A veteren electrician asked what the National Electrical Code (NEC) requirements are for installing cord-and-plug-connected room air conditioners. I thought the answer might be interesting to others.

Determining ampacity and rating

Section 440.6(A) requires the full-load current (FLC) in amps that is listed on the nameplate of the motor-compressor on the AC unit to be used to select the branch-circuit conductor, short-circuit protection, motor-overload protection, controller ratings and disconnecting means rating. The branch-circuit selection current (BCSC), if greater, must be used if shown instead of the FLC of the compressor components. As outlined in 440.4(C) and 440.2, the FLC rating must be used to select the motor’s overload protection (OLP) rating. Note that if the FLC rating listed on the compressor nameplate for the equipment does not list an FLC rating based on the BCSC, calculations must be performed.

Determining OCPD

Let’s consider the NEC and requirements that are necessary to size and select the overcurrent-protection device (OCPD) for the branch circuit supplying power to the receptacle outlet used to cord-and-plug connect a window AC unit. First, it is imperative that the branch-circuit fuse or circuit-breaker rating for a hermetically sealed motor be sized with enough capacity to allow the motor to start and accelerate without opening the OCPD, due to the momentary inrush current of the compressor and other operating elements. Usually, the best protection is provided when the ratings and settings of the OCPD being used are sized as low as possible. Hermetic refrigerant motor compressors, as recommended by the manufacturer or NEC, must be protected by properly sizing and selecting the ratings and settings of the OCPDs to protect the branch-circuit conductors and other elements in the circuit from short-circuit and ground-fault conditions. For example, if the nameplate calls for maximum size OCPD to be installed, then a larger size cannot be selected. Yes, a smaller rating of the same type can be used if it will start the unit and allow it to run.

Overload-protection devices are not normally field installed. They are provided by the manufacturer and are installed within the compressor compartment of the AC unit.

Selecting conductors

Section 440.32 governs the sizing of the conductors supplying power to a motor compressor. Branch-circuit conductors are sized at 125 percent of the FLC rating on the compressor’s nameplate or the BCSC, whichever is greater.

OCPDs larger than 15 or 20 amps

According to 440.2, 440.4(C) and 440.22(A), if the OCPD is larger than 15 or 20 amps, the device for hermetically sealed compressors must be selected at 175 percent or 225 percent of the compressor FLC rating or the BCSC, whichever is greater. A smaller device size can be used if it is capable of operating the unit under starting and running conditions.

Note, when 15- or 20-amp OCPDs are installed, specially designed units for starting and running are manufactured, so requirements of 440.22(B)(2), Ex. 1 must be referenced.

Installation tip: A normal circuit breaker must not be installed when the window unit is marked for a certain fuse size or heating, air conditioning and refrigerating (HACR) circuit-breaker rating. Only that specified fuse size or HACR circuit-breaker rating can be installed to protect the branch-circuit conductors.

Room AC supply cords

Section 440.64 covers the length requirements for a flexible cord used to cord-and-plug connect a room window air conditioning unit. The cord length is limited to 10 feet for a 120-volt circuit and 6 feet for 208- or 240-volt circuit. Long cord lengths must not be used, for they are subjected to physical damage and possible dangerous overheating conditions. Long lengths of cords also can be a shock hazard if they are not properly maintained.

Grounding requirements

When an air conditioner is installed where one of the following conditions exist, proper grounding of the metal must be accomplished as outlined in the NEC:

• Cord-and-plug connected

• Hard-wired (if within reach of the ground or grounded object)

• If in contact with metal

• Operating over a 150 volts to ground

• Wired with metal-clad wiring

• Located in a hazardous location

• Installed in damp location (within reach of the user)

Room AC disconnecting means

Section 440.63 contains conditions of use when a cord and plug serves as the disconnecting means for a room (window unit) air conditioner, only where all the following requirements are satified:

• Operates at 250 volts or less

• Controls are manually operated

• Controls are within 6 feet of the floor

• Controls are readily accessible to user

I hope this Code explanation is helpful to electricians field-wiring window air conditioning units.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.