I agree. The only thing other than the size of the foundation that my
inspector checked was that the tower had been properly bonded to the
rebar cage ... he was absolutely insistent about it. I bonded two #4
wires from each leg to the cage, and I used cadweld connections to do
it. I'm pretty certain that the requirement to use special rebar for
standard welding is to preserve strength, but I've never seen that
specified in the case of cadwelding an electrical connection. The molds
I used were just standard Cadweld molds (brand new, purchased dirt cheap
on eBay) and specifically stated for use with #4 wire to #5 and #4 rebar.
http://www.ab7e.com/AB7E_Tower_Page_5.html in case anyone is interested.
By the way, I also buried three ground rods underneath the foundation,
with #4 wires cadwelded to the rods and to the tower legs. I can't wait
for someone to trot out the other old wives' tale about not doing that
because of the chance for water migrating along the surface of the wire
to rust the rebar (in spite of the fact that concrete it self is porous
to moisture and typically is full of microcracks anyway) ... or because
the wire might act like a fuse and crack the concrete when a lightning
strike vaporizes it (in spite of the fact that the lower the impedance
path to ground the better).
73,
Dave AB7E
On 4/17/2011 8:09 PM, Dick Green WC1M wrote:
> 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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