Posted by bud-- on April 15, 2011, 3:24 pm
On 4/14/2011 10:33 AM, harry wrote:
>> On 4/12/2011 10:58 AM, m II wrote:
>>
>>
>>
>>
>>
>>
>>> On 4/11/2011 4:31 PM, daestrom wrote:
>>>> 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)
>>
>>> 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 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.
>>
>>> 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/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.
>>
>>> 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.
>>
>> Perhaps you should take reading lessons. Maybe you and harry could get
>> group rates.
>>
>> - You said "considering the street transformer as an infinite current
>> supply" which no one does.
>> - As a result your calculation is meaningless.
>> - You said Canadian house panels were protected with fuses. I disagree.
>> Perhaps a cite?
>> - You said "any approved O/C device in a panel these days is rated at
>> 100kA". I asked for a cite - still missing.
>>
>> - Daestrom said adding PV systems to residences could result in an
>> available fault current larger than the rating of existing service
>> panels. It is certainly an interesting point, but not likely for reasons
>> stated.
>>
>> I did agree with daestrom that most US house panels are likely to have a
>> 10kA IR.
>>
>> |Can you cite the percent impedance of the transformers
>>
>> 5% impedance would be common
>>
>> | or the code rules you discuss?
>>
>> I didn't discuss code rules.
>>
>> Your 'newsreader' is incompetent at treating sigs.
>>
>> --
>> bud--- Hide quoted text -
>>
>> - Show quoted text -
> Impedance is the vector sum of resistance and reactance measured in
> Ohms not a percentage.
> Regulation of a transformer mau be measured in percentage terms.
> What exactly are yo on about?
Use of "per-unit" values for voltage, current, impedance, ... is common
in the electric power field. It makes calculations easier, particularly
as the system gets more extensive. One of the "per-unit" values is "%
impedance".
A utility transformer is likely to be rated in "% impedance".
Daestrom has written about this. Looks like mII (or whoever) is familiar
with it. That leaves you. If you knew as much as you think you know you
would be familiar with % impedance.
You really need to go for some instruction. These things can't be worked
out by lying on your bed and thinking about it.
--
bud--
Posted by hubops on April 12, 2011, 10:50 pm
< super snips >
>> 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.
The utility needs to plan & build for all the peak-demand periods.
If every single home in a southern Canadian town installed 5-10
kw solar - the utility would still need to plan & design & build -
as if those panels didn't exist.
.. summer air conditioner peaks would be aces !
Air quality - big plus !
.. but the distribution system would still need to be built to
supply the peak at 5:45 pm December 20 -
.. solar output = zero. grid demand = near max.
John T.
Posted by bud-- on April 13, 2011, 3:20 pm
On 4/12/2011 11:59 AM, harry wrote:
>> On 4/11/2011 4:31 PM, daestrom wrote:
>>
>>
>>
>>
>>
>>> 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)
>>
>> 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 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.
>>
>> 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/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.
>>
>> 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--
> In theory transformers can carry any load.
If you are talking about normal load ratings - what a useful revelation.
I am sure no one had any idea...
> In practice there are losses due to resistance of the copper wire and
> in the iron core that cause heating. How swiftly this heat can be
> dissipated is one load carrying limtation. How much heat the
> insulation can stand is another.
There is a limit on the normal current for a transformer?
I had no idea....
But heating is not a limit on fault current (which my post was almost
entirely about).
> Fault currents are determined by assessing the "loop" resistance of a
> worst case electrical fault to earth and also as a dead short. These
> can be determined by calculation or by instruments.
Earth is not calculated because it is such a poor conductor. It may be
necessary in some of the screwier UK electrical systems with an RCD main.
> So any switch or circuit breaker has two current ratings.
> It's normal rating that it will carry continuously and interrupt many
> thousands of times.
> It's fault current rupturing capacity. Often several thousand amps. It
> will only break this current a very limited number of times and carry
> the current for milliseconds.
With minimal reading ability it is obvious that daestrom, mII (or
whoever) and I talked about the fault current ratings of circuit
breakers or fuses. Or did you think that houses have 10,000A services?
> You really need to go for some instruction. These things can't be
> worked out by lying on your bed and thinking about it.
I worked them out 40 years ago then worked with them the last 40 years.
You really should learn to read and think. Maybe when cows fly....
--
bud--
Posted by m II on April 7, 2011, 4:30 am
daestrom wrote:
> One issue that utilities worry about is the available fault current.
Please be informed that the Josepi clown has been forging my username
for a few weeks now. His provider is doing nothing to stop the forgeries.
Check the headers when in doubt. It's times like these I wonder about
the maturity levels of some, no doubt very ill, people.
mike
Posted by m II on April 7, 2011, 4:34 am
daestrom wrote:
> One issue that utilities worry about is the available fault current.
Please be informed that the Josepi clown has been forging my username
for a few weeks now. His provider is doing nothing to stop the forgeries.
Check the headers when in doubt. It's times like these I wonder about
the maturity levels of some, no doubt very ill, people.
mike
>>
>>
>>
>>
>>
>>
>>> On 4/11/2011 4:31 PM, daestrom wrote:
>>>> 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)
>>
>>> 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 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.
>>
>>> 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/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.
>>
>>> 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.
>>
>> Perhaps you should take reading lessons. Maybe you and harry could get
>> group rates.
>>
>> - You said "considering the street transformer as an infinite current
>> supply" which no one does.
>> - As a result your calculation is meaningless.
>> - You said Canadian house panels were protected with fuses. I disagree.
>> Perhaps a cite?
>> - You said "any approved O/C device in a panel these days is rated at
>> 100kA". I asked for a cite - still missing.
>>
>> - Daestrom said adding PV systems to residences could result in an
>> available fault current larger than the rating of existing service
>> panels. It is certainly an interesting point, but not likely for reasons
>> stated.
>>
>> I did agree with daestrom that most US house panels are likely to have a
>> 10kA IR.
>>
>> |Can you cite the percent impedance of the transformers
>>
>> 5% impedance would be common
>>
>> | or the code rules you discuss?
>>
>> I didn't discuss code rules.
>>
>> Your 'newsreader' is incompetent at treating sigs.
>>
>> --
>> bud--- Hide quoted text -
>>
>> - Show quoted text -
> Impedance is the vector sum of resistance and reactance measured in
> Ohms not a percentage.
> Regulation of a transformer mau be measured in percentage terms.
> What exactly are yo on about?