This article was posted 12/07/2012 and is most likely outdated.

Mike Holt - Motors and the NEC [EC&M]
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Motors and the NEC - Based on the 2011 NEC
Based on - NEC - 2011 Edition

Motors and the NEC – Based on the 2011 NEC

By Mike Holt for EC&M

Article 430 provides the requirements for electric motors. Article 440 provides amended and additional requirements for hermetic motors, such as those used in air-conditioning and refrigeration equipment.

Motor applications are complex because they’re inductive loads with a high-current demand at start-up. Because this inrush is typically six times the running current, overcurrent protection for motor applications must differ from that for other equipment.

You might be uncomfortable with some of the Article 430 allowances for overcurrent protection. But once you understand how motor protection works, you’ll understand why these allowances aren’t only safe, but necessary.

Table FLC versus Motor Nameplate Current Rating (FLA)
The motor full-load current ratings listed in Tables 430.247, 430.248, and 430.250 are used to determine:

  • Conductor ampacity [430.22].
  • Branch-circuit short-circuit and ground-fault overcurrent device size [430.52 and 430.62].
  • Ampere rating of disconnecting switches [430.110].

The table selected for this purpose depends on the type of motor being used. The correct FLC is selected from:

  • Table 430.247 Direct-Current Motors.
  • Table 430.248 Single-Phase Motors.
  • Table 430.250 Three-Phase Motors.

However, for some specific types of motors, the actual nameplate Full-Load Amperes (FLA) must be used instead of the Full-Load Current (FLC) (see Sidebar 1):
Use the nameplate FLA when sizing conductors, branch-circuit short-circuit and ground fault protection, and disconnecting switches for:

  • Motors built to operate under 1,200 RPM.
  • Motors that have high torques (and thus higher FLCs).
  • Multispeed motors (FLC varies with speed).
  • A listed motor-operated appliance.

Also use the nameplate FLA for sizing of separate motor overload protection [430.6(A)(2)].

Conductor size
If for a single motor, size conductors at least 125% 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 75°C branch-circuit conductor is required for a 7½ hp, 230V, three-phase motor?
(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 30A 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)]

Circuit conductors that supply several motors must not be smaller than the minimum ampacity found by adding [430.24]:

  • 125 percent of the FLC of the highest rated motor.
  • The FLCs of other motors.

For this purpose, the highest rated motor is the one with the highest FLC [430.17].

Question: What size 75°C 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.

Taps
For motor circuit conductors tapped from a feeder [430.28], determine the ampacity per 430.22. The tap conductors must terminate in a branch-circuit short-circuit and ground-fault protection device sized per 430.52.
Motor feeder tap conductors must have an ampacity that’s at least:

  • One-tenth the rating of the feeder protection device, if not over 10 ft.
  • One-third the ampacity of the feeder conductor, if over 10 ft but not over 25 ft.
  • As large as the feeder conductor ampacity.

Overload protection
A fault, such as a short circuit or ground fault, isn’t an overload [Article 100]. Overload is the operation of equipment in excess of its current rating, or where current is in excess of the conductor ampacity.

Sustained overload will dangerously overheat the equipment or even destroy it. So we want to protect motors, motor control equipment, and motor branch-circuit conductors against excessive heating from overload. You must install an overload protection device for each ungrounded conductor [430.37].

Motor circuit overload protection requirements are in Article 430, Part III. These are for overload and failure-to-start protection only. Overload protection isn’t required if it might introduce additional or increased hazards, as in the case of fire pumps (see 695.7).

Because of the difference between starting and running current (see Sidebar 2), overcurrent protection for motors differs from that of other circuits. Generally, the motor overload device is separate from the short-circuit and ground-fault protection device (Article 430 Part IV).

Overload devices come in many configurations. They can be conventional “heaters”or electronic. You can use a fuse sized per 430.32. If you use fuses for overload protection, provide one for each ungrounded conductor [430.36].

You can use a single overcurrent device to protect a motor against overload, short circuit, and ground fault [430.55]. But you must size it to the overload requirements in 430.32.

Continuous duty
Motors rated more than 1 hp, used in a continuous-duty application without integral thermal protection, must have an overload device sized to open at no more than 115% of the motor nameplate FLC rating [430.32(A)(1)]

But size the overload device no more than 125% of the nameplate FLC if:

  • The nameplate service factor (SF) is 1.15 or more.
  • The nameplate temperature rise is 40°C or less.

Branch-circuit short-circuit and ground-fault protection
A branch-circuit short-circuit and ground-fault protective device protects the motor, the motor control equipment, and the conductors against short circuits or ground faults. It does not protect against overload [430.51].

The motor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the motor’s starting current [430.52(B)]. Install a branch-circuit short-circuit and ground-fault protective device on each motor circuit, and ensure that it is sized no greater than the percentages listed in Table 430.52.

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

Overcurrent protection for motors is different than protection for other types of electrical loads, and the values you come up with might not seem right based on your experience with other types of applications. Protecting a 14 AWG conductor with a 30A circuit breaker, for example, just looks wrong. But keep in mind that motor branch-circuit conductors are protected against overloads by the overload device. That device is sized between 115% and 125% 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).

Feeder protection
You need to protect motor feeder conductors against short circuits and ground faults. But how do you size the protective device to handle this job?

First, determine which motor on the feeder has the largest rated branch-circuit short-circuit and ground-fault protective device. Then add up the FLCs of the other motors in the group. Thirdly, add that sum to the device rating from the first step [430.62(A)].

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. So you may need to round down to the protection device that “does not exceed” that calculated value.

An example problem helps illustrate this. What size feeder protection (inverse time breakers with 75°C terminals) and 75°C conductors do you need 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

Here’s the solution:

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)].
Answer: (b) 8 AWG, 80A breaker

Article 430 Road map
Compared to other NEC articles, Article 430 is long and complex. But it provides a road map, which is Figure 430.1 in the NEC. This reference shows the major steps from start to finish. It also shows the applicable Part for each step.

Now that you have a good overview of Article 430 requirements, you can combine that knowledge with the “map” of Figure 430.1 to boost your efficiency and accuracy.

Sidebar 1: FLA vs. FLC
The nameplate Full-Load Ampere (FLA) rating is the current the motor draws while producing its rated horsepower load at its rated voltage, based on its rated efficiency and power factor.

The current the motor actually draws depends upon the actual voltage at the motor terminals and upon the load the motor is trying to drive.The current increases if the load increases or if the voltage decreases.

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.

The Full-Load Current (FLC) tables are at the end of Article 430. In general terms, the permanent wiring installed for motor circuits is sized based on these tables, which are intended to be large enough for any motor design of a particular horsepower rating.

Sidebar 2: Starting, running, and locked
A motor draws significantly more current when starting than when running. It draws even more in a locked-rotor condition.

  • Starting Current. When voltage is first applied to the field winding of an induction motor, only the conductor resistance opposes the flow of current through the winding. Because conductor resistance is very low, the motor has a large inrush current.
  • Running Current. Once the rotor begins turning, the increase in counter-electromotive force (CEMF) reduces the starting current to running current
  • Locked-Rotor Current (LRC). If the rotating part of the motor winding (armature) can’t rotate (for instance, due to a jam), the winding produces no CEMF. Consequently, conductor impedance decreases until it’s effectively a short circuit. The motor operates at LRC, often six times the full-load ampere rating, depending on the motor code letter rating [430.7(B)]. The resulting over-heating of the motor winding will destroy the winding if the current isn’t quickly reduced or removed.

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Taken from Mike Holt's 2011 Understanding the NEC® Volume 1 Textbook. To order your copy please click here, or call 888-632-2633.

 

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Comments
  • Trying to understand this one stepat a time .... If the question ("Question: What size 75°C feeder conductor is required for two 7½ hp, 230V, three-phase motors, if the terminals are rated for 75°C?") remained as worded, but Instantaneous Trip Breakers were used would the following be correct? Conductor size would remain unchanged (#8AWG). Branch Circuit OCPD calulated: 22A x 8.00 (from T430.52)= 176A .... round up to 200A Feeder OCPD calculated: 200A + 176A = 376A .... round DOWN to 350A Also - is there a way that a line diagram indicating the various parts could be provided/included ... I think I'm having trouble differentiating the branch circuit from the feeder. In my mind ... I'm envisioning the Feeder OCPD as something like a fused switch at a Main Distribution Panel and the Branch-circuit OCPD as a fused disconnect.

    Ron Elliott  March 4 2013, 2:37 pm EST
    Reply to this comment

  • For equipment rated 100A or less would the terminations by UL be listed for only 60C thereby requiring us to size the branch circut conductors based on 60C ampacity tables and not 75C?

    Clay Calhoun  December 7 2012, 6:28 pm EST
    Reply to this comment

  • Thank you very mush for sharing this information in article 430

    gustavo  December 7 2012, 4:17 pm EST
    Reply to this comment


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