Skin effect can be important at 60 Hz.

Westinghouse's "Electrical Transmission and Distribution Reference Book,"
3rd ed (1944) notes:

"For small conductors this ratio [skin effect] is entirely negligible, but
for larger conductors it becomes quite appreciable and must be considered
when figuring the 60-cycle resistance of large conductors."

The following is extracted from the table associated with the foregoing
text:

Circular Mills                  Diameter (inches)               Skin Ratio
--------------                  -----------------               ----------
300 000                         0.630                           1.006
800 000                         1.031                           1.046
1 000 000                               1.152                           1.068
2 000 000                               1.631                           1.239
3 000 000                               1.998                           1.439


The skin ratio is the ratio of 60 Hz resistance to theoretical DC
resistance. 1,000,000 CM cable and larger is not uncommon in power
transmission.

In addition to skin effect, there is something called the "proximity
effect," where close spaced cables, cables in proximity to steel plates,
cables inside conduits, etc. have magnetic field distortions that force the
current into non-uniform distribution.  Proximity effect increases the
effective resistance, in some cases by a large factor.

In addition to skin effect, proximity effect is also found in radio
frequency inductors where it causes the effective inductance and Q deviate
from values calculated under uniform current sheet assumptions.


Jack


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