[TowerTalk] tower base grounding

[Jim Jarvis]

Dick, (TT):

You raise several questions:

"The (inspector's) note said all rebar must be bonded to the ground grid,
and inspection
will be required prior to backfill. "

I think an argument can be made for NOT bonding the rebar.  Rather, a robust
copper-strap
system should run OVER and OUTSIDE the foundation, and connect to your
planned lightning
dispersal ground system.  My recommendation would be to meet with the
inspector or planning
officer and discuss the technical merits of each approach...find out why
they want to route
a strike through the foundation, rather than around it...and then go with
what is required.

"A bigger concern is that I was going to try to dig the 2.5' x 2.5' x 4'
hole
by hand in order to get a nice square hole exactly to the required
dimensions and pour the concrete against undisturbed soil. "

Depending on your soil, and flood factors, this is a major consideration.
The ability of the foundation to withstand overturning moment is highly
dependent
upon the soil density.  Over-excavation and backfilling will diminish that
substantially, unless you layer the backfill and use a power-tamper as you
go.
I'd vote for hand digging, so long as it's YOU and not I who are digging.
heh.
On the other hand, a guyed tower's foundation basically supports vertical
compression
loads, not overturning moments...particularly true where you have a pin
base.
Personally, I'd excavate and tamp the refill, just to minimize the cosmetic
sinking
over the next couple of years.

" Do building codes address non-AC cable runs? Has anyone
out there been required to install conduit just for coax/hardline and
low-voltage control cables?"

As I understand it..and I am NOT a student of the NEC, the code DOES address
low voltage wiring.  My recommendation would be to indicate to the code
officer
that you're NOT going to use conduit,  that the nature of amateur
experimentation
requires flexibility with multiple low-voltage cables, and NO AC POWER...and
that
you're using direct-buriable hardline for RF feed, as is commonly done in
broadcast
installations, and the local cable company.  If you decide to use conduit,
it will
only be for cosmetic reasons, to keep the cables out of sight.  The 4' bury
depth
is to avoid personnel safety for folks like cable and telco, and irrigation
systems,
who use vibratory plows to pull stuff in 18" below the surface.

Someone with knowledge of code specifics may want to address this point,
but I think it's finessable.


n2ea
jimjarvis at ieee.org
jimjarvis at verizon.net