Sometimes a 250.52(A)(4) "ground ring" is spec'd out by architects. i just don't know how to 'encircle' a structure with only 20' though...
The operative phrase in that case is "at least." The reason for the restriction is that very small structures such as guard shacks and well houses could be encircled with less wire. Twenty Feet is the minimum length. The perimeter distance around most structures assures that the length will be adequate to provide a ground that is a lower impedance than the two driven rod minimum solution.
(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 2 AWG.
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Tom
