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is a clear description of the effect but ignoring the math, the chart shows that at 60Hz, the current in the middle of a 3/4" diameter copper conductor is still about 37% of the surface value. For typical wires 12 to 18 gauge wires at 60 Hz you can treat the current density as uniform as it is at DC. For high frequency RF the current is almost entirely in the outer few micro-inches which is why high performance RF conductors are silver plated to take advantage of the greater conductivity of silver compared to copper without the cost of solid silver.