The obvious one is the line cord on most lamps. For a 20 amp circuit, use 12 gauge wire. For a long run, though, you should use the next larger size wire, to avoid voltage drops. Here's a quick table for normal situations. Skin effect Skin effect is an effect that the electricity in high frequencies does not use the whole condictor area.
High frequencies tend to use only the outer parts of the conductor.
The higher the frequency, the less of the wire diameter is used and higher the losses. Sin effect must be taken care in high frequency coil designs. The frequency dependency of the resistance of a cylindrical conductor can be calculated by the following formula, which is surely valid for high frequencies and radii of approx. Skin depth is not an absolute, but only the depth where current through the wire or foil has fallen to a specific proportion of the current at the surface. In fact, current falls off exponenially as you move inward fromm the surface.