On 1/24/2013 3:35 AM, Robert wrote:
On Jan 24, 2:05 am, wrote:
I think that if there was good evidence that lightening rods DIDN'T
work, we would have heard about the controversy by now. If tall
building are all equipped with lightening rods, that's good enough
evidence for me to believe they do work.

Obviously, you want to ensure that you have a pretty massive cable to
transfer all of that current around the house and into the ground. But,
I'd rather fry a $300 cable than have lightening set my house on fire.

I'd put one in if it wuz my house.

--
nestork

The current may be many thousands of amps, but it only lasts
for a few thousandths of a second. Consequently, the conductor
doesn't have to be massive since the short conduction time
means shorter heating time. Something like #6 wire is usually
enough, even tho it may carry an instantaneous 50,000 amps
or so. This can be confirmed by checking the size of the
grounding wire at your service entrance. Local codes and the NEC call
out the required size.

Your comments on short duration are entirely right. But the NEC applies
to power systems, not lightning rod systems. (NFPA780 is the
installation standard for lighting rods.)

The average lighting strike is about 20,000A. About 5% are over
100,000A. Some are over 200,000A.

The numbers I have seen for down conductors are a little over #3 for
class 1 systems and over 1/0 for class 2. (I have no idea what the
difference between class 1 and 2 is.)

As far as a previous poster commenting on "bonding to the
structure", keep in mind that most houses have wood, non-conducting
construction, and that doesn't apply..... If the structure has steel
girders, it is another matter.

More than just structural steel. For commercial/industrial a rooftop
HVAC unit may have to be bonded if it is within 6 feet of the system
conductors. Cast iron plumbing stack? Service entrance riser? The point
was you can't effectively insulate the rod system from the building.