PART 1 NEC Motor Installation Requirements
Understanding the rules detailed in the National Electrical Code (NEC) is critical to the proper installation of motor control circuits. NEC Article 430 covers application and installation of motor circuits including conductors, short-circuit and ground fault protection, starters, disconnects, and overload protection. Figure 8-1 illustrates the basic elements of a motor branch circuit that the NEC addresses. A motor branch circuit includes the final overcurrent device (disconnect switch and fuses or circuit breaker), the motor starter and associated control circuits, circuit conductors, and the motor.
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PART 1 NEC Motor Installation Requirements 181 Sizing Motor Branch Circuit
Conductors
Installation requirements for motor branch circuit conduc- tors are outlined in NEC Article 430, Part II . Generally, motor branch circuit conductors that supply a single motor used in a continuous-duty application must have an ampacity of not less than 125 percent of the motor’s full-load current (FLC) rating as determined by Article 430.6. This provision is based on the need to provide for a sustained running cur- rent that is greater than the rated full-load current and for protection of the conductors by the motor overload protec- tive device set above the full-load current rating.
The full-load current rating shown on the motor name- plate is not permitted to be used to determine the ampac- ity of the conductors, the ampacity of switches, or motor branch-circuit short-circuit and ground fault protection.
The reasons for this are:
• The supply voltage normally varies from the voltage rating of the motor, and the current varies with the voltage applied.
• The actual full-load current rating for motors of the same horsepower may vary, and requiring the use of NEC tables ensures that if a motor must be replaced, this can be safely done without having to make changes to other component parts of the circuit.
Conductor ampacity must be determined by NEC Tables 430.247 through 430.250 and is based on the motor nameplate horsepower rating and voltage. Overload pro- tection, however, is based on the marked motor nameplate rating. Use of the term full-load current (FLC) rating
indicates the table rating while use of the term full-load ampere (FLA) rating indicates the actual nameplate rating. This makes it easier to clarify whether the table ampacity or the nameplate ampacity is being used.
Protecting against overloads Selecting a motor controller Protecting against grounds and shorts
Sizing branch circuit conductors Providing a control circuit Control circuit protection Providing a disconnect
Motor
Figure 8-1 Basic elements of a motor branch circuit that the NEC addresses.
E X A M P L E 8 - 1
Problem:
Using your edition of the NEC, determine the mini- mum branch-circuit conductor ampacity required for each of the following motors:
a. 2-hp, 230-V, single-phase motor
b. 30-hp, 230-V, three-phase motor with a name- plate FLA rating of 70 A
Solution:
a. NEC Table 430-248 shows the FLC as 12 A.
Therefore, the conductor ampacity required is 12 × 125% = 15 A.
b. NEC Table 430.250 shows the FLC as 80 A.
Therefore, the conductor ampacity required is 80 × 125% = 100 A.
Feeder conductors supplying two or more motors must have an ampacity not less than 125 percent of the full-load current rating of the highest-rated motor plus the sum of the full-load current ratings of the other motors supplied. When two or more motors of equal rating are on a feeder, one of the motors will be considered the largest and calculated at 125 percent, and the others will be added at 100 percent.
Once the required ampacity of the conductor has been determined, NEC Table 310.16 can be used to determine the American Wire Gauge (AWG) or thousand circular mil (kcmil) size of conductor required. NEC Table 310.16 applies to situations where you have three or less current-carrying conductors in a single wireway. You must select from the col- umn that shows the cable (identified by the insulating mate- rial letter designation) you intend to use, and choose between copper and aluminum. Bear in mind that all calculated con- ductor sizes based on ampacity are minimum, taking into consideration temperature rise only. The calculations do not take into account voltage dip during motor starting or volt- age drop during motor running. Such considerations often require increasing the size of the branch circuit conductors.
Branch Circuit Motor Protection
Overcurrent protection for motors and motor circuits is a lit- tle different from that for nonmotor loads. The most common method for providing overcurrent protection for nonmotor loads is to use a circuit breaker that combines overcurrent
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182 Chapter 8 Motor Control Circuits
Motor control circuit overcurrent protection
Motor branch circuit, short circuit and ground fault protection device
Motor overload protection device
Integral thermal protector device
To motor control circuit (if used) L1
CB M OL
L2 L3
Stop OL
M Start
M Motor X2
X1
Figure 8-3 Motor branch circuit protection.
protection with short-circuit and ground fault protection.
However, this isn’t usually the best choice for motors because they draw a large amount of current at initial start-up, usu- ally around 6 times the normal full-load current of the motor.
E X A M P L E 8 - 2
Problem:
Three 460-V, three-phase motors rated at 50, 30, and 10 hp share the same feeder (Figure 8-2). Using your edition of the NEC, determine the ampacity required to size the feeder conductors.
Solution:
50-hp motor—NEC Table 430.250 shows the FLC as 65 A.
400-V, three-phase feeder conductors Motor
starter panel
50 hp
30 hp
10 hp
Figure 8-2 Circuit for Example 8-2.
30-hp motor—NEC Table 430.250 shows the FLC as 40 A.
10-hp motor—NEC Table 430.250 shows the FLC as 14 A.
Therefore, the required ampacity of the feeder conduc- tors is (1.25)(65) + 40 +14 = 135.25 A.
With rare exceptions, the best method for providing overcur- rent protection for motors is to separate the overload protec- tion devices from the short-circuit and ground fault protection devices, as illustrated in Figure 8-3. Motor overload protection
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PART 1 NEC Motor Installation Requirements 183 rapidly in the fuse. Non-time-delay fuses usually hold about 5 times their rating for approximately ẳ second, after which the current-carrying element melts. Time- delay fuses provide overload and short-circuit protection.
Time-delay fuses usually allow 5 times the rated current for up to 10 seconds to allow motors to start.
NEC Article 430, Part III deals with motor and branch circuit overload protection. A motor overload condition is caused by excessive load applied to the motor shaft. For example, when a saw is used, if the board is damp or the cut is too deep, the motor may become overloaded and slow down. The current flow in the windings will increase and heat the motor beyond its design temperature. A jammed pump or an extra-heavy load on a hoist will have the same effect on a motor. The overload protection also guards against failure of a motor to start from a locked rotor and loss of a phase on a three-phase system. Overload pro- tection is not designed to break or may not be capable of breaking short-circuit current or ground fault current.
Motors are required to have overload protection, either within the motor itself or somewhere in close proximity to the line side of the motor. This overload protection is actually protecting the motor, the conductors, and much of the circuit ahead of the overloads. An overload in the circuit will trip the circuit overload devices, thus protect- ing the circuit from overload conditions. In the majority of applications, overcurrent protection is provided by the overload relays in the motor controller. All three-phase motors, except those protected by other approved means, such as integral-type detectors, must be provided with three overload units, one in each phase.
The nameplate full-load ampere (FLA) rating of the motor, rather than NEC tables, is used for sizing the overloads for the motor. Using this data, tables supplied by the starter manufacturer are consulted to find the correct overload relay thermal heater unit for the particular overload relay in use.
A separate overload device that is responsive to motor cur- rent is required by NEC Article 430.32. This device shall be selected to trip or shall be rated at not more than 125 percent of the motor nameplate full-load current rating. When the thermal element selected in accordance with NEC 430.32 is not sufficient to start the motor or to carry the load, a higher- size thermal element can be used, provided the trip current of the overload relay does not exceed 140 percent of the motor nameplate full-load current rating.
Motor control circuits carry the current that controls the operation of the controller, but do not carry the main power current to the motor. These circuits are permitted to be tapped from the motor branch circuit conductors or supplied from a separate source. NEC 430.72 deals with overcurrent protection of motor control circuits tapped from the motor branch circuit and NEC 725.23 is used for those from other sources of power. Where a lower voltage devices like heaters and integral thermal devices protect the
motor, the motor control equipment, and the branch circuit conductors from motor overload and the resultant excessive heating. They don’t provide protection against short-circuit or ground fault currents. That’s the job of the branch and feeder breakers. This arrangement makes motor calculations different from those used for other types of loads.
NEC Article 430, Part IV explains the requirements for branch circuit s hort-circuit and ground fault protection . The NEC requires that branch circuit protection for motor cir- cuits must protect the circuit conductors, the control appa- ratus, as well as the motor itself against overcurrent due to short circuits or ground faults. The protection device (circuit breaker or fuse) for an individual branch circuit to a motor must be capable of carrying the starting current of the motor without opening the circuit. The Code places maximum val- ues on the ratings or setting of these devices, as found in Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 must be used. In cases where the val- ues do not correspond to the standard sizes of fuses, to the ratings of nonadjustable circuit breakers, or to possible set- tings of adjustable circuit breakers, and the next lower value is not adequate to carry the motor load, the next higher size, rating, or setting may be used. The standard sizes of fuses and breakers are listed in NEC Article 240.6.
An instantaneous trip circuit breaker responds to a predetermined value of overload without any purposely delayed action. Most circuit breakers have an inverse time-delay characteristic. With an inverse time circuit breaker the higher the overcurrent, the shorter the time required for the breaker to trip and open the circuit.
Non-time-delay fuses provide excellent short-circuit protection. When an overcurrent occurs, heat builds up
E X A M P L E 8 - 3
Problem:
Determine the size of inverse time circuit breaker per- mitted to be used to provide motor branch circuit short- circuit and ground fault protection for a 10-hp, 208-V, three-phase squirrel-cage motor.
Solution:
NEC Table 430.250 shows the motor FLC as 30.8 A.
NEC Table 430.52 shows the maximum rating for an inverse time breaker as 250 percent of the FLC.
30.8 × 2.5 = 77 A
Because this is not a standard size, 80 A may be used if a 70-A inverse time circuit breaker is not adequate.
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184 Chapter 8 Motor Control Circuits
• A controller must be capable of starting and stop- ping the motor it controls as well as able to interrupt the locked-rotor current of the motor.
• Unless an inverse-time circuit breaker or molded case switch is used, controllers must have horse- power rating at the applied voltage not lower than the horsepower rating of the motor.
• For stationary motors rated at 2 hp or less and 300 V or less, the controller can be either of the following:
1. A general-use switch having an ampere rating not less than twice the full-load current rating of the motor.
2. On AC circuits, a general-use snap switch suit- able only for use on AC (not general use AC/DC snap switches) where the full-load current rating is not more than 80 percent of the amperage rat- ing of the switch.
• A controller that does not also serve as a disconnecting means must open only as many motor circuit conduc- tors as may be necessary to stop the motor, that is, one conductor for DC or single-phase motor circuits and two conductors for a three-phase motor circuit.
• Individual controllers must be provided for each motor unless the motor is under 600 V, or there is a single machine with several motors, or a single overcurrent device, or a group of motors located in a single room within sight of the controller.
Disconnecting Means for Motor and Controller
The ability to safely work on a motor, a motor controller, or any motor-driven machinery starts with being able to turn the power off to the motor and its related equipment.
NEC Article 430, Part IX covers the requirements for the motor disconnecting means. The Code requires that a means (a motor circuit switch rated in horsepower or a circuit breaker) must be provided in each motor circuit to discon- nect both the motor and its controller from all ungrounded supply conductors. All disconnecting switches must plainly indicate whether they are open (off) or closed (on) and no pole is allowed to operate independently. Separate discon- nects and controllers may be mounted on the same panel or contained in the same enclosure, such as a combination fused-switch, magnetic starter unit (Figure 8-5).
The disconnecting means other than the branch circuit short-circuit and ground fault protective device is used as a safety switch to disconnect the motor circuit. It should be in sight of the motor and can be unfused. If a person is work- ing on the motor, the disconnect will be where he or she can see it; that protects the person from a motor accidentally is desired, a control transformer may be installed in either
of the two supply methods used.
Selecting a Motor Controller
A motor controller is any device that is used to directly start and stop an electric motor by closing and open- ing the main power current to the motor. The controller can be a switch, starter, or other similar type of control device. Examples of motor controllers are illustrated in Figure 8-4. A magnetic starter consisting of a contac- tor and overload relay is considered to be a controller.
A properly rated snap switch that is permitted to turn a single-phase motor on and off is also considered to be a motor controller. Snap switches are permitted to serve as the motor controller as well as the disconnecting means.
A solid-state starter or an AC or DC drive is also classi- fied as a motor controller. With solid-state controllers, it is the power-circuit element, such as a triac or SCR, that meets the definition of controller.
The rating of the motor controller or starter is directly related to its NEMA size, with the electrical ratings of each being provided by the manufacturer’s data sheets.
A controller enclosure must be marked with the manufac- turer’s identification, the voltage rating, the current rating, or the horsepower rating. NEC Article 430, Part VII details the requirements for motor controllers. The follow- ing are some of the highlights of this section.
• The branch circuit and ground fault device can be used as the controller for stationary motors of 1/8 hp or less that are not normally left running and cannot be damaged by overload or failure to start.
A good example of this would be a clock motor.
• An attachment plug and receptacle can be used as a controller for a portable motor of 1/3 hp or less.
Magnetic starter Manual snap switch Adjustable-speed drive
Figure 8-4 Examples of motor controllers.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
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