Electrical design for HVAC systems

National Electrical Code Article 430 provides maximum protection for the equipment while limiting the nuisance tripping to a minimum. Failure to follow these requirements will not only result in unreliable system operation but will put the pump or fan motor at risk of significant damage in case of an overload or short circuit.

09/12/2011


For many engineers, contractors, and facility personnel, Article 430 of the National Electrical Code (NEC), Motors, Motor Circuits, and Controllers, is a real mystery. The rules governing the sizing of conductors and overcurrent protective devices (OPD) for pump and fan motors simply do not seem to agree with other articles of the NEC.

Due to the wide range of available motor types and applications, this article will address single-speed, nonreversing, single- and three-phase induction motors in fractional and integral horsepower sizes. Synchronous, multispeed, reversing, wound-rotor, duty-cycled, and other motors will be addressed in subsequent articles.

Ampacity

From the beginning, the engineer must determine the ampacity of the fan or pump motor or motors being circuited. While this appears somewhat simplistic, this is the first point of confusion in this motor protection morass. If the motor is available for examination, the full load ampacity is given on the motor nameplate, and this is the best possible method of determining the appropriate conductor and protective device sizes. Using the nameplate ampacity also will permit sizing of the protective device to prevent nuisance outages.

More typically, the fan or pump motor is specified by the mechanical engineer so there is no nameplate available, so the full load ampacity must be determined by using NEC tables 430.247 through 430.251. These tables give the full load amps for NEMA class, integral, and fractional horsepower motors at most typical voltage levels. For instance, a 1/3 hp single-phase, 115 V motor has a full load of 7.2 Amps (from table 430.248).

For refrigeration (hermetically sealed motors) equipment, the manufacturers’ catalog cut sheets must be used instead of the aforementioned tables, because refrigeration motors do not conform to the standard horsepower ratings.

Motor circuit conductor sizing

Sizing of the fan and pump motor conductors follows most of the sizing conventions for the NEC, but there are some quirks that are important to note. For a single motor on an individual overcurrent device, multiply the full load ampacity of the motor by 1.25 and then size the conductors the next size larger than the resultant value. NEC article 430.22 states in part, “shall have an ampacity not less than 125%.” This is one of the first differences from the standard, because for most loads under 800 Amps, the engineer is permitted to round down to the next common conductor size. For motors, he must round up to the next common size, even if 125% of the motor ampacity is only 0.1 Amps larger than the conductor ampacity from table 310.16.

For multiple pump or fan applications, the requirements of NEC 430.24 state that the largest motor ampacity is multiplied by 1.25 and the full load ampacity of each of the smaller motors are added to this value. Therefore, for six 5 hp pump motors operating at 200 V, multiply the largest motor ampacity of 17.5 Amps by 125% and then add the full load current of 17.5 Amps for each of the other motors. This results in a wire sizing ampacity of 109.4. From article 310.16, the ampacity of #2 copper with 75 C insulation is 115 Amps, but, with the correction factor of 0.88 for a 40 C ambient temperature, the conductor is rated at 101 Amps, so it is too small. However, #1 copper is rated at 130 Amps at 30 C and 114 Amps at 40 C, so it is adequate for the six motors.

Overload protection

Sizing the motor overload protection for our 1/3 hp exhaust fan motor is, typically, quite easy because most fractional horsepower motors come with integral, thermal overloads. When the motor is overloaded, the overload heats up, causing it to open the motor supply circuit and to shut down the motor. Once the motor has cooled sufficiently, the thermal switch will close, allowing the motor to restart and run.

For a three-phase fan motor, overload protection is just about as easy. Using the 5 hp motor from the example below, the full load ampacity is 17.5 Amps. If the motor starter uses thermal overload elements, one would select the thermal overload for that starter in which the full load ampacity falls. For example, a typical overload table has overload Ampere ranges of 14.4 to 15.7, 15.8 to 17.8, and 17.9 to 20.3. One would select the range of 15.8 to 17.8, in which the full load amperage of 17.5 falls. For the newer, electronic overload elements, the motor full-load amps can be programmed into the unit, eliminating the need for stocking multiple overloads or having to buy a different overload when the motor size changes.

Short-circuit protection

Sizing the short-circuit protective device for a fan or pump motor causes the most confusion for engineers and electricians because it seems to violate the breaker sizing rules that common sense would suggest. For single-phase motors, one would multiply the full-load current by the factor in Table 430.52, based on the type of motor and the type of OPD. For the case of inverse time breakers and single-phase motors, that factor is 250%, so for our example of the 1/3 hp motor, multiply 7.2 Amps by 2.5 giving 18 OPD Amps. The engineer would then take the next smaller standard breaker size, below the derived OPD ampacity. In our case, the next smaller breaker, below 18 Amps, is a 15 Amp breaker. It is important to note that any OPD ampacity that is smaller than19.9 Amps will always use a 15 Amp breaker. (See the specifics in Article 430.52(C); the multiplier may be increased to a maximum of 400%, which would result in a 15 Amp breaker for all motors with full-load currents of less than 5 Amps.) Sizing of the OPD at 250% will greatly reduce the possibility of nuisance tripping.

For the three-phase motor in the example on page xx (sidebar 1) with a full load ampacity of 17.5 Amps, the engineer would multiply this value by 250%, giving 43.75 OPD Amps. The standard trip setting of a thermal magnetic breaker just smaller than this OPD would be a 40 Amp breaker. Recalling that our conductor size was #12 (with a maximum rating of 25 Amps, per table 310.16), we now have a 40 Amp breaker protecting a conductor rated at 25 Amps.

When there are multiple motors on a single feeder, such as a motor control center or distribution panel, short-circuit protection takes a different methodology. For example, you have a 480 V motor control center with one 50 hp fan, two 30 hp pumps, one 20 hp pump, and three 10 hp fans. The full load ampacities for these motors are: 65, 40, 27, and 14, respectively, at 460 V. The largest motor ampacity is multiplied by 250% and the sum of the rest of the motors is taken at their full load ampacities. This results in an OPD ampacity of 311, so taking the next smaller size will result in a 300 Amp thermal magnetic breaker. The smallest conductor that meets our conductor calculation of 230 Amps is a 300 kcmil, with an ampacity of 250 Amps in a 40 C ambient room.

This is the point where one’s credulity could be stretched beyond its normal limits. How is it that the NEC permits a 40 Amp breaker to be the protective device for a conductor rated at only 25 Amps? How about the multiple motor example in which the conductor with an ampacity of 250 Amps is protected by a 300 Amp breaker? The key is the unique relationship between the motor conductor, the motor overloads, and the motor OPD. The OPD protects the motor (and the conductors) from a short-circuit condition either in the motor or in the motor branch circuit. By examining the time-current curve for a thermal magnetic breaker, the difference between the level of protection provided by a 25 Amp breaker and a 40 Amp breaker during short-circuit conditions is virtually identical. The overloads in the starter, or integral overloads on single-phase motors, protect the motor from overheating during an overloaded condition. 

Chiller motor electrical design

Designing the branch circuit and overcurrent protection for a refrigeration machine, whether a hermetic centrifugal, a screw compressor, a reciprocating compressor, or any other compressor, bears some resemblance to the aforementioned sizing for fan and pump motors. The nice part of this electrical design is that the engineer has to perform very few calculations since most of the work has already been done for him. Taking a 200 ton screw compressor as an example, each of two circuits has a full load ampacity of 168 Amps. From the nameplate information, the Minimum Circuit Amps (MCA, defined as wire sizing Amps) is 414 and the maximum overcurrent protection (MOCP) is a 500 Amp fuse. So, the only calculation required is correcting the conductor size for installation in a 40 C ambient condition. In table 310.16, 250 kcmil is rated at 255 Amps, so multiplying by 0.88 gives an ampacity of 224. Multiplying by 2 results in an ampacity of 448, which is the next size above the MCA of 414, or two sets of three 250 kcmil plus ground.

Determining the size of the fuse or breaker for a refrigeration machine is quite easy: Just size the MOCP the same as the value on the nameplate. In this example, the MOCP would be a 600 Amp fused switch with 500 Amp fuses, preferably dual element fuses. It is very important to understand that the notation on the nameplate, which requires that the MOCP be 500 Amp fuses, means that only fuses may be used for protecting this chiller. According to the NEC article 110.3(B): “Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.” Because the label (nameplate) designates that there is a maximum fuse size, this article requires that only a fuse may be used. If there had been no mention of a fuse, then a 500 Amp thermal magnetic breaker could be used instead.

Variable frequency drives

Variable frequency drives (VFDs) are quite common in HVAC applications. While a detailed description of all of the intricacies of VFD design is outside the scope of this article, there are some key considerations to ensure that your HVAC application goes more smoothly.

The most obvious element in applying a VFD to a fan or pump is to make sure the motor is rated for drive applications (inverter duty motor). Standard motors may do OK when connected to a VFD, or they may not. Due to the abnormal stresses applied by the VFD to the motor windings and rotor laminations, any small weakness of a motor will be amplified when the motor is operated at a lower speed/frequency. The result can range from just a very noisy motor to a shorted winding or complete motor failure.

One very important point is addressing harmonics that are produced by the HVAC VFDs. Many of the more modern drives produce either little or no harmful harmonics because they have an integral filter or an internal design that reduces the reflected wave harmonics. Those drives that do produce harmonics do not depend on the use of drive isolation transformers (DITs) to prevent these harmonics from entering the distribution system. DITs have some very specific applications where they are very beneficial, but mitigating harmonics is not an area in which they are very successful.

Distance between the VFD and the motor can become an opportunity for motor winding failure due to reflected wave high voltages caused by locating the motor distant from the VFD. Several drive manufacturers have Web-based calculators that will tell you if the distance between the motor and the drive is too far and give you mitigation means to help improve the installation. The common-sense approach would be to keep the VFD within sight of the motor (per NEC, that is less than 50 ft and within line of sight). If this is not possible, we can specify that the VFD should have an output dv/dt filter to mitigate the effects of reflections.

Example 1

Summary

Article 430 of the NEC is intended to provide maximum protection for the equipment while limiting the nuisance tripping to a minimum. Failure to follow these requirements will not only result in unreliable system operation but will place the pump or fan motor at risk of significant damage in case of an overload or short circuit. Additionally, failure to follow NEC requirements could jeopardize an engineer’s professional registration and put him at financial risk for any damage or modifications that are required to correct the improper sizing. 

Ken Lovorn, PE, is president of Lovorn Engineering Assocs., Pittsburgh, and a member of the Consulting-Specifying Engineer Editorial Advisory Board.


Example 1: Determining the ampacity of a three-phase motor

The first step to ascertain a motor’s ampacity is to know the motor voltage. For a 480 V system, the motor voltage should be 460 V, while a 200 V motor will operate on a 208 V system. This difference in voltage between the system and motor voltage is sometimes a mystery to engineers, because one would normally think that you would connect a 480 V motor to a 480 V electrical system. The reason for this difference is to accommodate the system voltage drop between the incoming service and the point of connection of the motor. On a 480 V system, Article 210.19 FPN suggests that the combined branch circuit and feeder have no more than a 5% voltage drop for efficient operation. This will result in a voltage of 456 V at the motor, if the incoming voltage from the electrical utility is exactly 480 V.

Example 2

For example, a three-phase, 5 hp motor connected to a 208 V system would have a motor voltage of 200 V. From NEC table 430.250, the full load ampacity for this motor would be 17.5 Amps.


Example 2: Tripping times for short circuit conditions

Figures 1 and 2 show partial, time-current curves for a 25 Amp and 40 Amp, Eaton Series C, F frame thermal magnetic breaker. In Figure 1 for the 25 Amp breaker, we have selected a 10,000 Amp fault occurring at the motor. The tripping time for this fault is 0.014 sec. In Figure 2 for the 40 Amp breaker, we selected the same, 10,000 Amp fault occurring at the motor. The tripping time for this fault is 0.014 sec. The two curves are virtually identical, and the differences between the two curves at this fault level are not discernable. For comparison, Figure 3 is for the same frame-size breaker with a 150 Amp trip, and the tripping time appears to be a bit below the 0.014 sec mark, perhaps at 0.0138 sec.


Example 3: NEC 310.16 wire sizing

Many engineers take the conductor ampacities from Table 310.16 at face value, that is, 75 C, #3 copper wire is listed as having an ampacity of 100 Amps. However, a sentence in the table heading says that the ampacities are “Based on Ambient Temperature of 30 C (86 F).” This means that a fan motor could be located in a typical mechanical room which commonly has a design temperature of 40 C (104 F). The ampere value in table 310.16 must be derated by the correction factor of 0.88, as noted in the Correction Factors table located below the ampacity table. For our single-phase, 1/3 hp motor with a full load of 7.2 Amps, multiplying by 1.25 gives 9 Amps, and dividing by 0.88 results in wire sizing Amps of 10.2 Amps, which would be a #12 wire (since that is the minimum size for most commercial wiring). For the 5 hp motor in Example 1, this same calculation gives wire sizing Amps of 24.85, which is also a #12 wire.

Example 3



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