Question

The National Electrical Code specifies safe current-carrying capacities for various sizes and types of wire. These ratings depend upon the length (“run”) of wire being used. For typical household use #12 AWG (American Wire Gauge) PVC-coated copper wire is rated at a maximum safe current of 20 A. The diameter of #12 AWG wire is 0.081”. At a current of 20 A find (a) the current density, (b) the electrical field strength, and (c) the potential difference for 25’ and 1000’ lengths of wire. Lastly, what is the rate of Joule heating for 25’ and 1000’ lengths of wire? What would you conclude about how the current capacity of wire might vary with the distance over which the power is delivered? Over very large distances, say the hundreds of miles over which PNM delivers your household power from the Four-Corners generating station, very high voltages (of order 200 kV) are used for the ‘transmission’ lines. Why is this?

Answer 1