Q. What is the rule for adjusting the ampacity of conductors based on the number of conductors in the raceway?

A. Where the number of current-carrying conductors in a raceway or cable exceeds three — or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in. — the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.

For example, if you wanted to know the ampacity of 10 THHN, where four current-carrying conductors are installed in a raceway or cable in a dry location, you would calculate it as follows:

Adjusted Ampacity = Table 310.16 Ampacity x Bundled Ampacity Adjustment Factor from Table 310.15(B)(2)(a)

Table 310.16 ampacity for 10 THHN is 40A at 90ºC. The bundled adjustment factor for four current-carrying conductors is 0.80.

Therefore, the adjusted ampacity would be:

Adjusted Ampacity = 40A x 0.80 = 32A