Posted by m II on April 11, 2011, 2:12 am
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. 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
Posted by daestrom on April 11, 2011, 9:31 pm
On 4/10/2011 22:12 PM, m II wrote:
> 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/assets/DIGEST/load-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
Posted by m II on April 12, 2011, 1:45 am
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/assets/DIGEST/load-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
-----------
Well that situation would be unfortunate and impossible to regulate as
`legal, not conforming`
This is not a problem here as 10kA hasn`t been passed for many years. I
believe any Canuck panels have to to have the class `R`or rejection fuse
holders so that only the nasty electricians can force an old `code` fuse
into the holder. The 100kA fuses have been promoted for a few decades with
the seriousness getting more severe in later years. I thought they were
actually not allowed, here, anymore. This may be incorrect. More research
would be required to verify.
Either way the 10kA doesn't take much more impedance to drop it down with a
few added factors mentioned in my previous text. Most of our service feeds
are (ACSR) aluminum conductor, steel reinforced, and present higher
impedances. I don`t see some small cogen circuit at the end of a few hundred
feet of grid (mine plus yours) being a fault capacity concern in a
residenial environment.
Having said that I guy up the street is just finishing installing 200kW or
more of PV panels. Wait until they produce nothing all winter as they were
snowed in most of last winter being at a low slope. The hook up wasn't`t
completed so he won`t find out until next winter...LOL See how much
harmonic crap we get on the street when they go online.
mike
Posted by m II on April 12, 2011, 2:07 am
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Posted by m II on April 14, 2011, 2:45 am
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> unless very old technology, like you.
> One hundred feet of twisted triplex supply cable limits faults to well
> within the fault tolerances.