**Motors**

The main purpose of a motor is to convert electrical energy to mechanical energy. Motors are unique as compared to other types of electrical equipment. Light bulbs, stoves, and ovens for instance, are classified as resistive loads. They commonly draw a very stable continuous current. Motors, on the other hand, are classified as inductive loads due to their use of coils of copper wire called windings. Windings utilize electromagnetism to produce mechanical rotation of a shaft. Remember, in electrical theory, whenever a copper wire is wound into a coil it becomes an inductor and produces what we know as inductance in a circuit.

What also separates motors from other types of loads is that they draw a lot of current, called surge or in-rush, at start-up. It seems to take a lot of work just to get the wheels spinning in a motor. The term for this surge at start-up is called Locked Rotor Current (LRC). The current in-rush at start-up can reach as much as six to eight times the normal running current, or Full Load Current (FLC), of a typical motor. Because of this we have to build a delay into the motor circuit protection. We use special “Dual-Element Time Delay Fuses” and “Inverse-Time Circuit Breakers” to solve this problem.

Another problem unique to motors is overheating due to freeze-up or failure to start. To protect motors we need special devices called overloads (heaters), or thermal protective relays. Keep in mind that branch circuit protection (breaker or fuse) safeguards only the branch circuit conductors. An overload, or thermal device is needed just to protect the motor itself.

The following requirements cover provisions for motors, motor branch circuits and feeders...

What also separates motors from other types of loads is that they draw a lot of current, called surge or in-rush, at start-up. It seems to take a lot of work just to get the wheels spinning in a motor. The term for this surge at start-up is called Locked Rotor Current (LRC). The current in-rush at start-up can reach as much as six to eight times the normal running current, or Full Load Current (FLC), of a typical motor. Because of this we have to build a delay into the motor circuit protection. We use special “Dual-Element Time Delay Fuses” and “Inverse-Time Circuit Breakers” to solve this problem.

Another problem unique to motors is overheating due to freeze-up or failure to start. To protect motors we need special devices called overloads (heaters), or thermal protective relays. Keep in mind that branch circuit protection (breaker or fuse) safeguards only the branch circuit conductors. An overload, or thermal device is needed just to protect the motor itself.

The following requirements cover provisions for motors, motor branch circuits and feeders...

Full-Load Current

Motors, as you may know, are rated in horsepower. I know it’s old fashioned but motors nowadays do the same work as horses used to do. Some traditions die hard and horsepower is one, fortunately, we’ve learned to convert horsepower to amps. In the old days one horsepower was equal to about 746 watts, but that was a rough guess. The Code has tables set up to accurately convert horsepower to what’s called “Full Load Current (FLC).” These tables take into account inefficiencies inherent in motors such as what’s called “power-factor losses.”

Table 430.247 converts Direct-Current (D.C.) motors rated 90, 120, 180, 240, 500 and 550 volts to Full-Load Current in amps. These values of full-load currents are for motors running at base (normal) speeds.

Table 430.248 converts Single-Phase Alternating-Currents (A.C.) motors rated 115, 200, 208 and 230 volts to Full-Load Current (FLC) in amps. These values of full-load currents are for motors running at usual speeds and motors with normal torque characteristics.

Table 430.247 converts Direct-Current (D.C.) motors rated 90, 120, 180, 240, 500 and 550 volts to Full-Load Current in amps. These values of full-load currents are for motors running at base (normal) speeds.

Table 430.248 converts Single-Phase Alternating-Currents (A.C.) motors rated 115, 200, 208 and 230 volts to Full-Load Current (FLC) in amps. These values of full-load currents are for motors running at usual speeds and motors with normal torque characteristics.

Table 430.250 converts three-phase alternating-currents (A.C.) motors rated 115 to 2,300 volts to Full-Load Current (FLC) in amps. These values of full-load currents are typical for motors running at speeds usual for belted motors and motors with normal torque characteristics.

For example: The Full-Load Current (FLC) of a 3/4 horsepower, single-phase, 115 volt motor according to Table 430.248 would be

13.8 amps.

For example: The Full-Load Current (FLC) of a 3/4 horsepower, single-phase, 115 volt motor according to Table 430.248 would be

13.8 amps.

Locked-Rotor Current For Motors

Locked-Rotor surge at start-up takes into account the in-rush current of a motor at start- up.You can use Table 430.251 to find the locked-rotor current for most general purpose motors. Table 430.251(A) is for single-phase motors. The rule of thumb for single-phase motors is six times (6 x) the F.L.C. from Table 430.248. Table 430.251(B) is for three-phase motors. Design Letters “B, C, and D” are efficiency ratings. Since 1991 all motors, installed or replaced, must be energy efficient. Design Letter “D” motors are the highest efficiency rating.

For example: The Locked-Rotor Current (LRC) of a 3/4 horsepower, single-phase, 115 volt motor according to Table 430.251(A) would be 82.8 amps (13.8 x 6).

For example: The Locked-Rotor Current (LRC) of a 3/4 horsepower, single-phase, 115 volt motor according to Table 430.251(A) would be 82.8 amps (13.8 x 6).

Motor Running Overload Protection

Each continuous-duty motor rated more than one horsepower shall be protected against overload by one of the following means: A separate overload device that is responsive to motor current. This device shall be selected to trip or rated at no more than the following percent of the motor nameplate full-load current rating.

(1) Overload Protection (Minimum) = Motor FLC x 430.32(A)(1) Percentage

Motors with a marked service factor 1.15 and up 125%

Motors with a marked temperature rise 40 ̊C. and down 125%

All other motors 115%

Where the overload relay selected (in accordance with 430.32(A)(1) and 430.32(B)(1) is not sufficient to start the motor or carry the load, the next higher size overload relay shall be permitted to be used (provided the trip current of the overload relay does not exceed the following percentage of motor nameplate full-load current rating).

(2) Overload Protection (Maximum) = Motor FLC x 430.32(C) Percentage

Motors with marked service factor 1.15 and up 140%

Motors with a marked temperature rise 40 ̊C. and down 140%

All other motors 130%

Example: The minimum overload protection for a 1 horsepower, single-phase, 115 volt motor is (16 amps x 115% = 18.4 amps). The maximum overload protection is (16 amps x 125% = 20 amps).

(1) Overload Protection (Minimum) = Motor FLC x 430.32(A)(1) Percentage

Motors with a marked service factor 1.15 and up 125%

Motors with a marked temperature rise 40 ̊C. and down 125%

All other motors 115%

Where the overload relay selected (in accordance with 430.32(A)(1) and 430.32(B)(1) is not sufficient to start the motor or carry the load, the next higher size overload relay shall be permitted to be used (provided the trip current of the overload relay does not exceed the following percentage of motor nameplate full-load current rating).

(2) Overload Protection (Maximum) = Motor FLC x 430.32(C) Percentage

Motors with marked service factor 1.15 and up 140%

Motors with a marked temperature rise 40 ̊C. and down 140%

All other motors 130%

Example: The minimum overload protection for a 1 horsepower, single-phase, 115 volt motor is (16 amps x 115% = 18.4 amps). The maximum overload protection is (16 amps x 125% = 20 amps).

Thermal Protection

A Thermal Protector is approved for use (with the motor it protects) on the basis that it will prevent dangerous overheating of the motor due to overload and failure to start. The ultimate trip current of a thermally protected motor shall not exceed the following percentage of motor full-load current given:

(3) Thermal Protection = Motor Full Load Current x 430.32(A)(2) Percentage

Motor full-load current 9 amps and under 170%

Motor full-load current 9.1 to and including 20 amps 156%

Motor full-load current 20 amps or more 140%

Example: Thermal protection for a 1 horsepower, single-phase, 115 volt motor is (16 amps x 156% = 24.96 amps).

(3) Thermal Protection = Motor Full Load Current x 430.32(A)(2) Percentage

Motor full-load current 9 amps and under 170%

Motor full-load current 9.1 to and including 20 amps 156%

Motor full-load current 20 amps or more 140%

Example: Thermal protection for a 1 horsepower, single-phase, 115 volt motor is (16 amps x 156% = 24.96 amps).

Motor Branch Circuit Conductor Amp Rating

Branch-circuit conductors supplying a single motor shall have an ampacity not less than 125% percent of the motor full-load current rating.

Branch Circuit Conductor Amp Rating = FLC x 125%

Note- The branch circuit conductor sizes based on Table 310.15(B)(16) for motor conductors are not limited by the asterisk (*) overcurrent protection limitations. Section 240.4(G) and Table 240.4(G) exempts the ampacity limitations of conductor sizes #14, #12 and #10. For instance; a #10 THW conductor for a motor can be used up to its 35 amp rating

Note- The branch circuit conductor sizes based on Table 310.15(B)(16) for motor conductors are not limited by the asterisk (*) overcurrent protection limitations. Section 240.4(G) and Table 240.4(G) exempts the ampacity limitations of conductor sizes #14, #12 and #10. For instance; a #10 THW conductor for a motor can be used up to its 35 amp rating

Motor Branch Circuit Protection

The motor branch-circuit, short-circuit, and ground-fault protective device shall be capable of carrying the starting current of the motor. The required protection shall be considered as being obtained, where the protective device has a rating or setting not exceeding the values given in Table 430.52.

430-52 exc.1: Where the values for branch-circuit, short-circuit, and ground-fault protective devices (determined by Table 430.52) do not correspond to the standard sizes or ratings of fuses (non-adjustable circuit breakers or thermal protective devices), the next higher size rating (or setting) shall be permitted. Standard amp ratings for fuses and circuit breakers are located in 240-6(A).

Branch-Circuit Protection Amp Rating = FLC x Table 430.52 Percentage

430-52 exc.1: Where the values for branch-circuit, short-circuit, and ground-fault protective devices (determined by Table 430.52) do not correspond to the standard sizes or ratings of fuses (non-adjustable circuit breakers or thermal protective devices), the next higher size rating (or setting) shall be permitted. Standard amp ratings for fuses and circuit breakers are located in 240-6(A).

Branch-Circuit Protection Amp Rating = FLC x Table 430.52 Percentage

Example: The minimum overload protection (breaker) for a 1 horsepower, single-phase, 115 volt motor is (16 amps x 250% = 40 amps) (240.6(A)).

Feeder Size

*Article 430.24: Conductors supplying two or more motors shall have an ampacity not less than 125% of the full-load current rating (FLC) of the highest rated motor in the group, plus the sum of the full-load currents of all of the other motors. Look for a conductor in Table 310.15(B)(16) based on 75 ̊C, which is the minimum rating for motors.*

Feeder Size = All Motor FLC's + 25% Of The Largest Motor FLC

Example: The feeder size for two motors, one 3/4 horsepower and one 1 horsepower (both single-phase, 115 volt) is (16 amps (+ 25%) + 13.8 amps = 33.8 amps). Use #10 THW (Table 310.15(B)(16)).

Feeder Protection

Feeders supplying several motor loads shall be provided with a protective device having a rating (or setting) not greater than the largest rating (or setting) of the branch-circuit, short- circuit, and ground-fault protective device for any motor of the group (based on Table 430.52), plus the sum of the full-load currents of all of the other motors of the group.

Whichever motor has the largest fuse, or breaker, is added to the full-load currents of all of the other motors of the group. Never add the full-load current of the motor with the largest branch circuit protective device though. Standard amp ratings for fuses and circuit breakers are located in Article 240.6(A). Remember that he total ampacity of all the motors cannot exceed the breaker, or fuse, rating.

Whichever motor has the largest fuse, or breaker, is added to the full-load currents of all of the other motors of the group. Never add the full-load current of the motor with the largest branch circuit protective device though. Standard amp ratings for fuses and circuit breakers are located in Article 240.6(A). Remember that he total ampacity of all the motors cannot exceed the breaker, or fuse, rating.

Feeder Protection = Largest Branch Circuit Protective Device + Sum Of All Other FLC's

Example: The feeder protection for two motors, one 3/4 horsepower and one 1 horsepower (both single-phase, 115 volt) is (40 amps (16 amps x 250%) + 16 amps = 53.8 amps). Use a 50 amp breaker. (240.6(A)) do not use next higher size.

Motor Sample Problem