Wiring an HVAC system is one of the most important installation steps. An air conditioner’s or furnace’s circuitry is sufficient only if the proper wires are chosen. Romex cable typically contains nonmetallic (NM) thermoplastic wires that are resistant to high heat, coated in nylon, and bundled into a polyvinyl chloride sheath. It’s best suited for indoor applications to connect electrical panels and large appliances. In this article, we’ll explain the Romex wire type you’d use with heating and cooling equipment.

HVAC Circuit Cable Wiring

An HVAC circuit generally uses NM cable. The higher the amperage of the system, the lower the gauge of wire you’ll need. For example, a smaller 20-amp unit will require a 12-gauge wire,  while a 30-amp unit will require a 10-gauge wire. The lower the gauge number, the thicker the wire. Other aspects of wiring vs. system size to consider include:

  • British Thermal Units (BTUs): An HVAC system’s capacity is rated in BTUs and tons. A 1-ton system equals 12,000 BTUs, so a 3-ton capacity system is 36,000 BTUs and a 5-ton system is 60,000 BTUs. The recommended wire size for a 5-ton unit is 10 gauge. A 1.5-ton air conditioner (18,000 BTUs) requires a 14-gauge wire, but that changes at 3.5 tons, which requires a 12-gauge wire.
  • 10/2 vs. 10/3 Wire: During HVAC installation, you’ll also encounter terms such as a 10/2 or 10/3 wire. A 10/2 wire has a hot, neutral, and ground wire (all 10 gauge). An AC technician typically uses a 10/2 wire size for a standard air conditioner. A 10/3 wire consists of one neutral, one ground, and two hot conductors and is suited for dual-voltage appliances (i.e., air conditioners with cooling and heating options).

How Air Conditioner Outlets Are Wired

While many ACs can run on a 120-volt circuit, central air conditioning systems require 240- or 250-volt circuits.

If the circuit is 120 volts, the black wire carries the hot current and the white wire is neutral. Both wires in a 240/250-volt circuit are hot; each carries 120 volts. Therefore, the white wire may be labeled with black or red tape at the screw terminal so that a service technician can identify the hot connection. 

Also, the receptacle, circuit, and air conditioner must have matching amperage ratings. That’s because the configurations between, for example, 20-amp and 30-amp receptacles, usually differ. The ground wire should connect with the outlet and the electrical box (if it’s made of metal).

In addition to the correct Romex wire type, you’ll also need the following to set up your HVAC system:

  • A double pole circuit breaker that matches the brand of the breaker box (and your central AC system’s amperage).
  • An outdoor-mounted disconnect box that’s located near the condenser unit.
  • A waterproof electrical wire that runs from the condenser to the disconnect box (ready-made electrical whips are designed for this purpose).
  • Low voltage wires that connect the condenser to the furnace’s control board.

Other considerations regarding Romex wire type include wire size. The larger the wire, the more difficult it can be to bend and connect in tight spaces. A cable clamp should be installed at the main breaker panel. It secures the wire to avoid movement that can cause it to loosen or be damaged. Also, use electrical staples to keep the wire secure along its length. They hold the wire to the wood framing of your home.

Call Over the Moon for HVAC Wiring Help

Over the Moon can help with any electrical wiring installation or repair. Our technicians are also trained to manage the complexities of wiring an HVAC system. Continue reading to learn more about how Romex wire is used. In addition to heating and cooling system installation and repair, Over the Moon also provides electrical services in the Milwaukee and Oconomowoc area. To request help, call (262) 510-0956 today.