Here's what the EZNEC manual says: "Horizontal wires should not
be placed exactly on the ground, but should be at least 1/1000
wavelength above (and in the case of EZNEC/4, also below) the
ground."

Cecil, Probably the 1/1000 WL limit contains a safety margin.
This does not appear to be addressed by either the NEC 2,
or NEC 4 user manual.

Cebik's book "Intermediate Antenna Modeling", p 1-12,
states: "The minimum height for wires above
a Sommerfeld-Norton ground has two dimensions. The first
relates the height above ground limit to the wire radius.
The wire height (h) should be several times the wire radius
(a), that is, h~a. As well, the minimum height is related to
the wavelength for the frequency in use:
(h^2 + a^2)^(1/2)10^(-6)Lambda. If a is very small
compared to h, the wires may approach 10^(-6) Lambda
toward ground. ......reflection Coefficient approximation....
.... the general recommendation is that ......
horizontal wires should be () 0.4 Lambda above ground".

Obviously, from the manual quote, EZNEC can invoke a
Sommerfeld-Norton ground.

Since I do not have GNEC I usually test my NEC 4 models
with NEC-Win Pro. Interestingly NEC-Win Pro actually
runs, with no errors, on below ground wires. The results
are usually pretty weird though.

73,

Frank