Code Quiz

Jun 23, 2005 9:01 AM, By Steve Owen

What’s the allowable ampacity of each of the current-carrying conductors of a 4-conductor, 2 AWG, type SO cord that’s rated 75° C by the manufacturer and connected to a 3-phase, 4-wire piece of equipment (no neutral load)? The conductors are installed in an area where the ambient temperature has been determined to be 50° C by a documented engineering study.

A) 60A
B) 71.25A
C) 72.80A
D) 86.45A

Answer: A)

Explanation: Per a change in the 2005 NEC, 400.5(A) was revised to allow for adjustment of current-carrying capacity (ampacity) of conductors when installed in areas where the ambient temperatures exceed the 30°C / 86°F, as described in the bold print header of Table 310.16.

The ampacity adjustments from the adjustment table on the bottom of Table 310.16 are almost always reduced to lower values than the initial values established in Tables 400.5(A) and (B) or the ampacities of Tables 310.16-19.

The adjustment factors beneath Table 310.16 shall be applied as stated in this explanation.

The ampacity for a 2 AWG Type SO cord (3-phase, 4-wire application, 3-current-carrying conductors) is 80A, as listed in Table 400.5(A), Column A. The correction factor for an ambient temperature of 50°C / 122°F, as noted in the bottom of Table 310.16 is 0.75 (from the 75°C column, based on the manufacturer's 75°C rating of the type SO cord).

Therefore, the allowable ampacity under the conditions specified in this question would be 80A × 0.75 = 60A.

Note: This doesn’t address terminations, only the allowable ampacity under the specified conditions.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.