When installing feeder conductors for portable switchboards at a facility like the Grand Ole Opry in Nashville, Tenn., an issue arises regarding the size of the supply neutral conductor. Here's the situation.

Single-conductor feeder cables (not installed in raceways) are used on multiphase circuits feeding portable switchboards containing solid-state phase-control dimmers. This arrangement calls for the neutral conductor to have an ampacity of at least 130% of the ungrounded circuit conductors feeding the portable switchboard. The size of the largest ungrounded phase conductor is 500kcmil type W single-conductor cable. The ampacity, which is 620A from Table 400.5(B), is based on the 75 degree temperature rating of the cable (Column D1), and the 75 degree C terminals that the cable is connected to at both ends.

What is the minimum ampacity and size required for the supply neutral conductor is this example?

  1. Two sizes smaller (i.e., 350kcmil)
  2. Same size (i.e., 500kcmil)
  3. One size larger, just to be safe (i.e., 600kcmil)
  4. Minimum 800kcmil

Answer: D

As per 520.53(O)(2), "Where single-conductor feeder cables, not installed in raceways, are used on multiphase circuits feeding portable switchboards containing solid-state phase-control dimmers, the neutral conductor shall have an ampacity of at least 130% of the largest ungrounded circuit conductors feeding the portable switchboard."

The calculation for this example would go as follows:
Largest circuit conductor = 500kcmil type W; 75 degree C temperature rating of the cable; 75 degree C terminals (with no ampacity adjustments) = 620A.
620A x 130% = 806A.
806A is the minimum ampacity required for this example.

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.