As we approach the end of the year, the 2005 NEC has been adopted by most jurisdictions. This month, we'll look at a few of the changes that are likely to have impact on design and specification of electrical systems and equipment for HVACR and industrial projects. Further information on these and other changes can be found in Analysis of Changes - 2005 NEC, published by the International Association of Electrical Inspectors.
Motors And ControlsThe code recognizes a new development in providing disconnecting means for motors on large multi-motor machines. Many machines require frequent operator intervention, which, to comply with safety regulations, first requires that the disconnect switch for every motor that can operate the machine be opened and locked and tagged. The time required and inconvenience of this procedure can produce a strong incentive to shortcut it and a resulting risk of injury or death.
Article 430.2 now recognizes "System Isolation Equipment" as a type of permitted motor disconnecting means. This is a UL-listed system by which contactors with carefully designed control circuit lockout schemes are used to disconnect power to multiple motors from a single location. It is important to note the requirement that these be listed systems, and not as a standard motor control circuit. In all other applications, it is still unacceptable, under safety rules, to lockout a motor control circuit in lieu of opening and locking the disconnect switch.
Another change affecting motor disconnecting means appears in 430.102. When using one of the exceptions to permitting elimination of the disconnect within sight of the motor and driven machinery, the controller disconnect must be capable of being locked open. The change requires the locking means be installed on the switch or circuit breaker and remain when the lock is removed. This ensures that all a technician needs to properly lockout the equipment is a padlock, not an assortment of breaker locking accessories, one of which may fit the if they are lucky.
Design E motors are no more. Actually, according to the Analysis of Changes, they never were. The rationale for deleting all references to Design E motors is that in spite of the standards developed and accommodations made for them in the NEC, they were never produced. The current NEMA "Premium Efficiency" motors are based on standard Design B. It's good to know that all of the concerns we had with the expected high inrush currents of Design E motors will not materialize.
Motor controllers are now required to be marked with their short circuit rating in addition to the voltage, current, and horsepower markings that were previously required. I have discussed the need to specify short circuit current ratings of controllers in previous columns. This requirement is intended to make it easier for the inspector to determine whether the starters, drives, and other controllers installed are properly rated. The requirement was also added to Article 440, making it applicable to refrigeration and air conditioning equipment that contains a hermetic compressor.
The 2005 NEC sees the demise of the term "HACR" as applied to circuit breakers which are suitable for supplying HVACR equipment. The UL standard for molded case circuit breakers no longer makes a distinction between standard breakers and those marked "HACR." Equipment nameplates will change to delete mention of HACR breakers, and this requirement can be deleted from specifications.
Variable-Frequency DrivesVFDs or "adjustable-speed drives" as the NEC defines them, now have their own section of Article 430, Part X, which includes requirements for sizing conductors, overload and over-temperature protection, and disconnecting means. Some highlights include:
- Supply conductors to be sized based on 125% of the rated input current of the drive, or on 125% of the motor full load current per 430.6 if there is a bypass starter.
- Overload protection integral to the drive is acceptable, but the drive must be marked to indicate that includes overload protection (again, making it easier for the inspector)
- Motor thermal protection is now required; as a sensing device in the motor or a thermal memory function in the drive that is retained on shutdown or power loss.
- Disconnecting means may be sized on the basis of 115% of the rated input current of the drive.
Note that requirements in Part X supplement, but don't replace, the requirements of other parts of Article 430 applicable to VFDs, which are considered a controller under the NEC. ES