In the last issue of ELECTRICAL CONTRACTOR, I discussed the National Electrical Code (NEC) requirements for cord-and-plug-connected air conditioners (AC units). In this issue, I thought it would be appropriate to talk about the installation of permanently (hard-wired) installed units. Having been asked what the procedures are for wiring in an AC unit, I decided that it was time to address the various issues.
Wiring an AC unit in the field is not complicated. However, because there are various types of installations, and numerous types of equipment, application and adherence to the NEC is mandatory for a safe and reliable operation of the unit.
Article 440 of the NEC covers the techniques for properly sizing conductors, disconnecting means and controllers for individual or group installations having hermetically sealed-motor compressors.
The components used for the branch circuit supplying heating, air conditioning and refrigeration (HACR) equipment may be selected by the branch-circuit selection current listed on the nameplate of such equipment as outlined in 110.3(B) and 440.4(C) of the NEC.
Importance of nameplate information
The nameplate may provide information for sizing and selecting major components that are necessary for safe and reliable operation. Therefore, it is easy to understand why it is so important to those with the task of installing and maintaining these units. For this reason, the nameplate should never be covered or removed from the unit.
When required, electricians use the nameplate information on an AC unit to determine the ratings of the branch-circuit conductors, are ground-fault protection devices, short circuits and overloads. The disconnecting means as well as other components of the circuit are selected.
The conductors supplying HACR equipment as with cord-and-plug type are sized from the full load current (FLC) listed on the compressor’s nameplate. For a hard-wired AC unit, however, the condenser motor is also considered. The NEC requires these FLC ratings of the compressor to be increased by 125 percent plus 100 percent of the condenser motor’s FLC rating per 440.32 and 440.33. For example, 125 percent × 21.5 amps (compressor FLC) plus 2.5 amps (condenser FLC) at 100 percent is equal to 29.4 amps. Referring to Table 310.16, 10 THWN, AWG, copper conductors are required.
The overcurrent-protection devices (OCPDs) protect the branch-circuit conductors and compressor windings from short circuits and ground faults. When calculated, the OCPDs are sized from the provisions listed in 440.22(A) and (B), which requires the FLC values in amps to be increased from 175 percent up to 225 percent to allow the starting and running of the unit and prevent nuisance tripping of the upstream OCPD.
For example, 21.5 amps × 175 percent plus 2.5 amps equals 40 amps. Therefore, a 40-amp OCPD is permitted per 240.6(A). If the unit fails to operate properly, multiply 21.5 amps by 225 percent plus 2.5 amps. That equals 50.9 amps. This rule permits a maximum 50-amp circuit breaker (CB) or set of fuses to be installed.
The NEC in 440.51 permits the overload protection (OLP) for a compressor to be accomplished by installing OCPDs in separate enclosures, separate overload relays or thermal protectors that are an integral part of the compressor. For example, a disconnect switch can be installed with time-delay fuses that either back up the OLP in the AC unit or serve as the OLP scheme. In other words, time-delay fuses sized at 125 percent of 21.5 amps per 440.52(A)(3) produce 26.9-amps. Therefore, 25-amp fuses as the OLP for the AC unit are permitted. Often, installers use 30-amp fuses that the manufacturer provides in the compressor of the AC unit as outlined in 440.52(A)(1) and 440.53 to back up the OLP.
The FLC ratings on the nameplate or the nameplate branch circuit selection current of the compressor and condenser motor, whichever is greater, must be used at 115 percent to size the disconnecting means. For example, 21.5 amps plus 2.5 amps times 115 percent is equal to 27.6 amps. Therefore, the NEC requires a 30-amp nonautomatic CB or 30 amp nonfusible disconnect.
A horsepower-rated switch, CB, or other type switch may be used as the disconnecting means per 440.12(A) through (E) of the NEC.
A minimum amperage is derived when applying 115 percent per 440.12(A)(1) and (B)(2) to size the disconnecting means, and it may not be capable of starting larger AC units. In such cases, the disconnecting means may have to be determined by the horsepower rating outlined in the NEC per 440.12(A)(2) and (B)(1).
Electricians installing AC units with permanent wiring methods are welcome to use this information as well as other pertinent NEC requirements as an installation guideline.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.