On Apr 11, 10:31*pm, daestrom wrote:
On 4/10/2011 22:12 PM, m II wrote:







"daestrom" wrote in ...

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 haven’t figured in any of the other impedances (very generous)
and any approved O/C device in a panel these days is rated at 100kA.

Only problem with that is that many home service panels use breakers
with an AIR of only 10kA, not 100kA. (my old house, built in 2000 was
10kA, and my new one, built in 2010 is also 10kA, both perfectly correct
by code)

Here's are some modern service panels that come with 10k AIR breakers.http://static.schneider-electric.us/...ad-centers.pdf

And how many homes in the utilities service area are even up to current
code? *I'd bet many homes in many service areas have only 10kA AIR.

The utility that is being ultra-conservative may have to consider that
older homes in their service area may not even support this.

Can you just imagine the hue and cry when some homeowners are told they
have to spend a couple hundred bucks to upgrade their service panel
because of changes in the utility's distribution?

daestrom- Hide quoted text -

- Show quoted text -

Small scale locally generated power makes no difference to these
problems. In fact it helps.