I don't agree about not connecting to the rebar. That's the standard way to do
a Ufer ground and I believe that's the way it's described in the NEC. My local
jurisdiction required a visual inspection to verify the rebar connections
before the concrete was poured.
I would use standard ground rod clamps to attach the ground wire to the rebar.
It's a no-no to weld rebar unless you use special rebar made for that purpose.
That's why rebar is normally just wired together to form the cage. But I don't
know if that restriction applies to exothermic welding. I vaguely remember
reading about special kits for exothermically welding wire to rebar, but I'm
not sure about that. Something you could research. Clamps should be OK because
little or no air will penetrate through the concrete to corrode the
connections. Also, you can coat them with conductive grease to further reduce
the chance of corrosion.
For redundancy and lower impedance, I would use more than one wire to connect
the rebar to the tower. I used three, one for each tower leg and radial run. I
tied the rebar, tower legs and radial ground wires together. To do this, I cut
three lengths of wire long enough to reach from a ground rod a foot or two from
the tower, through the side of the concrete base to the rebar, and from there
up through the concrete to each tower leg. Each wire is exothermically welded
to the ground rod (50' radials extend from there out to a series of
exothermically welded 8' ground rods spaced 16' apart.) The middle of each Ufer
wire is clamped to the rebar using standard aluminum ground clamps coated with
coppper paste. That's how the Ufer wires are attached to the tower legs, too. I
could have used two separate wires for each connection, one connecting the
radial system to the rebar and one connecting the rebar to the tower, but the
way I did it ensures that the tower legs remain connected
to the radial system even if there's a failure in the connections to the rebar.
I do agree with Jim that you shouln't use use stranded wire. I believe the
standard for lightning ground is #2/0 solid. Not knowing this at the time, I
used #1/0 stranded wire. What happens is that corrosion forms in the spaces
between the conductors and they lose connectivity with each other. This is
plainly visible above the concrete. I'm not too worried about it because the
outer surface of the indivudual conductors are still connected at each end of
the wire, effectively providing multiple parallel connections. But those
conductors may or may not be touching each other anymore, and any inner
conductors are probably no longer connected at each end. I haven't measured it,
but I would guess the impedance is not as low as it would be if the conductors
were in full contact with each other. Given the size of the #1/0 conductors,
though, I think the impedance is still pretty low.
73, Dick WC1M
-----Original Message-----
From: Jim W7RY [mailto:w7ry@inbox.com]
Sent: Sunday, April 17, 2011 10:58 AM
To: Mat Eshpeter; towertalk@contesting.com
Subject: Re: [TowerTalk] grounding quesitons: Ufer, strap, exothermic welds
I would put some #2AWG tinned solid in the foundation and not rely on the
rebar.
I have used a clamp type unit to clamp the copper strap to the ground rod.
If it's lightning your concerned with, I would not use strap. Stick with
#2AWG tinned bare copper, copper clad rods and exothermic attachment.
73
Jim W7RY
--------------------------------------------------
From: "Mat Eshpeter" <kk1c@live.com>
Sent: Sunday, April 17, 2011 7:24 AM
To: <towertalk@contesting.com>
Subject: [TowerTalk] grounding quesitons: Ufer, strap, exothermic welds
>
> I am about to pour concrete for the base of my HDX589 tower. I am going to
> go with a Ufer ground as well as the Polyphaser recommended eight 75'
> radials with multiple ground rods (8' rods every 16'). Location is on top
> of a 1600' ridge in WV and lightning is a very real concern. My QRZ web
> page gives a feel for the location.
>
> To build the Ufer ground, I am thinking about connecting a single bare 2/0
> stranded copper wire to the rebar. Questions:
> 1) is one enough?
> 2) how do I properly connect the copper to the rebar? Is exothermic weld
> the only way? Any problem with dissimilar metals?
> 3) how should the copper exit the base slab - above grade or below grade?
>
> Also, seems that copper strap is the preferred ground material for
> connecting rods together. Will 0.032" thick strap have significantly
> better longevity than 0.022" strap? I am refering to the strap sold by
> Georgia Copper. Copper is expensive and I don't want to invest in copper
> that won't add any value to my installation.
>
> Where can I find exothermic weld products to connect copper strap to 5/8"
> copper clad ground rods? I am exhausted reading Harger, Erico, Terraweld
> info, and can only find wire-to-ground rod products, no strap-to-rod
> products.
>
> 73
> Mat
> KK1C
>
>
>
>
> _______________________________________________
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>
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