Frank,

The efficiency formula is incomplete rather than being in error.

In the case of radial systems it could be something like -

Efficiency = Rr / ( Rr + Rradials + Rsoilsurface ) .

When soil resistivity becomes very small, efficiency approaches 100
percent and the error when compared with NEC4 reduces to zero. The
error is therefore a function of soil resistivity.
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Reg.