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Posted to alt.energy.homepower,alt.energy.renewable,alt.home.repair
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Feeding solar power back into municipal grid: Issues and finger-pointing
On Apr 12, 4:44*pm, bud-- wrote:
On 4/11/2011 4:31 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
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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)
This is a fatal flaw in your argument. Transformers are not infinite
sources. A utility transformer might supply a fault current 20x the
rated current (for a "5% impedance" transformer). (While a transformer
will supply a fault current larger than the rated current that is not
likely with PV. PV is basically a constant current source.)
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.
Using a real transformer houses will have far less available fault current.
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