[TowerTalk] From the perspective of a concrete technician

[Jim Lux]

David Gilbert wrote:
> 
> I think this is the official IRC requirement for a "concrete encased 
> electrode".  It always seemed kind of marginal to me, but this is 
> minimum spec:
> 
> "An electrode encased by at least 2 inches 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 20 feet of one or 
> more bare or galvanized or other electrically conductive steel 
> reinforcing rods or bars not less than 1/2 inch diameter, or consisting 
> of at least 20 feet of bare copper conductor not smaller than No. 4 
> shall be considered as a grounding electrode."
> 

That's because it's more than adequate as an electrical safety ground 
which isn't going to need to take 100kA fault currents.. more like 100s 
of amps.  The NEC ground (and other codes, other than lightning 
protection) requirement is aimed at things like powerlines inadvertently 
contacting a conductor/antenna, shorts of line to grounded case of an 
appliance, someone driving a nail through the wire, etc.  The goal is to 
have the resistance low enough that
a) the breaker will trip or fuse will blow
b) the voltage drop is small enough so that someone won't get hurt or a 
fire start.  Imagine the scenario of standing in bare feet on a wet 
concrete floor and touching something metal. Now assume that there's a 
line/case short, so there's current flowing through the green wire 
ground.  You want the voltage difference between the case and your feet 
to be down in the <10 volt range



> In our county they want to see 20 feet of #4 solid copper laid out along 
> the footing and more or less wrapped around the rebar.  When I built 
> this house I ran 20 feet of #4 copper wire in BOTH directions from the 
> service entrance and I clamped the wire to the rebar with bronze clamps, 
> but who knows whether that added anything.

redundancy is never bad, assuming you've got the budget...

> 
> 73,
> Dave   AB7E
> 
> p.s. to the list:  Yes, I realize that the spec says to keep the 
> electrodes encased by at least 2 inches of concrete, but I figured that 
> was because they accept steel as the electrode and everyone already 
> agrees (I think) that it is a bad idea to have reinforcing steel exposed 
> to a rusting environment.  In this part of the country (true, this isn't 
> Florida), the #4 copper wire often exits the footing below grade before 
> coming up along the wall (usually protected by a PVC tube or something) 
> to the service panel.
> 

probably also for mechanical protection against accidental shovel and 
pick impacts...