Why is THHN bad for open wiring?

Discussion in 'General Electronics Chat' started by thingmaker3, Oct 29, 2007.

  1. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
    I know the NEC prohibits THHN for open wiring, requiring NM or UF instead. THHN must only be run in conduit or raceway.

    But when my brother-in-law asked for the rationale behind this, I realized I had never contemplated such.

    Anyone know the reasoning behind the rule?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    I may be misremembering, but I think THHN is not encased. Your standard building wiring is essentially double insulated by the jacketing that covers it and the associated wiring for that circuit. I believe the protection given by the jacketing and the simple identification of the wiring makes for the requirement. If a house were done all in THHN, imagine trying to identify all three wires in any circuit.