310.14 Ampacities for Conductors Rated 0 Volts — 2000 Volts
Informational Note No. 1: Ampacities provided by this section do not take voltage drop into consideration. See 210.19(A), Informational Note No. 4, for branch circuits and 215.2(A), Informational Note No. 2, for feeders.
Exception: Where different ampacities apply to portions of a circuit, the higher ampacity shall be permitted to be used if the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit,
Informational Note: See 110.14(C) for conductor temperature limitations due to termination provisions.
Informational Note No. 1: The temperature rating of a conductor [see Table 310.4(A) and Table 311.10(A)] is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The ampacity tables of Article 310 and the ampacity tables of Informative Annex B, the ambient temperature correction factors in 310.15(B), and the notes to the tables provide guidance for coordinating conductor sizes, types, ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows:
- Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time.
- Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.
- The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.
- Adjacent load-carrying conductors — adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.
Informational Note No. 2: Refer to 110.14(C) for the temperature limitation of terminations.
|Tc||= conductor temperature in degrees Celsius (°C)|
|Ta||= ambient temperature in degrees Celsius (°C)|
|Rdc||= dc resistance of 305 mm (1 ft) of conductor in microohms at temperature, Tc|
|Yc||= component ac resistance resulting from skin effect and proximity effect|
|Rca||= effective thermal resistance between conductor and surrounding ambient|