A single-phase 240-volt residential central air-conditioning unit, the minimum Romex NM cable size to the unit, with THHN/THWN insulation, is which AWG?

A single-phase 240-volt residential central air-conditioning unit, the minimum Romex NM cable size to the unit, with THHN/THWN insulation, is which AWG?

• A. 10 AWG
• B. 6 AWG
• C. 12 AWG
• D. 8 AWG

Answer: (D)

The main concept here is how to size the feed for a 240 V single‑phase residential air conditioner using the unit’s minimum circuit ampacity (MCA) and the overcurrent protection rules, while recognizing the effect of conductor insulation on ampacity. To determine the correct conductor size, you start with the unit’s nameplate, which lists the MCA (the minimum current the service must be able to carry) and the required overcurrent device. The conductor must have an ampacity at least equal to that MCA, and the overcurrent device is sized according to the MCA (often 125% of the continuous load, depending on the unit and installation specifics). Because the run to the outdoor unit may involve THHN/THWN conductors in conduit, those conductors can be rated at higher temperature ratings (75°C or 90°C) than NM-B insulation rated at 60°C, which increases the allowable ampacity for a given conductor size. For typical residential central air units, the MCA is in a range where 10 AWG NM-B (30 A) often becomes borderline or insufficient once you apply the sizing rules and provide margin for the protection device. Using copper conductors with THHN/THWN insulation in conduit gives you higher ampacity for the same inch size, so eight AWG copper is commonly the minimum that provides enough ampacity to meet the MCA requirements while still fitting within practical wiring methods. This is why eight AWG is the standard minimum in this scenario. If the unit’s MCA were smaller, a smaller conductor could suffice; if the unit demanded more current, larger conductors (such as six AWG) would be used.

The main concept here is how to size the feed for a 240 V single‑phase residential air conditioner using the unit’s minimum circuit ampacity (MCA) and the overcurrent protection rules, while recognizing the effect of conductor insulation on ampacity.

To determine the correct conductor size, you start with the unit’s nameplate, which lists the MCA (the minimum current the service must be able to carry) and the required overcurrent device. The conductor must have an ampacity at least equal to that MCA, and the overcurrent device is sized according to the MCA (often 125% of the continuous load, depending on the unit and installation specifics). Because the run to the outdoor unit may involve THHN/THWN conductors in conduit, those conductors can be rated at higher temperature ratings (75°C or 90°C) than NM-B insulation rated at 60°C, which increases the allowable ampacity for a given conductor size.

For typical residential central air units, the MCA is in a range where 10 AWG NM-B (30 A) often becomes borderline or insufficient once you apply the sizing rules and provide margin for the protection device. Using copper conductors with THHN/THWN insulation in conduit gives you higher ampacity for the same inch size, so eight AWG copper is commonly the minimum that provides enough ampacity to meet the MCA requirements while still fitting within practical wiring methods. This is why eight AWG is the standard minimum in this scenario.

If the unit’s MCA were smaller, a smaller conductor could suffice; if the unit demanded more current, larger conductors (such as six AWG) would be used.