Jim Higgins wrote:
On Mon, 19 Jul 2010 09:23:53 -0700, Jim Lux
wrote:

Tom Horne wrote:

Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke
Perhaps you could explain why you think it's a bad joke? Do you think a
13 square mm conductor couldn't carry the strike current? (it can)

Or, perhaps, you're thinking that there are some other design criteria
that might push one towards a larger conductor (mechanical strength in
the face of icing and storms might be one).

Maybe E=IR has something to do with wanting a larger conductor. The
voltage between the strike point and true ground is going to be the 20
- 100 kA of the strike times the resistance of the down conductor from
the strike point to true ground. With a smaller conductor,
fewer/shorter ground rods, or other conditions that raise the
resistance of the path to ground that voltage will be higher and if
high enough the strike will seek additional paths to ground by arcing
to nearby objects closer to ground potential.


Resistance isn't actually a big deal here. It's all about inductance on
that microsecond rise time pulse. And there's not much difference in
inductance between a AWG 6 and 2/0 (it's very weakly dependent on
diameter and strongly dependent on length.. 1 microhenry/meter is a good
estimate, pretty much independent of diameter)

The other problem is that for fast transients, skin effect means that
the AC resistance goes more as the diameter than as the cross sectional
area (hollow tubes work just as well as solid conductors).

So, the net effect is that you need to design for several things:
1) the wire not melting..
2) The wire not breaking from mechanical impact (ladders hitting it,
lawnmowers, etc.
3) The wire not breaking under electromagnetic forces (this is why you
don't want loops and why NFPA 780 says 8" bend radius.. while a 1
microsecond pulse at 10kA won't melt a AWG 10 wire, if it's in a loop,
it will destroy it from EM forces)

You'll see heavier conductors where they have to be able to move (say on
a gate or actuated device), not only for mechanical life, but also
because the flexible wire is more subject to destruction by EM forces.

Side flash is a consideration, but usually accommodated by making sure
your downleads are far from potential victim circuits.