On Jul 13, 2:18*am, Owen Duffy wrote:
Tom Horne wrote in news:e802f6fa-b0e1-471b-bf31-
:

Can anyone make a recommendation, based on actual training and
experience, as to what width and thickness of copper strap would be ...

In this part of the world, we have an Australian / New Zealand Standard
(our version if you like of ANSI, BS etc) which explains the rationale
behind lightning protection, a method of estimating the downcurrent for
protection design purposes and a process for designing down conductors.

Broadly, the scheme is that downconductors are designed to withstand a
few donwstrokes in quick succession without melting the down conductor.
If you work from a peak current of 20kA, it would lead to a down
conductor in copper of at least 25mm^2 which is about #2 to you folk.

I regularly see hams recommend much thinner down conductors, and can only
assume that there is not regulatory guidance or requirement, and I wonder
at the effectiveness of using #6 as often recommended, especially
aluminium as is often the case. Note that reducing conductor size is a
double whammy, you increase the resistance (so the power), and decrease
the mass that has to be heated to melting point, and so the energy
required.

But, firstly, you should determine if there are regulatory requirements,
such as NEC etc.

The question of equipotential bonding conductors ought be dealt with in
the same way, though that is not to imply that they will be the same
size.

Owen

Owen
Believe it or not the NEC only calls for number ten American Wire Gage
(AWG) or 5.261 (mm)2 for protective grounding conductors. Bonding
conductors between electrodes are only required to be number six AWG
or 13.30 (mm)2. So leaving aside the bad joke that is the NEC
requirements I'm trying to get some idea of what best practice might
be.
--
Tom Horne, W3TDH

Tom Horne wrote:

Owen
Believe it or not the NEC only calls for number ten American Wire Gage
(AWG) or 5.261 (mm)2 for protective grounding conductors. Bonding
conductors between electrodes are only required to be number six AWG
or 13.30 (mm)2. So leaving aside the bad joke that is the NEC
requirements I'm trying to get some idea of what best practice might
be.


Tom is a bit confused here about the purpose of NEC vs NFPA 780..

The bonding requirements in the NEC are designed to keep the building
from burning down in the event of an accidental fault to an energized
conductor. The basic requirement is that it carry enough fault current
for long enough to trip the overcurrent protection device on the
energized conductor.

It's not for lightning protection, per se. (although NEC bonding will,
incidentally, provide some degree of protection against induced transients)

I'd also note that AWG 10 wire is more than sufficient to carry a 50kA
pulse for the 20 microseconds or so that a lightning stroke lasts
without melting.
Using the Onderdonk equation, you can calculate that a AWG16 copper wire
will carry about 90kA for a 20 microsecond pulse. AWG10 should be able
to carry 4 times that much. AWG6, 10 times, because it scales with cross
sectional area.

Having only really paid attention to this recently, I noticed that in
Rome (a place with a fair number of thunderstorms), they use fairly
small down conductors (AWG 10 or 6, just by eye), and similar for 7
story tall wooden pagodas in Nara, Japan (another place with lots of
thunderstorms).

I'm not quite sure where the fashion for 2/0 grounding conductors comes
from (maybe Phelps-Dodge has a representative on the NFPA 780 review
committee?grin)