For a 208Y/120-volt, 110-ampere feeder supplying a single dwelling unit, what is the minimum size THWN-2 grounded feeder conductor required?

To determine the minimum size THWN-2 grounded feeder conductor for a 208Y/120-volt, 110-ampere feeder supplying a single dwelling unit, we refer to the National Electrical Code (NEC) tables for the allowable ampacity of conductors based on the material and insulation type. THWN-2 is a type of thermoplastic wire rated for high-temperature and wet locations.

According to NEC Table 310.16, conductors’ ampacity must be checked against the load they are expected to carry. For copper conductors with THWN-2 insulation, the ampacity can vary based on whether the conductors are in a raceway, free air, or bundled together.

For a residential feeder supporting a continuous load, it is common practice to apply a demand factor. However, since the load here is given as 110 amperes, we must ensure the conductor size accommodates the full load, taking into account any relevant derating factors.

For a 110-ampere load in typical conditions, a 3 AWG THWN-2 copper conductor is adequate as it can handle up to 100 amperes under typical conditions. However, because of the need to account for continuous loads where conductors may