310.15Ampacities for Conductors Rated 0–2000 Volts. (A) General. (1) Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15 (B) or under engineering supervision, as provided in 310.15 (C).

Section 310.15 (B) (7) contains rules on calculating the conductor size for 120/240-volt, 3-wire, single-phase dwelling services and feeders. In the 2011 NEC ®, a simple table existed showing the service size on the left and the minimum size conductor required to supply the service on the right.

The new 2014 NEC language states: 310.15 (B) (7) For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted be sized in accordance with 310.15 (B) (7) (a) through (d).

Table 310.15 (C) (1) provides adjustment factors when installing more than three current-carrying conductors (CCC). Years ago, the table only applied when there were more than three CCC “in a raceway or cable”. The adjustment factors in the table are required for more than just raceway or cable installations containing more than three CCC.

In the 2020 NEC ®, any raceways or cables exposed to direct sunlight on or above rooftops that cannot be installed at least 7/8 inch above the roof to the bottom of the raceway or cable, require a temperature adder of 33°C (60°F) to be added to the outdoor temperature to determine the applicable ambient temperature for applying the correction factors in Table 310.15 (B) (1) or Table 310.15 (B) (2).