#2 will carry the energy and is physically big enough to withstand abuse. Many projects with long life will spec 4/0 to compensate for long term corrosion.
#2 stranded will take more abuse than solid. But in corrosive soils solid is better and tinned even better.
#2 stranded is today $US 2 and more per foot. Cost issue.
Inductance per foot is another issue. With lightning and L di/dt the inductance becomes dominant.
Oldagpe, you have good point regarding use of cable #2 for grounding application. In regular non corrosive soil with low SC<40 kA with several shield wires or UG feeders. Other criteria to check grid size is withstand capability for fusing during ground fault.
I am not sure if the inductance of #2 cable makes any difference for buddies grid since the overall impedance is virtually resistive (X/R ratio very low)
"Grounding ring" I'm assuming is a buried conductor surrounding the perimeter of the facility. It distributes energy from the lightning protection system; or provides an electrical ground; or distributes energy from lightning electric and magnetic fields, and ground potential rise from external lightning events.
Soares book on grounding, Fig 11-14 shows a bolted connection, 5 seconds, of 2281 amps for #2. Fig 11-12 shows a 16 msec withstand for #2 of over 30 thousand amps.
Lightning could be 20 to 120 thousand amps, with a rise time of from 1/2 to about 6 microseconds. Regular cable being about 1/2 microhenry per foot, some high voltages are developed on the down conductors and ties to earth electrodes, even after being divided in current paths.
A grounding ring will have a step voltage associated with it in a lightning event.
So, that's why #2 is popular as a grounding ring.
Low frequency 60 hz bolted connection current will not be as inductance sensitive, true. The electrical ground conductor (different than ring conductor) does need to be sized to handle fault currents.
It is not entirely correct. The National Lightning Safety Institute in Section 5.3.2 "Definition of Terms Used in Grounding"
notes indeed:
""Ground Ring: A ground wire of No. 2 size encircling or surrounding a building, tower or other above-ground structure. Usually the ground ring should be installed to a minimum depth of 2.5 ft. and should consist of at least 20 ft. of bare copper conductor. It should be installed beyond the building drip line."
But NFPA-780 divides the "Ordinary Structures" in two classes:
Section: 4.1.1
Ordinary Structures. An ordinary structure shall be any structure that is used for ordinary purposes, whether commercial, industrial, farm, institutional, or residential. Class I not exceeding 75 ft height, class II exceeding.
The main size lighting conductor shall be[at least] 57,400 circ.mils for class I and 115,000 for class II.
Further, section 4.13.4.2 defines:
"The ground ring electrode shall be a main-size lightning conductor". :..
So, for class I the minimum copper size shall be #2 [66,360 circ.mils] but for class II #2/0 [133,100] circ.mils
The size is connected with possible mechanical abuse as oldagpe explained very well.
The corrosion is another issue. The highest part -24 ft from top-of the down coming conductor has to be lead or tin covered in order to avoid fume attack from a high stack.
Copper is enough corrosion resistant, but in a moisturized corrosive electrolytic soil it could attack steel rods in vicinity....
