Conductor Protection

Every size of electric wire has a maximum safe current-carrying capacity or rating. The current-carrying capacity is slightly different depending on:

  • whether the wire is aluminum or copper;
  • whether the conductors are used individually or in groups;
  • and where the wire or cable is placed
    • out in the open air,
    • direct sun or shade
    • inside a well-ventilated, normally heated building, or
    • near a hot furnace or boiler rooms, etc.

There is a point to all of this specifying current ratings in wires.

Breaker and Fuse GraphicThe breaker or fuse selected to protect a particular wire size from overload must trip or melt before the current flow will cause conductor heating that could be damaging to the wire or insulation. Depending on the way the interrupting device works, the wire or cable may carry momentary overloads, provided the overload time is very short so that the total heat produced cannot build up to dangerous levels. These are called "slow blow" protectors. But, be aware that a small amount of overheating, repeated numerous times can produce deterioration in the conductor insulation.

Obviously, the idea of placing a penny or other metal object behind a fuse, or taping a breaker down so that it cannot trip, is absolutely foolhardy.