More on Grounding...and Concrete [RE-wrenches]

[David Blecker]

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Allan, Travis, Don, all

Thanks for the feedback.  The PSI article on Ufer Grounding was 
particularly interesting.  It seems that the Ufer concrete ground "can" be 
an effective method, but only if done right.

 From the NEC

250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 
in.) of concrete, located within and near the bottom of a concrete 
foundation or footing that is in direct contact with the earth, consisting 
of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other 
electrically conductive coated steel reinforcing bars or rods of not less 
than 13 mm (½ in.) in diameter, or consisting of at least 6.0 m (20 ft) of 
bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be 
permitted to be bonded together by the usual steel tie wires or other 
effective means.

So, a typical pier and pad foundation with a 5- 6 foot square pad would not 
meet NEC's requirement to have a 20 ft minimum length of rebar.      It's 
not clear from the code whether or not the 20 feet of Cu ground wire would 
be in compliance if it were snaked or threaded back and forth in a 5-6 ft 
square concrete pad.

Another concern is the structural integrity of the foundation.  Rohn says 
all rebar has to be covered by at least 3 inches of concrete but the Ufer 
method would have one piece of rebar extending up and out from the 
foundation to provide an attachement point for the ground wire that then 
attaches to the tower leg.  Would the exposed rebar create a path for 
moisture that would lead to cracking from the freeze/thaw 
cycle?     Probably no more so than the embedded 
reporters...errrr...J-bolts for the tower.   Perhaps if we added an extra 
piece of rebar specifically for this purpose, then it would be ok.

If we build a foundation that allows a 20 foot rebar rod, I may be tempted 
to try the Ufer method.  (see HP#75 Page 11 for one such example) but I 
would probably sink conventional ground rods too.  For a regular pier and 
pad foundation, it seems like ground rods are the way to go.

Dave

----------------------------------------------------------
David Blecker, P.E., Director
Seventh Generation Energy Systems
608-424-1870 (ph)  424-1810 (fax)

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