"daestrom" wrote in message ...

On 4/6/2011 19:31 PM, m II wrote:
The fault capacity of a household main breaker or fuses is not an issue,
unless very old technology, like you.
One hundred feet of twisted triplex supply cable limits faults to well
within the fault tolerances.


Got some numbers/calculations to support that? Is that including the
next door neighbors with their PV installation?

daestrom

-------------------

Sure! Basic Ohms lawa and a wire resistance table

http://en.wikipedia.org/wiki/American_wire_gauge

A 200 ampere service running 240 Vac and only considering the straight
resistance of copper (many use AL outside conductors these days).
and considering the street transformer as an infinite current supply (0 Ohms
impedance)

The chart shows we would use 2/0 copper (assuming solid copper, but it won't
be)

In a 100 feet of overhead run to a house, down the stack and through the
meter to the main panel, where the fuses or breakers are, not considering
the impedance of the overcurrent devices (that allegedly cannot handle a
fault this big) we come up a with a minimum copper resistance of

200 feet (has to return) x 0.07793 x 10^-3 Ohms / foot (oh look ...your old
units too) = 0.015586 Ohms

Using 240 Vac as the fault supply (it won't be under a faulted condition)
the max fault current would be

240 Vac / 0.015586 Ohms = 15.4 kA.

Now we havent figured in any of the other impedances (very generous) and
any approved O/C device in a panel these days is rated at 100kA. The old
"code" fuses were 10kA and no amount of lowering the impedance of the grid
source using a PV generator attempting make it lower than 0 Ohms impedance
is going to increase that fault current. In real life this fault current
would be below 5kA after connection impedances, transformer winding
impedance, primary impedance, ferrous openings, smaller conductors used by
the utility that uses free air rating on smaller conductors, etc.. etc...

Engineering people do not worry about fault currents at residential services
unless special circumstances apply, like within a few feet of a commercial
busbar splitter without enough wire in between. Then they know how to close
their eyes and say "Nobody told me."


Mike