Aerial Ampacity Calculations and the NEC Code
Does the NEC Code Allow Alternate Ampacity Calculations?
While it is possible to compute higher cable ampacities with more refined and industry accepted calculation procedures, is this something that the NEC Code allows, or is strict adherence to the tables required?
The Code itself clearly indicates that use of the tables is not required, and that engineering ampacity calculations can be undertaken for any specific application. In Section 310.15 "Ampacities for Conductors Rated 0-2000 Volts", it is stated in paragraph (A)(1) that, "Ampacities for conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.15(B) and (C)."
Similarly, for cables rated 2001 to 35,000 Volts, the following wording is offered in Section 310.60.(B), "Ampacities of Conductors Rated 2001 to 35,000 Volts. Ampacities for solid dielectric-insulated conductors shall be permitted to be determined by tables or under engineering supervisionů"
The following generalized formula is provided for use in these calculations:
The real complexity in the use of this formula is in the determination of the Rca factor, which is the overall thermal resistance between the conductor and ambient. For a cable in conduit, this thermal resistance is the sum of the following individual thermal resistances:
The determination of these parameters, especially the latter value, is generally where the differences between the various calculation procedures come into play.