In certain parts of the country, the summer of 2024 was extremely hot, with record temperatures registered daily and for long periods of the season. Electric utility bills reached almost unheard-of costs. People shut off unused rooms in their homes, adjusted thermostats to higher levels during peak use and used ceiling fans or small, portable floor fans to stay cool.

Window air conditioners are another efficient way to stay cool, with each room having either a 120V or 240V receptacle located directly below the window to supply the unit. This type of system only cools one room, which saves on costs. 

However, window units are easy to access from outside and may make the home vulnerable to break-ins. To counter these issues, some people are now installing a mini-split air conditioning system. Mini-split systems are easy to install and do not require air conditioning supply and return air ducts.

Mini-split systems

Mini-split systems have their pros and cons, of course. They can be ideal for retrofits, adding rooms or homes where a central air conditioning system has not been installed with supply and return air ducts. Small mini-splits (12,000 British thermal units or smaller) typically only require 120V supply, with larger ones needing 240V, and vary with the efficiency based on the unit’s SEER (seasonal energy-efficiency ratio) rating. 

Mini-splits require a compressor located outside the house and a disconnecting means and power supply from a panelboard or the electrical service. The hermetic refrigerant motor-­compressor for the mini-split will be provided with a nameplate indicating the manufacturer’s name, trademark or symbol, identifying designation, phase, voltage and frequency, as required in 440.4(A) of the NEC. 

The equipment manufacturer must mark the rated load current of the motor compressor, in amperes, on the motor compressor nameplate. The rated load current must also be located on the inside unit and any other equipment that is used with the motor compressor. 

Sizing and specifics

The overcurrent protective device for the mini-split unit would be sized based on 440.22, as follows: The motor-­compressor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175% of the motor-­compressor rated-load current or branch-circuit selection current—whichever is greater—is permitted. Depending on the mini-split’s manufacturer, the unit may contain a surge protector that is installed in the junction box at the outside compressor. 

Some mini-splits will also have a power whip already attached to the unit from the factory that can be connected to the disconnecting means at the compressor. If this whip is supplied by the manufacturer and the unit is properly listed, the conductors should be sized based on 440.32 at 125% of the motor-compressor rated load current or 125% of the branch circuit selection current. 

If a whip is not included, then a normal liquidtight flexible metal conduit could be used with the conductors’ size based on 440.33 for motor-compressors with an additional motor or motors in the inside wall-mounted unit. 

Those conductors must have an ampacity not less than the sum of each of the following: the rated-load or branch-circuit selection current—whichever is greater—of all motor-compressor(s), the full-load current rating of all other motors and 25% of the highest motor-compressor or motor full load current in the group. 

Remember, when mounting the mini-split disconnecting means on the outside of the home, ensure that proper working clearance is provided based on 110.26(A). 

The manufacturer of the mini-split usually provides a connecting four-conductor cable that connects the outside compressor unit to the inside wall-mounted unit and provides it with control and power. This cable assembly must be installed in an acceptable wiring method, sized based on Table 1 in NEC Chapter 9. Table 1, Note 9 can be used for sizing the raceway for this cable assembly. However, do not forget that the hole in the wall to the inside unit must be large enough for the coolant lines and power to the unit and must be sealed against any water intrusion. 

I see many of these units being installed lately, especially on garages. Following the NEC requirements in Article 440 is the key for safety on mini-split unit installations.

Ale / stock.adobe.com