Another lesson learned

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JFletcher

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Williamsburg, VA
Looks like pre 1982. Coinflation.com

Actually mid-82. Easy way to tell w/o looking at the coin: copper pennies ring when dropped on glass or another hard surface; zinc pennies make a "thud". It takes 154 copper pennies to get a pound of copper (and some zinc) so anytime copper is >$1.54/pound, a penny is worth more than a penny in metal value.

Forget getting rich yanking copper pennies out of circulation: you might see one or two out of every roll. and last time I checked, melting pennies was illegal.
 
I would think that everything on the customer side of the point of common coupling would be governed by the NEC and a consistent set of rules within it.

Everything looked fine until I looked up at the pole transformer......10kva transformer. That unit is going to get warm on a bright sunny day.
Luckily the POCO swapped out for a 25kva without charge to the homeowner.
Good example of of what ggunn is talking about.

ggunn- Thanks for the real world example there, I just narrowly avoided the problem by accident, strangely enough.

Carultch- Yes on the first part and yes on the other comments about the consistent-ness! Combined with Dan's quote, *now* I understand a POCO "guideline"- there's a "simplified process" for systems under 10kWAC.
Simple is either under 10kW single phase and under 25kW 3ph...
The common "providing 10kVA xfmrs for residential services" thing is the reason for that- 10kVA is only 83.3A @ 120V.
edit: Which is kind of cool- 90% of 83.3 is 75A @ 120v- which is exactly the max output of the 9kW inverter! Nice.

Dan- So how could you tell by just looking? Because of the size? I'm looking at one- it does look a little smaller than a nearby 25kVA...but it's also quite a bit newer. Hmm.

Another real world example, regarding the simplified process.
Inverters =<10kWAC/10kVA (or =<25kWAC/kVA 3ph) are the simplified.
However, one particular brand of inverter says the inverter(s) have to be =<90% the size of the interconnection xfmr.
So you could not technically use a 10kW inverter of that brand on a house that has a 10kVA xfmr.
I was comparing 8,9 and 10kW models (with 10kwDC of panels on each) and decided on 9kW *before* I saw this thread. They all ended up with pretty similar outputs, there was a little too much clipping with the 8kW, and the 10kW put out such a small amount more it didn't seem worth it. (Yes, the 10kW put out more than the 9kW inverter, with 9.977kWDC of panels on each, which I don't think it would everywhere- something about the weather I suppose)

Depending on which type of panels and differences in amps @lsc, the 10kW was a option- but now I know it isn't really a possibility with a 10kVA service xfmr.

Thanks, people! :)
 
Good answer. However, service conductor sizing is governed by the NEC. Not by the utility.

Ok, here's another somewhat related question, about service main disconnect breakers. (which the AC PV disco counts as, #2 of 2)
If the POCO requires - amperes RMS Symmetrical =>10,500 for service disconnects...that's *not* the NEC, correct?
But...when the choices are 10kAIC or 14kAIC breakers, you'd have to go with 14, even if the load-side fault current was very low?

Just like as in the NEC, 90.5A would require a 100A breaker when there's no 95A option but there is a 90A?
I'm not seeing the fault current part of the code.

The POCO rules do say NEC, just not which part:
Consideration for future load growth and subsequent transformer change-out may require initial installation
of service equipment to have a larger fault current interrupting rating to ensure its suitability according to the
NEC.
 
Ok, here's another somewhat related question, about service main disconnect breakers. (which the AC PV disco counts as, #2 of 2)
If the POCO requires - amperes RMS Symmetrical =>10,500 for service disconnects...that's *not* the NEC, correct?
But...when the choices are 10kAIC or 14kAIC breakers, you'd have to go with 14, even if the load-side fault current was very low?

NEC 110.9 requires that equipment intended to interrupt a fault have a rating at or above the Fault current. A utility could have their own minimum requirements

Just like as in the NEC, 90.5A would require a 100A breaker when there's no 95A option but there is a 90A?
I'm not seeing the fault current part of the code.

I dont think you mean 90.5(A), maybe you mean 240.4(B)? See 110.9

The POCO rules do say NEC, just not which part:
Consideration for future load growth and subsequent transformer change-out may require initial installation
of service equipment to have a larger fault current interrupting rating to ensure its suitability according to the
NEC.

That is a reasonable issue, that the POCO changes or reroutes their equipment which results in a higher Fault current that may exceed the NEC governed equipment.
 
1.NEC 110.9 requires that equipment intended to interrupt a fault have a rating at or above the Fault current. A utility could have their own minimum requirements

2. I dont think you mean 90.5(A), maybe you mean 240.4(B)? See 110.9

3. That is a reasonable issue, that the POCO changes or reroutes their equipment which results in a higher Fault current that may exceed the NEC governed equipment.

1. Thanks! 110.9 is what I was looking for. So when the POCO is saying 10,500A RMS, 10kAIC is a no-go.

2. No! Sorry, I meant as in if you have a circuit of 90.5 amps, you'd have to use a 100 amp breaker, since there's no 95 amp breaker.
I was trying to compare that to having to up-size to the next kAIC breaker- if the POCO says 10,500A, you have to go with a 14kAIC breaker.
Yep, thanks, I was looking at 240.4(B)- it mentions straight amps, not kAIC, but I was thinking that of course kAIC would be the same.

3. So...240.4(B) says "next highest OCPD rating". That would mean that a 14kAIC breaker is the only option, unless the fault current is over 14kAIC, then you'd have to use 22kAIC. I think?
 

DanS26

Member
Location
IN
Dan- So how could you tell by just looking? Because of the size? I'm looking at one- it does look a little smaller than a nearby 25kVA...but it's also quite a bit newer. Hmm.


In my neck of the woods here in SE Indiana the local REMC has the transformer size painted in rather large numbers on the side of the transformer....10 15 25 or 50 for residences and most farms.

Sometimes when I'm driving around I compare the transformer size to the residence. It gets a little suspisious when a little side by side mobile home with aluminum foil on the windows has a 25 or 50 kVa hanging outside;).

Anyone here watch "Trailer Park Boys"?
 
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Smart $

Esteemed Member
Location
Ohio
So then in another example, if the POCO is calling for 10,000A RMS protection for a different xfmr bank, and the customer side is under 500A fault current, a 10kAIC breaker is fine?
The interrupting rating of an overcurrent device, the one typically with kAIC appended, is the level of fault current under which the overcurrent device is claimed to operate as designed. If the Available Short Circuit Current (a rating oft abbreviated SCCR) at the line terminals is greater than the interrupting rating, the device could fail to operate and possibly explode.

Do not confuse the interrupting rating with the overcurrent rating. They have very little to do with each other.
 

Smart $

Esteemed Member
Location
Ohio
The interrupting rating of an overcurrent device, the one typically with kAIC appended, is the level of fault current under which the overcurrent device is claimed to operate as designed. If the Available Short Circuit Current (a rating oft abbreviated SCCR) at the line terminals is greater than the interrupting rating, the device could fail to operate and possibly explode.

Do not confuse the interrupting rating with the overcurrent rating. They have very little to do with each other.
Let me add a little clarification to the above, and to correct my own terminological errors.

Interrupting Rating is covered under 110.9.

Short-Circuit Current Ratings are covered under 110.10 and made a title term in the 2011 edition. The definition was added in the 2008 edition.

The next evolution in this area was the addition of 110.24 Available Fault Current in the 2011 edition. It entails field marking requirements and remarking after modifications.
 

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mivey

Senior Member
Good answer. However, service conductor sizing is governed by the NEC. Not by the utility.

I would think that everything on the customer side of the point of common coupling would be governed by the NEC and a consistent set of rules within it.
If it is customer side then NEC. I believe GoldDigger was talking about the POCO transformer and POCO service conductors, which are not necessarily sized using NEC calcs or tables.
 
Let me add a little clarification to the above, and to correct my own terminological errors.

Interrupting Rating is covered under 110.9.

Short-Circuit Current Ratings are covered under 110.10 and made a title term in the 2011 edition. The definition was added in the 2008 edition.

The next evolution in this area was the addition of 110.24 Available Fault Current in the 2011 edition. It entails field marking requirements and remarking after modifications.

Thanks for the help...a bit more clarification?
So... 110.9 is referring to the fault current available from POCO transformers, plus any on the customer side, and the kAIC of the breaker needed for that, whereas 110.10 is referring to...short circuits on the customer's circuits only?

If so, the kAIC of the OCPD breaker for PV inverter protection (line side connection) and the kAIC of the main breaker used for loads supplied by the grid and the PV could be the same kAIC, or different, depending on the situation/level of short current from 110.10?

Another thing that's confusing- if for instance you had 12,200A of fault current available from the POCO and another 10,000A on the customer side for a total of 22,200...
That's also 22.2 kAIC- do you drop the .2 and use a 22kAIC breaker, or jump up to 25kAIC?

Thanks again.
 

Smart $

Esteemed Member
Location
Ohio
Another thing that's confusing- if for instance you had 12,200A of fault current available from the POCO and another 10,000A on the customer side for a total of 22,200...
That's also 22.2 kAIC- do you drop the .2 and use a 22kAIC breaker, or jump up to 25kAIC?

Thanks again.
You can't drop anything. Your interrupting rating must equal or exceed, and much preferably exceed the maximum available fault current at that location.
 
You can't drop anything. Your interrupting rating must equal or exceed, and much preferably exceed the maximum available fault current at that location.

A ha! Thanks, it makes perfect sense now. Because you use the 125% calc for the load amperage, but not for the fault current! So a 90A breaker is OK for 90.2A of load, if 90.5A you go up a size, while 22.2kAIC always means a 25kAIC breaker.

Thanks again.
 
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