Source: https://www.powerelectronics.com/content/applying-new-1996-nec-rules-motor-circuits
Timestamp: 2019-11-20 07:19:19
Document Index: 195590582

Matched Legal Cases: ['Art. 430', 'Art. 100', 'Art. 430', 'Art. 430', 'Art. 240', 'Art. 430', 'Art. 430']

Applying the new 1996 NEC rules for motor circuits | Power Electronics
Hartwell, Frederic P. | Mar 01, 1999
Fundamental changes are on tap for motor circuits as provisions, largely unaltered since the 1940 NEC, are drastically modified or deleted entirely.
Motor circuits have always had a special place in the Code because they are, by far, the most important example of a circuit where the short-circuit and ground-fault protection usually occurs at a different point in the circuit, and uses a different device than the overload protection. This article focuses on how we will be applying the far-reaching changes that have been made in Art. 430 for 1996. To do this, we will look at the elements of the motor circuit from the motor back through its supply conductors, the devices that protect the motor, its conductors from overcurrent, and finally the controller and disconnecting means.
As motors become more energy-efficient by reducing internal resistance (and thereby wasted [I.sup.2]R), the side effect is increasing inrush current. This is because until the motor begins to turn, the only impedance is the winding resistance, and as that goes down, the current goes up. Motors have always had varying inrush currents, which is why, ever since the 1940 NEC, there have been "code letters" that expressed locked-rotor kVA per horsepower. Now the code letters are no longer the governing factor in judging elements of the motor circuit that depend on inrush values; they are being replaced by the NEMA design letter.
Code letters supplanted. To this end, Sec. 430-7(a)(8) no longer requires the code letter on the nameplates; instead the actual locked-rotor amperes can be used instead. Then, a new paragraph in Sec. 430-7(a)(9) requires the design letter to be marked on the nameplate for all Design B, C, D, or E motors. Completing these changes, the mandatory rule that used to be in Sec. 430-7(b) that required the use of code letters for determining the permitted short-circuit and ground-fault protection has been deleted [ILLUSTRATION FOR FIGURE 1 OMITTED].
Probably 80% of all motors sold in the U.S. are Design B, and these can be made quite energy efficient. The NEMA standard (MG-1) requires these motors to meet certain energy efficiencies if they are marked "energy efficient." For example, a 50-hp motor could be marked that way if it had a nominal efficiency rating of 93.0%, with a minimum efficiency of 91.7%.
By contrast, the Design E version of the same motor must have a nominal efficiency of 95.4 % and a minimum efficiency of 94.5 %, an appreciable improvement. In October of 1997, the federal energy efficiency legislation becomes effective and requires that replacement motors meet at least the minimum energy efficiency standards required for the "energy efficient" marking. Although this doesn't necessarily mean a Design E motor, such motors may often end up as the ones specified because of their greater efficiency. Be very careful, because often such motors cannot be used as horsepower-for-horsepower replacements. Even if the torque characteristics of the replacement motor will drive the load, as shown in subsequent portions of this article, the controller and the disconnect may not be able to handle the new motor.
Table 430-151 changed. A graphic illustration of the dramatic differences in inrush current appears in the new version of the locked-rotor current table, Table 430-151. This table is now two tables. Table 430-151A gives locked-rotor ampere values for single-phase motors; Table 430-151B gives the 3-phase locked-rotor currents in two columns per voltage, one for Design E, and one for Designs B, C, and D. Note the significant differences between Design E motors and the conventional motors, with Design E currents running close to 50% higher in many cases.
Terminal housings get larger. The energy efficiency isn't the only change we'll be seeing on motors; they'll also be easier to wire because the terminal housings (for wire-to-wire connections on AC motors) are slated to increase in size by 40%. All of the minimum dimensions in Table 430-12(b), both with regard to dimensions and volumes in the AC portions of the table, have been increased.
The basic rules for sizing motor circuit conductors, generally based on a minimum of 125% of the full-load current of the motor, haven't changed. The idea is to roughly coordinate the wire size and the running overload protection, which allows the running overload protection to protect both the motor and the branch-circuit conductors from damage due to overloads. Nevertheless, there are important changes that apply to conductors involved with adjustable speed drives and wye-delta starters.
Adjustable speed drive conductors. With a new mandatory Ex. 3 to Sec. 430-22(a), adjustable speed drive conductors must now be sized at 125% of the rated input to the drive. Prior to this change, the only Code language on this topic was the general requirement in Sec. 430-2 that the "incoming branch circuit...shall be based on the rated input..." The usual inference was to size the conductors at 100% of the rated input. Note that the Code is making a distinction between "based on" and the final result. The calculation is still "based on" the rating of the drive, but "based on" means that rating is the first step, and an additional 25% of that rating must be added.
Often adjustable speed drives provide overload protection for the motor (and its conductors). Sec. 430-2 has been changed to clarify that if this is the case, and the equipment is so marked, additional overload protection is unnecessary. In addition, a new FPN points out that electrical resonance can result from the nonsinusoidal currents that result from variable frequency drive loads interacting with power factor correction capacitors.
A resonant condition can cause extensive damage. Harmonic traps or other means may be necessary to prevent these problems, and it takes skilled design to specify the correct equipment.
Power conductor sizing in wye-delta applications. Wye-delta starting cuts the initial current to one-third the usual value, and is very useful on loads with long acceleration times or for loads with comparatively low starting torque requirements such as fans. It has been an important industrial mainstay for reduced-voltage starting because no special equipment is required to vary system voltages or frequencies; all that is required is a motor with all 12 (or six for a single voltage) leads brought out.
Another change in Sec. 430-22(a) spells out for the first time the minimum required conductor sizing between a wye-start/delta-run controller and the motor. These conductors must now be based on a minimum of 58% of the motor full-load current. These conductors each carry a portion of the full-load current (FLA [divided by] 1.73 = 0.58 x FLA). Note that the phrasing and sentence structure on the line and load side of the controller are very similar.
* Line side: "The selection...shall be based on the motor full-load current."
* Load side: "The selection...shall be based on 58 percent of the motor full-load current."
Here again, don't confuse the expression "based on" with the final result. In the case of the line side of the controller, the conductors will usually end up at 125% of the full-load current; however, if duty cycle service is involved, that percentage could vary greatly. For example, if the motor were rated continuous and used for intermittent duty, Table 430-22 (a) Exception requires a multiplier of 140%. Similarly, in the case of the conductors going to the motor, the usual result will be 73% of the full-load current (58% x 12.5 = 72.5%); but again, in the case of duty cycle service, that result will vary.
Overload is defined in Art. 100 as:
Running overload protection in motor circuits is effective at preventing damage to components of the motor circuit that results from sustained current that exceeds component ratings. It is useless for protecting a circuit from a short circuit or a ground fault. These failures lead to very high current values that can easily cause catastrophic damage in a fraction of a second, much faster than most running overload protection could possibly react.
In general, running overload protection is individualized to each motor, based on the full-load current marked on the nameplate. Even Sec. 430-6(a), which generally disallows nameplate current ratings information from being used for sizing elements of motor circuits, makes an exception for running overload protection. Otherwise, the tables at the end of the article must be used for determining conductor sizes, ground-fault and ranch-circuit protection ratings, disconnect ratings, etc; accordingly, Sec. 430-34 has been revised to correlate with this principle and now specifically refers to "motor nameplate full-load current rating."
Although we often provide this protection with fuses or inverse-time circuit breakers that could qualify as overcurrent protection (because they have both overload and short-circuit protective elements), for the purposes of Art. 430, these devices are short-circuit and ground-fault protective devices only. They follow the sizing rules in Sec. 430-52 and Table 430-152, even where those rules lead to settings several times the ampacity of the conductors. The basic procedure is to multiply the ampere rating of the motor as given in the tables at the end of Art. 430 and not on the nameplate by the percentage given in Table 430-152 for the type of overcurrent device being used. Note that the rated full-load currents of 3-phase AC motors 1 1/2 hp and below have been revised upward in Table 430-150. In addition, the range of the table has been extended to cover motors up to 500 hp.
Table 430-152. There probably isn't any clearer example of the dramatic changes taking place in the Code with respect to motors than Table 430-152. This table has been extremely simplified, as all references to code letters have been deleted. In fact, this table now has approximately the same degree of complexity as it did in the 1937 NEC, just before the advent of code letters. All polyphase motors are now treated alike, except for synchronous or wound rotor motors. The allowable percentages for instantaneous trip circuit breakers, which we will discuss shortly, have been revised upward, with a special allowance for Design E motors.
Note that a time-delay Class CC fuse is now to be calculated the same as a nontime-delay fuse. A Class CC fuse is closer in performance to a nontime-delay fuse. For example, a Class CC fuse will hold 200% of its rating for 12 sec, as opposed to the traditional time-delay fuse that will hold 500% of its rating for 10 sec.
Sec. 430-52(c)(1). Sec. 430-52(c) contains the specific rules that refer to Table 430-152, and the first paragraph effectively requires conventional overcurrent devices (fuses and inverse-time circuit breakers) to be sized so as to not exceed the Table 430-152 calculations. Ex. 1 covers the procedure to follow when that calculation results in a size or rating that doesn't correspond to standard size of overcurrent protective device.
Now (following a crucial 1996 change) you can round upward to the next higher standard sized device by right, and without showing that the next lower size would be unable to carry the load. The process is similar to Sec. 240-3(b), although there is no 800A upper limit on this exception as there is in Art. 240.
Another result of the change is that small motors whose required protection calculated below 15A can round up to 15A. It isn't necessary to consider restricting the overcurrent protective devices to small fuses [for which 1A, 3A, 6A, and 10A are additional standard sizes in Sec. 240-6(a) Ex.] in this case, which was an inadvertent result of the 1993 NEC. Be aware, however, of product listing restrictions. Some small overload relays won't take the amount of current that a large short-circuit and ground-fault protective device will allow into the circuits. They have restrictions on the upper limit. Sec. 430-52(c)(2) and Sec. 110-3(b) combine to make these published limits enforceable.
If the short-circuit and ground-fault protective device selected under these calculations won't allow the motor to start, there is another exception (Ex. 2) that allows even higher ratings than those allowed by Table 430-152. Another important change, however, is that the exception now only applies if the rounded up rating (from Ex. 1) is not sufficient to start the motor. This increases safety by ensuring that the next higher device will be tried before jumping all the way up to the significantly higher limits permitted under this exception. Fig. 2 also shows some examples of these calculations. Note that there is no permission to round up to the next higher standard size above these limits.
Sec. 430-52(c)(3). So far, we have only focused on traditional overcurrent devices (fuses and inverse-time circuit breakers) being used as short-circuit and ground-fault protective devices. There are other protective arrangements that only function in that way. That is, they trip almost instantaneously on a specified fault current, but provide no overload protection.
As in the case of conventional devices, Table 430-152 sets the basic parameters for the allowable settings, and there are important changes on tap here as well, as illustrated in Fig. 3. The allowable percentage now depends on the motor's design letter, and not the old "code letter." Most motors now can use 800% instead of the former 700% by right, and Design E motors can now use 1100% by right.
Inrush currents on energy-efficient motors can often exceed even these more generous percentages, however. If the values taken from Table 430-152 won't work, the even higher allowances in Sec. 430-52(c)(3) Ex. 1 can be used. You must show, however, by actual field trial or by engineering evaluation, that the parameters in the Table won't work. For other than Design E, the upper limit remains at 1300% of full-load current, but for Design E you can go up to 1700% of full-load current. Motor short-circuit protectors (which involve fusable elements) aren't included in this exception, and must not exceed 1300 % of the motor's full-load current, regardless of the design letter.
Great care needs to be taken when short-circuit and ground-fault protective devices are set as high as now allowed. A 30-hp 460V motor (Table FLA = 40A) could now be wired on a circuit with protection as high as 680A. A 200-hp 600V motor could be protected as high as 3200A. Although this will probably allow the motors to start (and indeed, as long as these currents are confined to the motor windings there isn't any problem), this setting may exceed the current flow in an arcing ground fault.
If that happens at 277V (or 347V or 600V) to ground, the fault will only be cleared by the running overload protection. Arcing faults at this voltage tend to restrike and continue, and over the course of the time delay built into running overload protection, the result will be an almost certain fire. There is no substitute for a very high standard of workmanship and careful attention to Sec. 250-51. You must do everything possible to ensure the lowest possible impedance in any ground return path, so any fault returns enough current to exceed the trip setting of the circuit breaker.
There is also another exception, Sec. 430-52(c)(3) Ex. 2, which now allows small motors with full-load currents of 8A or less to use instantaneous trip circuit breakers with continuous current ratings of 15A or less for protection. This exception applies irrespective of the actual percentage involved. The actual setting can be increased "to the value marked on the controller." The marked settings will have been carefully evaluated and coordinated by qualified testing laboratories to ensure that low-level faults are cleared. These small breakers can be set to give closer protection than 15A inverse-time breakers. The 8A threshold was chosen because it encompasses all 3-phase motors 2 hp and below.
Remember, instantaneous trip circuit breakers may only be used as part of a listed combination controller. They must never, in any size, be purchased and field-installed separately.
Control and disconnection
Now that the elements of circuit protection have been settled, the next step is to be certain that we can control the motor and disconnect it for maintenance. The ratings of controllers and disconnecting means are now heavily dependent on the design letter of the motor. For both motor controllers and disconnects, a horsepower rating may no longer be the required horsepower rating for Design E motors. Unless specifically marked as acceptable for Design E, the devices must be derated when used on such motors.
As noted before, Design E motors have higher locked rotor currents. These currents run about eight times full-load current instead of the conventional six times. The rules for equipment that must be capable of interrupting stalled rotor currents (controllers and disconnects) reflect this by shifting the requirements for using conventional equipment on Design E motors by approximately the ratio of 8:6, or 1.3+. Specifically, you can use a conventionally rated controller or disconnect with a horsepower rating that exceeds that of the motor in question by a factor of 1.4 from 3 to 100 hp, and by a factor of 1.3 for larger motors. Of course, you can also use equipment that has been tested and marked for the higher Design E currents.
As this is written, the effects will vary by manufacturer. Take, for example, a 50-hp 460V motor. This would fit into the upper end of a NEMA 3 starter. If you change it to a Design E motor, you would need a NEMA 4 starter, capable of handling 50 hp x 1.4 = 70 hp, unless the starter were marked as suitable for Design E motors. In fact, many such starters are probably suitable now, although the markings aren't out yet.
For example, one major manufacturer advertises its Size 3 starters as good for 947A maximum inrush. By way of contrast, the locked-rotor current on this motor, from Table 430-151B, is 363A for Design B etc., and 515A for Design E. That manufacturer has told EC&M that it expects no problems getting its NEMA-rated equipment listed, but it thinks it may have trouble with some of its IEC-rated equipment that has closer tolerances. Another major manufacturer advertises its starters as all being capable of inrush currents equal to 8.4 times full-load current.
Disconnects. Disconnecting devices will have a similar problem. For example, a standard heavy-duty switch rated 100A at 480V has a 60-hp motor rating. That is fine for the conventional 50-hp motor just described, but it wouldn't be acceptable for the same hp motor in a Design E configuration. (As before, the rating would have to go up to 70 hp unless the switch were marked for Design E.) As in the case of controllers, there will likely be changes in marking down the road as various manufacturers assess the relative market share of Design E motors.
Receptacles. Another problem area will be many cord- and plug-connected motors. If an attachment plug and receptacle is a disconnecting means, and it supplies a Design E motor over 2 hp, then its rating must exceed the horsepower rating of the motor it disconnects by a factor of 1.4.
Remember, Sec. 430-108 requires every disconnecting means in a motor branch circuit right up to the "point of connection to the motor" (which could be on the load side of the outlet) to qualify as a disconnect under Sec. 430-109 (and also Sec. 430-110). You can't opt out of this requirement by pretending you won't open a plugged-in connection under load; the Code makes no such assumption. Short of obtaining some sort of waiver from the AHJ, you will need to show that cord- and plug-connections to a motor meet the applicable horsepower rating requirements.
Example: Your specification calls for a 7 1/2-hp 460V motor to be cord- and plug-connected. If it is other than Design E, you can use a conventional locking plug and receptacle configuration because they fall within the blanket horsepower rating recognitions (they run up to 10 hp) in the UL Electrical Construction Materials Directory. If it is a Design E motor, what configuration receptacle will be permitted? Answer: 7.5 hp x 1.4 = 10.5 hp; this is beyond the generic horsepower ratings of standard plugs and receptacles, at least as the new Code goes into effect, and you have a problem. Check with various manufacturers to see if they offer (or intend to offer) improved horsepower ratings on any of their devices. Some 480V, 30A pin and sleeve devices, for example, are rated up to 15 hp.
In this code cycle, Art. 430 is probably one of the best examples of how emerging technology can dramatically affect everyday trade practice. Often Code changes result from fresh engineering outlooks on old technology that really hasn't changed. Consider, for example, the new rule on wiring wye-delta motors.
This time, however, most of the major changes in Art. 430 are resulting from the advent of the Design E motor, which was only on the drawing board when the 1993 NEC was being revised.