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NEC Questions and Answers - January 2016  

 
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By Mike Holt for EC&M Magazine

 

Here's the follow up to yesterday's newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Note: These questions are based on the 2014 NEC®. Any underlined text indicates a change to the Code rule for the 2014 NEC.

 

 

Q1. When sizing Motor circuits, when does the code require the use of the Code tables as opposed to the nameplate current?

A1.  According to 430.6(A)(1), the motor full-load current ratings listed in Tables 430.247, 430.248, and 430.250 are used to determine the conductor ampacity [430.22], the branch-circuit short-circuit and ground-fault overcurrent device size [430.52 and 430.62], and the ampere rating of disconnecting switches [430.110].

Author’s Comment:

The actual current rating on the motor nameplate full-load amperes (FLA) [430.6(A)(2)] isn’t permitted to be used to determine the conductor ampacity, the branch-circuit short-circuit and ground-fault overcurrent device size, nor the ampere rating of disconnecting switches.

Motors built to operate at less than 1,200 RPM or that have high torques may have higher full-load currents, and multispeed motors have full-load current varying with speed, in which case the nameplate current ratings must be used.

430.6(A)(1) Ex 3: For a listed motor-operated appliance, the actual current marked on the nameplate of the appliance must be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection.

430.6(A)(2) Motor Nameplate Current Rating (FLA). Overload devices must be sized based on the motor nameplate current rating in accordance with 430.31.

Author’s Comment:

The motor nameplate full-load ampere rating is identified as full-load amperes (FLA). The FLA rating is the current in amperes the motor draws while producing its rated horsepower load at its rated voltage, based on its rated efficiency and power factor.

The actual current drawn by the motor depends upon the load on the motor and on the actual operating voltage at the motor terminals. That is, if the load increases, the current also increases, or if the motor operates at a voltage below its nameplate rating, the operating current will increase.

CAUTION: To prevent damage to motor windings from excessive heat (caused by excessive current), never load a motor above its horsepower rating, and be sure the voltage source matches the motor’s voltage rating.

 

Q2. What is the Code rule for sizing conductors to a single continuous duty motor?

A2. Conductors to a single motor must be sized not less than 125 percent of the motor FLC rating as listed in Table 430.247 Direct-Current Motors, Table 430.248 Single-Phase Motors, or Table 430.250 Three-Phase Motors [430.22].

Question: What size branch-circuit conductor is required for a 7½ hp, 230V, three-phase motor with 75ºC terminals?

(a) 14 AWG       (b) 12 AWG      (c) 10 AWG     (d) 8 AWG

Answer: (c) 10 AWG

Motor FLC = 22A [Table 430.250]

Conductor’s Size = 22A x 1.25

Conductor’s Size = 27.50A, 10 AWG, rated 35A at 75°C [Table 310.15(B)(16)]

Note: The branch-circuit short-circuit and ground-fault protection device using an inverse time breaker is sized at 60A according to 430.52(C)(1) Ex 1:

Circuit Protection = 22A x 2.50

Circuit Protection = 55A, next size up 60A [240.6(A)]

 

Q3. When sizing conductors for more than one motor, what does the Code consider to be the highest rated motor? 

A3. When sizing motor circuit conductors, the highest rated motor is the motor with the highest rated full-load current rating (FLC) [430.17].

Question: Which of the following motors has the highest FLC rating?  

(a) 10 hp, three-phase, 208V   (b) 5 hp, single-phase, 208V(c) 3 hp, single-phase, 120V   (d) none of these

Answer: (c) 3 hp, single-phase, 120V

10 hp = 30.80A [Table 430.250]

5 hp = 30.80A [Table 430.248]

3 hp = 34.00A [Table 430.248]

 

 

Q4. What is the Code requirement for sizing conductors that supply several motors?

A4.  Circuit conductors that supply several motors must not be sized smaller than the sum of the following [430.24]:

(1) 125 percent of the full-load current of the highest rated motor

(2) The full-load current ratings of other motors

Question: What size feeder conductor is required for two 7½ hp, 230V, three-phase motors, if the terminals are rated for 75°C?

(a) 14 AWG       (b) 12 AWG      (c) 10 AWG     (d) 8 AWG

Answer: (d) 8 AWG

Motor FLC = 22A [Table 430.250]

Motor Feeder Conductor = (22A x 1.25) + 22A

Motor Feeder Conductor = 49.50A, 8 AWG rated 50A at 75°C [Table 310.15(B)(16)]

Author’s Comment:

The feeder overcurrent device (inverse time circuit breaker) must comply with 430.62 as follows:

 

Step 1: Determine the largest branch-circuit overcurrent device rating [240.6(A) and 430.52(C)(1) Ex 1]:

22A x 2.50 = 55A, next size up 60A

Step 2: Size the feeder overcurrent device in accordance with 240.6(A) and 430.62:

Feeder Inverse Time Breaker: 60A + 22A = 82A, next size down, 80A

Author’s Comment:

The “next size up protection” rule for branch circuits [430.52(C)(1) Ex 1] doesn’t apply to motor feeder short-circuit and ground-fault protection device sizing.

 

 

Q5. What is the Code requirement for protecting a motor from short circuit, ground fault, and overload with a single device?

A5.  A motor can be protected against overload, as well as short circuit, and ground fault by a single overcurrent device sized to the overload requirements contained in 430.32 [430.55].

Question: What size dual-element fuse is permitted to provide overload as well as short circuit and ground fault protection for a 5 hp, 230V, single-phase motor with a service factor of 1.15 and a nameplate current rating of 28A?

(a) 20A               (b) 25A             (c) 30A            (d) 35A

Answer: (d) 35A

Overload Protection [430.32(A)(1)]

28A x 1.25 = 35A

 

 

Q6. What is the Code rule for sizing short circuit and ground fault protection for a single motor branch circuit?

A6.  The motor branch-circuit short-circuit and ground-fault protective device must comply with 430.52(B) and 430.52(C).

(B) All Motors. A motor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the motor’s starting current.

(C) Rating or Setting.

(1) Table 430.52. Each motor branch circuit must be protected against short circuit and ground faults by a protective device sized no greater than the following percentages listed in Table 430.52.

 

Table 430.52

 

 

 

 

Motor Type

 

Nontime Delay

 

Dual-Element Fuse

 

Inverse Time Breaker

 

Wound Rotor

150%

150%

150%

Direct Current

150%

150%

150%

All Other Motors

300%

175%

250%

 

Question: What size conductor and inverse time circuit breaker are required for a 2 hp, 230V, single-phase motor with 75ºC terminals?  

(a) 14 AWG, 30A breaker          (b) 14 AWG, 35A breaker(c) 14 AWG, 40A breaker       (d) 14 AWG, 45A breaker

Answer: (a) 14 AWG, 30A breaker

Step 1: Determine the branch-circuit conductor [Table 310.15(B)(16), 430.22, and Table 430.248]:

12A x 1.25 = 15A, 14 AWG, rated 20A at 75°C [Table 310.15(B)(16)]

Step 2: Determine the branch-circuit protection [240.6(A), 430.52(C)(1), and Table 430.248]:

12A x 2.50 = 30A

Author’s Comment:

I know it bothers many in the electrical industry to see a 14 AWG conductor protected by a 30A circuit breaker, but branch-circuit conductors are protected against overloads by the overload device, which is sized between 115 and 125 percent of the motor nameplate current rating [430.32]. The small conductor rule contained in 240.4(D) which limits 15A protection for 14 AWG doesn’t apply to motor circuit protection. See 240.4(D) and 240.4(G).

Ex 1: If the motor short-circuit and ground-fault protective device values derived from Table 430.52 don’t correspond with the standard overcurrent device ratings listed in 240.6(A), the next higher overcurrent device rating can be used.

Question: What size conductor and inverse time circuit breaker are required for a 7½ hp, 230V, three-phase motor with 75ºC terminals?

(a) 10 AWG, 50A breaker          (b) 10 AWG, 60A breaker(c) a or b        (d) none of these

Answer: (b) 10 AWG, 60A breaker

Step 1: Determine the branch-circuit conductor [Table 310.15(B)(16), 430.22, and Table 430.250]:

22A x 1.25 = 27.50A, 10 AWG, rated 35A at 75°C [Table 310.15(B)(16)]

Step 2: Determine the branch-circuit protection [240.6(A), 430.52(C)(1) Ex 1, and Table 430.250]:

22A x 2.50 = 55A, next size up = 60A

 

 

Q7. What is the Code rule for sizing short circuit and ground fault protection for a motor feeder?

A7. Feeder conductors must be protected against short circuits and ground faults by a protective device sized not more than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor, plus the sum of the full-load currents of the other motors in the group [430.62(A)].

Question: What size feeder protection (inverse time breakers with 75°C terminals) and conductors are required for the following two motors?  

Motor 1—20 hp, 460V, three-phase = 27A FLC [Table 430.250]

Motor 2—10 hp, 460V, three-phase = 14A FLC

(a) 8 AWG, 70A breaker            (b) 8 AWG, 80A breaker (c) 8 AWG, 90A breaker         (d) 10 AWG, 90A breaker

Answer: (b) 8 AWG, 80A breaker

Step 1: Determine the feeder conductor size [430.24]:

(27A x 1.25) + 14A = 48A

8 AWG rated 50A at 75°C [110.14(C)(1) and Table 310.15(B)(16)]

Step 2: Feeder protection [430.62(A)] isn’t greater than the largest branch-circuit ground-fault and short-circuit protective device plus the other motor FLC.

Step 3: Determine the largest branch-circuit ground-fault and short-circuit protective device [430.52(C)(1) Ex]:

20 hp Motor = 27A x 2.50 = 68, next size up = 70A

10 hp Motor = 14A x 2.50 = 35A

Step 4: Determine the size feeder protection:

Not more than 70A + 14A, = 84A, next size down = 80A [240.6(A)]

Author’s Comment:

The “next size up protection” rule for branch circuits [430.52(C)(1) Ex 1] doesn’t apply to a motor feeder protection device rating.

 

 

Q8. How do I size the equipment grounding conductor to a motor branch circuit?

A8.  Equipment grounding conductors of the wire type must be sized not smaller than shown in Table 250.122, based on the rating of the circuit overcurrent device; however, the circuit equipment grounding conductor isn’t required to be larger than the circuit conductors [250.122(A)].

 

Table 250.122 Sizing Equipment Grounding Conductor

 

Overcurrent Device Rating

Copper Conductor

15A

14 AWG

20A

12 AWG

25A—60A

10 AWG

70A—100A

8 AWG

110A—200A

6 AWG

225A—300A

4 AWG

350A—400A

3 AWG

450A—500A

2 AWG

600A

1 AWG

700A—800A

1/0 AWG

1,000A

2/0 AWG

1,200A

3/0 AWG

 

250.122 (D)(1) specifically addresses sizing the equipment grounding conductor for motor branch circuits.

       Question: What size equipment grounding conductor is required for a 2 hp, 230V, single-phase motor?

(a) 14 AWG       (b) 12 AWG      (c) 10 AWG     (d) 8 AWG

Answer: (a) 14 AWG

Step 1: Determine the branch-circuit conductor size [430.22(A) and Table 310.15(B)(16)]

2 hp, 230V Motor FLC = 12A [Table 430.248]

12A x 1.25 = 15A, 14 AWG, rated 20A at 75°C [Table 310.15(B)(16)]

Step 2: Determine the branch-circuit protection [240.6(A), 430.52(C)(1), and Table 430.248]

12A x 2.50 = 30A

Step 3: The circuit equipment grounding conductor must be sized to the 30A overcurrent device—10 AWG [Table 250.122], but it’s not required to be sized larger than the circuit conductors—14 AWG [250.122(A)].

 

Q9. What is the Code requirement for the rating of a motor controller?

A9. The controller must have one of the following ratings [430.83(A)]:

(1) Horsepower Rating. Controllers, other than circuit breakers and molded case switches, must have a horsepower rating not less than that of the motor.

(2) Circuit Breakers. A circuit breaker can serve as a motor controller [430.111].

Author’s Comment:

Circuit breakers aren’t required to be horsepower rated.

(3) Molded Case Switch. A molded case switch, rated in amperes, can serve as a motor controller.

Author’s Comment:

A molded case switch isn’t required to be horsepower rated.

430.83(C) Stationary Motors of Two Horsepower or Less. For stationary motors rated at 2 hp or less, the controller can be:

(2) General-Use Snap Switch. A general-use alternating-current snap switch, where the motor full-load current rating isn’t more than 80 percent of the ampere rating of the switch.

Author’s Comment:

A general-use snap switch is a general-use switch constructed for installation in device boxes or on box covers [Article 100].

 

 

Q10. What are the Code requirements for motor disconnects?

A10. A disconnecting means is required for each motor controller, and it must be located within sight from the controller [430.102(A)].

Author’s Comment:

According to Article 100, within sight means that it’s visible and not more than 50 ft from one to the other.

A motor disconnect must be provided in accordance with 430.102(B)(1) or (B)(2).

(1) Separate Motor Disconnect. A disconnecting means is required for each motor, and it must be located in sight from the motor location and the driven machinery location.

(2) Controller Disconnect. The controller disconnecting means [430.102(A)] can serve as the disconnecting means for the motor, if the disconnect is located in sight from the motor location.

Ex to (1) and (2): A motor disconnecting means isn’t required under either condition (a) or (b), if the controller disconnecting means [430.102(A)] is lockable, as described in 110.25.

(a) If locating the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.

(b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment.

Informational Note 2: For information on lockout/tagout procedures, see NFPA 70E, Standard for Electrical Safety in the Workplace.

     

 

 

 

 

 

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Comments
  • In an open manufacturing (electronics assembly) area what is the correct method to power benches. Each bench has a power strip to operate several low amperage items ie. computer, display, small torque wrench, a hand soldering iron and maybe a small personal fan. Are drops to be supplied to each bench, can several power strips be assigned to a drop? Or should the bench be connected by a twist lock and to the drop?

    Dave G  January 28 2016, 8:46 am EST

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