"bud--" wrote in message ...
On 4/11/2011 4:31 PM, daestrom wrote:
On 4/10/2011 22:12 PM, m II wrote:
"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)
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.
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.
Cite where 100kA is required.
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.
I agree that is very likely. One reason is that a higher rating is not
necessary.
(SquareD, if I remember right, has a rating of 20kA downstream from both
the main and branch circuit breaker.)
I doubt many Canadian house panels have fuse protection, or are
different from US panels with circuit breaker protection rated around 10kA.
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
The interrupt rating required goes up with the service current rating.
For a house, the utility is not likely to have over 10,000kA available
fault current. The transformers become too large, many houses are
supplied with longer wires and higher resistance losses, and the system
is much less safe.
I believe it would take a rather massive amount of PV installations to
cause a problem. The PV installations would all have to be on the
secondary of the same utility transformer. The transformer is then not
likely to support the PV current back to the grid. If the fault current
is 20x the transformer full load current, and the PV current is equal to
the transformer full load current, the PV supply would increase the
fault current by about 5% (assuming the inverter doesn't shut down). If
there were too many PV installations the utility could put fewer houses
on a transformer. Seems like a problem that is not that hard to handle
for the utility, at least until PV generation becomes rather common.
--
bud--
-----------------
Perhaps re-read ( or just read ) the last few posts. Your objection is
mostly agreement with items already covered.
Can you cite the percent impedance of the transformers or the code rules
you discuss?
mike