for a particular motor.
Determining Motor Current
There are different types of motors, such as direct current, single-phase AC, two-phase AC, and three-phase AC. Different tables from the National Electrical Code (NEC) are used to determine the running cur rent for these different types of motors. Table 430.247 ( Ill. ftysvn-1) is used to determine the full load running current for a direct current motor. Table 430.248 (Ill. ftysvn-2) is used to determine the full load running current for single-phase motors; Table 430.249 (Ill. ftysvn-3) is used to determine the running cur rent for two-phase motors; and Table 430.250 (Ill. ftysvn-4) is used to determine the full load running current for three-phase motors. Note that the tables list the amount of current that the motor is expected to draw under a full load condition. The motor will exhibit less current draw if it is not under full load. These tables list the ampere rating of the motors according to horsepower and connected voltage. It should also be noted that NEC Section 430.6(A)(1) states these tables are to be used to in determining conductor size, short circuit protection size, and ground fault protection size instead of the nameplate rating of the motor. The motor overload size, however, is to be determined by the nameplate rating of the motor.
Direct Current Motors
Ill. ftysvn-1 Table 430.247 is used to determine the full load current for direct current motors. (Reprinted with permission from NFPA 70, National Electrical Code, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.)
Table 430.247 lists the full load running currents for direct current motors. The horsepower rating of the motor is given in the far left-hand column. Rated volt ages are listed across the top of the table. The table shows that a 1 horsepower motor will have a full-load current of 12.2 amperes when connected to 90 volts DC.
If a 1 horsepower motor is designed to be connected to 240 volts, it will have a current draw of 4.7 amperes.
Single-Phase AC Motors
The current ratings for single-phase AC motors are given in Table 430.248. Particular attention should be paid to the statement preceding the table. The statement asserts that the values listed in this table are for motors that operate under normal speeds and torques. Motors especially designed for low speed and high torque, or multispeed motors, should have their running current determined from the nameplate rating of the motor.
The voltages listed in the table are 115, 200, 208, and 230. The last sentence of the preceding statement says that the currents listed shall be permitted for volt ages of 110 to 120 volts and 220 to 240 volts. This means that if the motor is connected to a 120 volt line, it is permissible to use the currents listed in the 115 volt column. If the motor is connected to a 220 volt line, the 230 volt column can be used.
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EXAMPLE:
A 3 horsepower single-phase AC motor is connected to a 208 volt line. What will be the full load running current of this motor? Locate 3 horsepower in the far left-hand column. Follow across to the 208 volt column. The full load current will be 18.7 amperes.
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Ill. ftysvn-2 Table 430.248 is used to determine the full load current for single-phase motors. (Reprinted with permission from NFPA 70, National Electrical Code, Copyright 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.) Table 430.248 Full-Load Currents in Amperes, Single-Phase Alternating-Current Motors: The following values of full-load currents are for motors running at usual speeds and motors with normal torque characteristics. The voltages listed are rated motor voltages. The current listed shall be permitted for system voltage ranges of 110 to 12 and 220 to 240 volts.
Ill. ftysvn-3 Table 430.249 is used to determine the full load current for two-phase motors. (Reprinted with permission from NFPA 70™, National Electrical Code, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.) Table 430.249 Full-Load Current, Two-Phase Alternating-Current Motors (4-Wire): The following values of full-load current are for motors running at speeds usual for belted motors and motors with normal torque characteristics. Current in the common conductor of a 2-phase, 3-wire system will be 1.41 times the value given. The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
Two-Phase Motors
Although two-phase motors are seldom used, Table 430.249 lists the full load running currents for these motors. Like single-phase motors, two-phase motors that are especially designed for low speed, high torque applications and multi-speed motors, use the nameplate rating instead of the values shown in the table. When using a two-phase, three-wire system, the size of the neutral conductor must be increased by the square root of 2, or 1.41. The reason for this is that the voltages of a two phase system are 90 degrees out-of-phase with each other, as shown in Ill. ftysvn-5. The principle of two phase power generation is shown in Ill. ftysvn-6. In a two-phase alternator, the phase windings are arranged 90 degrees apart. The magnet is the rotor of the alternator. When the rotor turns, it induces voltage into the phase windings, which are 90 degrees apart. When one end of each phase winding is joined to form a common terminal, or neutral, the current in the neutral conductor will be greater than the current in either of the two phase conductors. An example of this is shown in Ill. ftysvn-7.
In this example, a two-phase alternator is connected to a two-phase motor. The current draw on each of the phase windings is 10 amperes. The current flow in the neutral, however, is 1.41 times greater than the current flow in the phase windings, or 14.1 amperes.
Ill. ftysvn-4 Table 430.250 is used to determine the full load current for three-phase motors. (Reprinted with permission from NFPA 70, National Electrical Code, Copyright 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.) Table 430.250 Full-Load Current, Three-Phase Alternating-Current Motors: The following values of full-load currents are typical for motors running at speeds usual for belted motors and motors with normal torque characteristics. The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
Ill. ftysvn-5 The voltages of a two-phase system are 90 degrees out of phase with each other.
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EXAMPLE:
Compute the phase current and neutral current for a 60 horsepower, 460 volt two-phase motor.
The phase current can be taken from Table 430.249.
Phase current = 67 amperes
The neutral current will be 1.41 times higher than the phase current.
Neutral current = 67 x1.41
Neutral current = 94.5 amperes
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Ill. ftysvn-6 A two-phase alternator produces voltages that are 90 deg. out of phase with each other.
Ill. ftysvn-7 The neutral conductor of a two-phase system has a greater current than the other two conductors.
Three-Phase Motors
Table 430.250 is used to determine the full load current of three-phase motors. The notes at the top of the table are very similar to the notes of Tables 430.248 and 430.249. The full load current of low speed, high torque and multispeed motors is to be determined from the nameplate rating instead of from the values listed in the table. Table 430.250 has an extra note that deals with synchronous motors. Notice that the right-hand side of Table 430.250 is devoted to the full load cur rents of synchronous type motors. The currents listed are for synchronous type motors that are to be operated at unity, or 100%, power factor. Since synchronous motors are often made to have a leading power factor by over excitation of the rotor current, the full load current rating must be increased when this is done. If the motor is to be operated at 90% power factor, the rated full load current in the table must be increased by 10%. If the motor is to be operated at 80% power factor, the full load current is to be increased by 25%.
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EXAMPLE:
A 150 horsepower, 460 volt synchronous motor is to be operated at 80% power factor. What will be the full load current rating of the motor? The table indicates a current value of 151 amperes for this motor. To determine the running cur rent at 80% power factor, multiply this current by 125%, or 1.25. (Multiplying by 1.25 results in the same answer that would be obtained by dividing by 0.80.) 151 x 1.25 = 188.75 or 189 amperes
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EXAMPLE:
A 200 horsepower, 2300 volt synchronous motor is to be operated at 90% power factor. What will be the full load current rating of this motor? Locate 200 horsepower in the far left-hand column. Follow across to the 2300 volt column listed under synchronous type motors. Increase this value by 10%: 40 x 1.10 _ 44 amperes
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EXAMPLE:
A 30 horsepower three-phase squirrel cage induction motor is connected to a 480 volt line. The conductors are run in conduit to the motor. The motor does not have a NEMA design code listed on the nameplate. The termination temperature rating of the devices is not known. Copper conductors with THWN insulation are to be used for this motor connection. What size conductors should be used? The first step is to determine the full load cur rent of the motor. This is determined from Table 430.250. The table indicates a current of 40 amperes for this motor. The current must be increased by 25% according to Section 430.22(A).
40 x 1.25 = 50 amperes
Table 310.16 is used to determine the conductor size. Locate the column that contains THWN insulation in the copper section of the table. THWN is located in the 75°C column. Since this circuit is less than 100 amperes and the termination temperature is not known, and the motor does not contain a NEMA design code letter, the conductor size must be selected from the ampacities listed in the 60°C column. A #6 AWG copper conductor with type THWN insulation will be used.
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Determining Conductor Size for a Single Motor
NEC Section 430.6(A)(1) states that the conductor for a motor connection shall be based on the values from Tables 430.247, 430.248, 430.249, and 430.250 instead of the motor nameplate current. Section 430.22(A) states that conductors supplying a single motor shall have an ampacity of not less than 125% of the motor full load current. NEC Section 310 is used to select the conductor size after the ampacity has been determined. The exact table employed will be determined by the wiring conditions. Probably the most frequently used table is 310.16 ( Ill. ftysvn-8).
Ill. ftysvn-8 Table 310.16 is used to determine the ampacity of conductors. (Reprinted with permission from NFPA 70, National Electrical Code, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269.
Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)
Termination Temperature
Another factor that must be taken into consideration when determining the conductor size is the temperature rating of the devices and terminals as specified in NEC Section 110.14(C). This section states that the conductor is to be selected and coordinated as to not exceed the lowest temperature rating of any connected termination, any connected conductor, or any connected device. This means that, regardless of the temperature rating of the conductor, the ampacity must be selected from a column that does not exceed the temperature rating of the termination. The conductors listed in the first column of Table 310.16 have a temperature rating of 60°C, the conductors in the second column have a rating of 75°C, and the conductors in the third column have a rating of 90°C. The temperature ratings of devices such as circuit breakers, fuses, and terminals are often found in the UL (Underwriters Lab oratories) product directories. Occasionally, the temperature rating may be found on the piece of equipment, but this is the exception and not the rule. As a general rule, the temperature rating of most devices will not exceed 75°C.
When the termination temperature rating is not listed or known, NEC Section 110.14(C)(1)(a) states that for circuits rated at 100 amperes or less, or for #14 AWG through #1 AWG conductors, the ampacity of the wire, regardless of the temperature rating, will be selected from the 60°C column. This does not mean that only those types of insulations listed in the 60°C column can be used, but that the ampacities listed in the 60°C column must be used to select the conductor size.
For example, assume that a copper conductor with type XHHW insulation is to be connected to a 50 ampere circuit breaker that does not have a listed temperature rating. According to NEC Table 310.16, a #8 AWG copper conductor with XHHW insulation is rated to carry 55 amperes of current. Type XHHW insulation is located in the 90°C column, but the temperature rating of the circuit breaker is not known. Therefore, the wire size must be selected from the ampacity ratings in the 60°C column. A #6 AWG copper conductor with type XHHW insulation would be used.
NEC Section 110.14(C)(1)(a)(4) has a special provision for motors with marked NEMA design codes B, C, or D. This section states that conductors rated at 75°C or higher may be selected from the 75°C column even if the ampacity is 100 amperes or less. This code will not apply to motors that do not have a NEMA de sign code marked on their nameplate. Most motors manufactured before 1996 will not have a NEMA de sign code. The NEMA design code letter should not be confused with the code letter that indicates the type squirrel cage rotor used in the motor.
For circuits rated over 100 amperes, or for conductor sizes larger than #1 AWG, Section 110.14(C)(1)(b) states that the ampacity ratings listed in the 75°C column may be used to select wire sizes unless conductors with a 60°C temperature rating have been selected for use. For example, types TW and UF insulation are listed in the 60°C column. If one of these two insulation types has been specified, the wire size must be chosen from the 60°C column regardless of the ampere rating of the circuit.
Overload Size
When determining the overload size for a motor, the nameplate current rating of the motor is used instead of the current values listed in the tables (NEC Section 430.6(A)(1)). Other factors such as the service factor (SF) or temperature rise (°C) of the motor are also to be considered when determining the overload size for a motor. The temperature rise of the motor is an indication of the amount of temperature increase the motor should experience under a full load condition and should not be confused with termination temperature discussed in Section 110.14(C). NEC Section 430.32 ( Ill. ftysvn-9) is used to determine the overload size for motors of 1 horsepower or more. The overload size is based on a percentage of the full load current of the motor listed on the motor nameplate.
If for some reason this overload size does not permit the motor to start without tripping out, Section 430.32(C) permits the overload size to be increased to a maximum of 140% for this motor. If this increase in overload size does not solve the starting problem, the overload may be shunted out of the circuit during the starting period in accordance with Section 430.35(A)&(B).
Determining Locked Rotor Current
There are two basic methods for determining the locked rotor current (starting current) of a squirrel cage induction motor, depending on the information avail able. If the motor nameplate lists code letters that range from A to V, they indicate the type of rotor bars used when the rotor was made. Different types of bars are used to make motors with different operating characteristics. The type of rotor bars largely determines the maximum starting current of the motor. NEC Table 430.7(B) ( Ill. ftysvn-10) lists the different code letters and gives the locked-rotor kilovolt-amperes per horse power. The starting current can be determined by multiplying the kilovolt-ampere rating by the horsepower rating and then dividing by the applied voltage.
The second method of determining locked rotor current is to use Tables 430.251(A)&(B) ( Ill. ftysvn-11) if the motor nameplate lists NEMA design codes. Table 430.251(A) lists the locked rotor currents for single phase motors and Table 430.251(B) lists the locked rotor currents for poly-phase motors.
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EXAMPLE:
A 15 horsepower, three-phase squirrel cage motor with a code letter of K is connected to a 240 volt line. Determine the locked-rotor current.
The table lists 8.0 to 8.99 kilovolt-amperes per horsepower for a motor with a code letter of K. An average value of 8.5 will be used.
8.5x 15 x 127.5 kVA or 127,500 VA
127,500 / 240 sqr. rt 3 = 306.7 amperes
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EXAMPLE:
A 25 horsepower three-phase induction motor has a nameplate rating of 32 amperes. The nameplate also shows a temperature rise of 30°C. Determine the ampere rating of the overload for this motor.
NEC Section 430.32(A)(1) indicates the over load size is 125% of the full load current rating of the motor.
32 x1.25 = 40 amperes
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Ill. ftysvn-9 Table 430.32 is used to determine overload size for motors. (Reprinted with permission from NFPA 70™, National Electrical Code, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.)
Ill. ftysvn-10 Table 430.7(B) is used to determine locked rotor current for motors that do not contain a NEMA code letter.
Short-Circuit Protection
The rating of the short-circuit protective device is determined by NEC Table 430.52 ( Ill. ftysvn-12).
The far left-hand column lists the type of motor that is to be protected. To the right of this are four columns that list different types of short-circuit protective devices; non-time delay fuses, dual-element time delay fuses, instantaneous trip circuit breakers and inverse time circuit breakers. Although it is permissible to use non time delay fuses and instantaneous trip circuit breakers, most motor circuits are protected by dual-element time delay fuses or inverse time circuit breakers.
Each of these columns lists the percentage of motor current that is to be used in determining the ampere rating of the short-circuit protective device.
The current listed in the appropriate motor table is to be used instead of the nameplate current. NEC Section 430.52(C)(1) states that the protective device is to have a rating or setting not exceeding the value calculated in accord with Table 430.52. Exception No. 1 of this section, however, states that if the calculated value does not correspond to a standard size or rating of a fuse or circuit breaker, it shall be permissible to use the next higher standard size. The standard sizes of fuses and circuit breakers are listed in NEC Section 240.6 ( Ill. ftysvn-13). Starting in 1996, Table 430.52 has listed squirrel cage motor types by NEMA design letters instead of code letters. Section 430.7(A)(9) requires that motor nameplates be marked with design letters B, C, or D.
Motors manufactured before this requirement, how ever, do not list design letters on the nameplate. Most common squirrel cage motors used in industry actually fall in the design B classification and for purposes of selecting the short-circuit protective device are considered to be design B unless otherwise listed.
If for some reason this fuse will not permit the motor to start without blowing, NEC Section 430.52(C)(1) Exception 2(b) states that the rating of a dual-element time delay fuse may be increased to a maximum of 225% of the full load motor current.
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EXAMPLE:
A 100 horsepower three-phase squirrel cage induction motor is connected to a 240 volt line. The motor does not contain a NEMA design code. A dual-element time delay fuse is to be used as the short-circuit protective device. Determine the size needed.
Table 340.250 lists a full load current of 248 amperes for this motor. Table 430.52 indicates that a dual-element time delay fuse is to be calculated at 175% of the full load current rating for an AC polyphase (more than one phase) squirrel cage motor, other than design code E. Since the motor does not list a NEMA design code on the nameplate, it will be assumed that the motor is design B.
248 x 1.75 x 434 amperes
The nearest standard fuse size above the computed value listed in Section 240.6 is 450 amperes, so 450 ampere fuses will be used to protect this motor.
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Ill. ftysvn-11 Table 430.251(A) & (B) are used to locked rotor current for motors that do contain NEMA code letters. (Reprinted with permission from NFPA 70, National Electrical Code, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.) Table 430.251(A) Conversion Table of Single-Phase Locked-Rotor Currents for Selection of Disconnecting Means and Controllers as Determined from Horsepower and Voltage Rating Table 430.251(B) Conversion Table of Polyphase Design B, C, and D Maximum Locked-Rotor Currents for Selection of Disconnecting Means and Controllers as Determined from Horsepower and Voltage Rating and Design Letter For use only with 430.110, 440.12, 440.41, and 455.8(C).
Ill. ftysvn-12 Table 430.52 is used to determine the size of the short circuit protective device for a motor.
Ill. ftysvn-13 Section 240.6 lists standard fuse and circuit breaker sizes.
Starter Size Another factor that must be considered when in stalling a motor is the size of starter used to connect the motor to the line. Starter sizes are rated by motor type, horsepower, and connected voltage. The two most common ratings are NEMA and IEC. A chart showing common NEMA size starters for alternating current motors is shown in Ill. ftysvn-14. A chart showing IEC starters for alternating current motors is shown in Ill. ftysvn-15. Each of these charts lists the minimum size starter designed to connect the listed motors to the line. It is not uncommon to employ larger size starters than those listed. This is especially true when using IEC type starters because of their smaller load contact size.
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EXAMPLE:
A 40 horsepower three-phase squirrel cage motor is connected to a 208 volt line. What are the mini mum size NEMA and IEC starters that should be used to connect this motor to the line? NEMA: The 200 volt listing is used for motors rated at 208 volts. Locate the NEMA size starter that corresponds to 200 volts and 40 horsepower.
Since the motor is three-phase, 40 horsepower will be in the polyphase column. A NEMA size 4 starter is the minimum size for this motor.
IEC: As with the NEMA chart, the IEC chart lists 200 volts instead of 208 volts. A size N starter lists 200 volts and 40 horsepower in the three-phase column.
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Example Exercises
Exercise 1
A 40 horsepower 240 volt DC motor has a nameplate current rating of 132 amperes. The conductors are to be copper with type TW insulation. The short-circuit protective device is to be an instantaneous trip circuit breaker. The termination temperature rating of the connected devices is not known. Determine the conductor size, overload size, and circuit breaker size for this installation. Refer to Ill. ftysvn-16.
The conductor size must be determined from the current listed in Table 430.247. This value is to be in creased by 25%. (NOTE: multiplying by 1.25 has the same effect as multiplying by 0.25 and then adding the product back to the original number (140 x 0.25 = 35) (35 x 140 = 175 amperes)
140 x 1.25 = 175 amperes
Table 310.16 is used to find the conductor size. Al though Section 110.14(C) states that for currents of 100 amperes or greater, the ampacity rating of the conductor is to be determined from the 75°C column, in this instance, the insulation type is located in the 60°C column. Therefore, the conductor size must be determined using the 60°C column instead of the 75°C column. A 4/0 AWG copper conductor with type TW insulation will be used.
The overload size is determined from NEC Section 430.32(A)(1). Since there is no service factor or temperature rise listed on the motor nameplate, the heading ALL OTHER MOTORS will be used. The motor name plate current will be increased by 15%.
132 x 1.15 = 151.8 amperes
The circuit breaker size is determined from Table 430.52. The current value listed in Table 430.247 is used instead of the nameplate current. Under DC motors (constant voltage), the instantaneous trip circuit breaker rating is given at 250%.
140 x 2.50 = 350 amperes Since 350 amperes is one of the standard sizes of circuit breakers listed in NEC Section 240.6, that size breaker will be employed as the short-circuit protective device.
Exercise 2
A 150 horsepower three-phase squirrel cage induction motor is connected to a 440 volt line. The motor nameplate lists the following information:
Amps 175 SF 1.25 Code D NEMA code B. The conductors are to be copper with type THHN insulation. The short-circuit protective device is to be an inverse time circuit breaker. The termination temperature rating is not known. Determine the conductor size, overload size, circuit breaker size, minimum NEMA starter size, and IEC starter size. Refer to Ill. ftysvn-17.
The conductor size is determined from the current listed in Table 430.250 and increased by 25%.
180 x 1.25 = 225 amperes
Table 310.16 is used to determine the conductor size.
Type THHN insulation is located in the 90°C column.
Since the motor nameplate lists NEMA code B, and the amperage is over 100 amperes, the conductor will be selected from the 75°C column. The conductor size will be 4/0 AWG.
Ill. ftysvn-16 Example problem #1.
The overload size is determined from the name plate current and NEC Section 430.32(A)(1). The motor has a marked service factor of 1.25. The motor nameplate current will be increased by 25%.
175 x 1.25 = 218.75 amperes
The circuit breaker size is determined by Tables 430.250 and 430.52. Table 430.52 indicates a factor of 250% for squirrel cage motors with NEMA design code B. The value listed in Table 430.250 will be in creased by 250%.
180 x 2.50 = 450 amperes
One of the standard circuit breaker sizes listed in NEC Section 240.6 is 450 amperes. A 450 ampere inverse time circuit breaker will be used as the short-circuit protective device.
The proper motor starter sizes are selected from the NEMA and IEC charts shown in Ill. ftysvn-14 and Ill. ftysvn-15. The minimum size NEMA starter is 5 and the minimum size IEC starter is R.
Ill. ftysvn-17 Example circuit #2
Multiple Motor Calculations
The main feeder short-circuit protective devices and conductor sizes for multiple motor connections are set forth in NEC Section 430.62(A) and 430.24. In this example, three motors are connected to a common feeder. The feeder is 480 volts three-phase and the conductors are to be copper with type THHN insulation.
Each motor is to be protected with dual-element time delay fuses and a separate overload device. The main feeder is also protected by dual-element time delay fuses. The termination temperature rating of the connected devices is not known. The motor nameplates state the following:
- Motor #1 Phase 3 HP 20 SF 1.25 NEMA code C Volts 480 Amperes 23 Type Induction
- Motor #2 Phase 3 HP 60 Temp. 40°C Code J Volts 480 Amperes 72 Type Induction
- Motor #3 Phase 3 HP 100 Code A Volts 480 Amperes 96 PF 90% Type Synchronous
Motor #1 Calculation
The first step is to calculate the values for motor amperage, conductor size, overload size, short-circuit protection size, and starter size for each motor. Both NEMA and IEC starter sizes will be determined. The values for motor #1 are shown in Ill. ftysvn-18.
The ampere rating from Table 430.250 is used to determine the conductor and fuse size. The amperage rating must be increased by 25% for the conductor size.
27 x 1.25 = 33.75 amperes
The conductor size is chosen from Table 310.16. Al though type THHN insulation is located in the 90°C column, the conductor size will be chosen from the 75°C column. Although the current is less than 100 amperes, NEC Section 110.14(C)(1)(d) permits the conductors to be chosen from the 75°C column if the motor has a NEMA Design Code.
33.75 amperes = #10 AWG
The overload size is computed from the nameplate current. The demand factors in Section 430.32(A)(1) are used for the overload calculation.
23 x 1.25 = 28.75 amperes
Ill. ftysvn-18 Motor #1 calculation
Ill. ftysvn-19 Motor #2 calculation
The fuse size is determined by using the motor cur rent listed in Table 430.250 and the demand factor from Table 430.52. The percent of full load current for a dual-element time delay fuse protecting a squirrel cage motor listed as Design C is 175%. The current listed in Table 430.250 will be increased by 175%.
27 x 1.75 = 47.25 amperes
The nearest standard fuse size listed in Section 240.6 is 50 amperes, so 50 ampere fuses will be used.
The starter sizes are determined from the NEMA and IEC charts shown in Ill. ftysvn-14 and Ill. ftysvn-15. A 20 horsepower motor connected to 480 volts would require a NEMA size 2 starter and an IEC size F starter.
Motor #2 Calculation
Ill. ftysvn-19 shows an example for the calculation for motor #2. Table 430.250 lists a full load current of 77 amperes for this motor. This value of current is increased by 25% for the calculation of the conductor current.
77 x 1.25 = 96.25 amperes
Table 310.16 indicates a #1 AWG conductor should be used for this motor connection. The conductor size is chosen from the 60°C column because the circuit cur rent is less than 100 amperes in accord with Section 110.14(C), and the motor nameplate does not indicate a NEMA design code. (The code J indicates the type of bars used in the construction of the rotor.) The overload size is determined from Section 430.32(A)(1). The motor nameplate lists a temperature rise of 40°C for this motor. The nameplate current will be increased by 25%.
72 x 1.25 = 90 amperes
The fuse size is determined from Table 430.52. The table current is increased by 175% for squirrel cage motors other than design E.
77 x 1.75 = 134.25 amperes
The nearest standard fuse size listed in Section 240.6 is 150 amperes, so 150 ampere fuses will be used to protect this circuit.
The starter sizes are chosen from the NEMA and IEC starter charts. This motor would require a NEMA size 4 starter or a size L IEC starter.
Ill. ftysvn-20 Motor #3 calculation.
Motor #3 Calculation
Motor #3 is a synchronous motor intended to operate with a 90% power factor. Ill. ftysvn-20 shows an example of this calculation. The notes at the bottom of Table 430.250 indicate that the listed current is to be in creased by 10% for synchronous motors with a listed power factor of 90%.
101 x 1.10 = 111 amperes
The conductor size is computed by using this cur rent rating and increasing it by 25%.
111 x 1.25 = 138.75 amperes
Table 310.16 indicates that a #1/0 AWG conductor will be used for this circuit. Since the circuit current is over 100 amperes, the conductor size is chosen from the 75°C column.
This motor does not have a marked service factor or a marked temperature rise. The overload size will be calculated by increasing the nameplate current by 15% as indicated in Section 430.32(A)(1) under the heading all other motors.
96 x 1.15 = 110.4 amperes
The fuse size is determined from Table 430.52. The percent of full load current for a synchronous motor is 175%.
111 x 1.75 = 194.25 amperes
The nearest standard size fuse listed in Section 240.6 is 200 amperes, so 200 ampere fuses will be used to protect this circuit.
The NEMA and IEC starter sizes are chosen from the charts shown in Ill. ftysvn-14 and Ill. ftysvn-15.
The motor will require a NEMA size 4 starter and an IEC size N starter.
Main Feeder Calculation
An example of the main feeder connections is shown in Ill. ftysvn-21. The conductor size is computed in accord with NEC Section 430.24 by increasing the largest amperage rating of the motors connected to the feeder by 25% and then adding the ampere rating of the other motors to this amount. In this example, the 100 horsepower synchronous motor has the largest running current. This current will be increased by 25% and then the running currents of the other motors as deter mined from Table 430.250 will be added.
111 x 1.25 = 138.75 amperes
138.75 x 77 = 27 x 242.75 amperes
Table 310.16 lists that 250 KC mil copper conductors are to be used as the main feeder conductors. The conductors were chosen from the 75°C column.
The size of the short-circuit protective device is determined by Section 430.62(A). The code states that the rating or setting of the short-circuit protective device shall not be greater than the largest rating or setting of the largest branch circuit short-circuit and ground fault protective device for any motor supplied by the feeder plus the sum of the full load running currents of the other motors connected to the feeder. The largest fuse size in this example is the 100 horsepower synchronous motor.
The fuse calculation for this motor is 200 amperes. The running currents of the other two motors will be added to this value to determine the fuse size for the main feeder.
200 x 77 = 27 _ 304 amperes
Ill. ftysvn-21 Main feeder calculator.
The closest standard fuse size listed in Section 240.6 without going over 304 amperes is 300 amperes, so 300 ampere, so fuses will be used to protect this circuit.
Quiz
1. A 20 horsepower, DC motor is connected to a 500 volt DC line. What is the full load running current of this motor?
2. What rating is used to find the full load running current of a torque motor?
3. A 3/4 horsepower, single-phase squirrel cage motor is connected to a 240 volt AC line. What is the full load current rating of this motor and what is the minimum size NEMA and IEC starters that should be used?
4. A 30 horsepower, two-phase motor is connected to a 230 volt AC line. What is the rated current of the phase conductors and the rated current of the neutral?
5. A 125 horsepower, synchronous motor is connected to a 230 volt three-phase AC line. The motor is intended to operate at 80% power factor.
What is the full load running current of this motor? What is the minimum size NEMA and IEC starters that should be used to connect this motor to the line?
6. What is the full load running current of a three phase, 50 horsepower motor connected to a 560 volt line? What minimum size NEMA and IEC starters should be used to connect this motor to the line?
7. A 125 horsepower, three-phase squirrel cage induction motor is connected to 560 volts. The nameplate current is 115 amperes. It has a marked temperature rise of 40°C and a code letter J. The conductors are to be type THHN copper and they are run in conduit. The short-circuit protective device is dual-element time delay fuses. Find the conductor size, overload size, fuse size, minimum NEMA and IEC starter sizes, and the upper and lower range of starting current for this motor.
8. A 7.5 horsepower, single-phase squirrel cage induction motor is connected to 120 volts AC.
The motor has a code letter of H. The nameplate current is 76 amperes. The conductors are copper with type TW insulation. The short-circuit protection device is a non-time delay fuse. Find the conductor size, overload size, fuse size, minimum NEMA and IEC starter sizes, and upper and lower starting currents.
9. A 75 horsepower, three-phase, synchronous motor is connected to a 230 volt line. The motor is to be operated at 80% power factor. The motor name plate lists a full load current of 185 amperes, a temperature rise of 40°C, and a code letter A. The conductors are to be made of copper and have type THHN insulation. The short-circuit protective device is to be an inverse time circuit breaker. Determine the conductor size, overload size, circuit breaker size, minimum size NEMA and IEC starters, and the upper and lower starting current.
10. Three motors are connected to a single branch circuit. The motors are connected to a 480 volt three-phase line. Motor #1 is a 50 horsepower induction motor with a NEMA code B. Motor #2 is 40 horsepower with a code letter of H, and motor #3 is 50 horsepower with a NEMA code C. Deter mine the conductor size needed for the branch circuit supplying these three motors. The conductors are copper with type THWN-2 insulation.
11. The short-circuit protective device supplying the motors in question #10 is an inverse time circuit breaker. What size circuit breaker should be used?
12. Five 5 horsepower, three-phase motors with NEMA code B are connected to a 240 volt line. The conductors are copper with type THWN insulation. What size conductor should be used to supply all of these motors?
13. If dual-element time delay fuses are to be used as the short-circuit protective device, what size fuses should be used to protect the circuit in question #12?
14. A 75 horsepower, three-phase squirrel cage induction motor is connected to 480 volts. The motor has a NEMA code D. What is the starting current for this motor?
15. A 20 horsepower, three-phase squirrel cage induction motor has a NEMA code B. The motor is connected to 208 volts. What is the starting current for this motor?