[TowerTalk] [Bulk] Antenna to Shack Ground Connection

[Jim Lux]

On 2/2/15 9:30 AM, Grant Saviers wrote:
> I'm considering what gage wire to use for grounding tower, ground rods
> and Ufer guy anchors together, and need about 500' so cost is an issue.
> The best pricing for solid bare copper I have found is $1.80/ft 2ga;
> $0.85/ft 4ga; and $0.42/ft 6ga.
>
> I suspect the code requirement for 2ga on 200/400a entry panels is to
> handle the maximum interruption current of the affiliated main breaker.
> The fusing currents for 2/4/6 awg copper are 72Ka/36Ka/28Ka respectively
> which makes some sense.  (although with the usual 2 to 4 ohm rod to
> ground connection, how can current ever get beyond a few hundred amps?
> Not so if the ground is domestic water supply iron or copper.)
>
> However, in the case of a lightning strike is fusing current important
> since currents are likely an order of magnitude larger or more?  My
> historical examination of HV tower grounds often found big wire 2/0, 4/0
> etc. so is that needed for a strike and it really doesn't matter much
> between 2/4/6 ga, they fuse?  Or is the strike current so short that
> fusing isn't a concern?  Then 6 ga would be ok to tie stuff together.
>
> Is there some engineering basis for 2ga for ground rod and tower to
> shack wiring?
>


Lightning is high peak, but low "action" (the averaged power 
dissipation, in units of amps^2*seconds) because it's a short pulse (50 
microseconds)

Upshot is AWG 10 will handle a pretty large lightning discharge.  (and 
lots of lightning rod grounds around the world are in that sort of size 
range.. a few mm in diameter)