I have a situation where a new service was installed, the available fault current at the load side of the service disconnect at a main lug panel is 50k, and 22k branch breakers were installed. i failed the installation, the engineer sent me a calculation that the let thru current of the current limiting fuses at the service disconnect is 14k at the main lug panel and says that panel is fully rated, i disagree, i say he must use a series rated combination or fully rated would be branch breakers rated 50k or more, I have cited 240.86 but he disagrees, looking for some info, thanks in advance
let thru current
- Thread starter mpd
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I agree with you. "Fully rated" would be with each and every breaker rated at the system AIC. If he wants to "series rate" thru a fuse fine and dandy if he provides you with the manufacturer documentation showing the listing.
Cold Fusion
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This is always an interesting topic. Most here attack with a religious fever bordering on the inquisition.
Always fun to read.:smile:
Anyway:
Have you seen the fault current study - Etap, maybe skm? A lot of the reasoning is probably in that.
The details of the calculation matter a lot. You have only given us three pieces of information:
50kA SSCA at the service
Current limiting fuses in the service disconnect
Panel (located somewhere) has 22kA CBs
That is not enough information to give you good answers.
I'd be curious on how far it is from the fused disconnect to the MLO panel - and the wiring method.
Side note, just curious: As the AHJ representative, would an existing building with 22kA rated equipment getting a new service fall under the 240.86.A?
Note: Yes I'm familiar with the issues of overlapping instaneous trips with fuse clearing times.:smile:
cf
Cold Fusion
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And if the panel is five hundred feet away on 4/0 in rigid steel, would the panel cbs still have to be rated at the service AIC? Perhaps there is some impedance between the two
I'm not saying you are wrong - I'm saying we have incomplete information to make blanket judgement calls. And you could well be right - but not just because there current limiting fuses upstream of breakers.
cf
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mivey
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Cold Fusion
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Even if it wasn't Friday, it would still be true.Well, that has never stopped us before.
Sorry, it's Friday.
cf
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There are no currently available UL 489 listed breakers that may be applied, at less than rated AIC, using the "let-through" of a fuse. All series ratings of molded-case circuit breakers require testing, not engineered calculations.
(UL does allow the use of some series ratings based on "let-through" currents in UL508A control panels.)
(UL does allow the use of some series ratings based on "let-through" currents in UL508A control panels.)
the electrical panel is about 30ft away from the fused service disconnect, it is 600 amp, parallel 350's, the fault current was calculated by the engineer, its about 70k at the pad mount which is only about 15ft from the service disconnect, if this was an existing service that the fault current had increased I agree the engineer could use 240.86 (A), but this is brand new from the pad mount to inside, if he is going to design a service using let thru currents isn't that still a series rated system and the NEC only permits that on existing installations
thanks jim I typing as you were writing
Cold Fusion
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That's a good start, now how about the rest of it. As I asked earlier, have you seen the fault current study? Is there any impedance shown for the fused disconnect - or the feeder from the fused disconnect to the panel?
What jim said is true - which is significantly different from "current limiting fuses upstream from CBs is wrong"
cf
as the inspector i do not need to get involved in all that, the engineer submitted the calculation to me, and wants to use the let thru current he calculated as the available fault current at the main lug panel, and call it a fully rated system, i disagree
Cold Fusion
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i would agree with that, but when i read this part:
That says that since you did not need to get involved with understanding the calculation, maybe all you heard was, 'blah, blah, current limiting fuses, blah, blah, cbs downstream are fully rated" And you promptly said, "I disagree"
And you are likely right. But with out understanding the calculation - I couldn't say that.
I don't see that as a problem, as mivey said there are plenty here that will agree with you without having the data.
cf
what you have to understand the engineer put all the available fault current calculations on paper from the transformer to the service disconnect and to the load side panel, and calculated the let thru current at the branch breakers at 14k, all i care about is the fault current he shows, how he does the calculation is his responsibility, i disagree because he wants to call it a fully rated system, maybe i am wrong but didn't he design a series rated system, that is only permitted on existing installations, if this is permitted why would you need series rated systems
Cold Fusion
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First, I agree that (2008) 240.86.B requires tested combinations.
However I am surprised with your statement about, 'There are no currently available UL 489 listed breakers that may be applied, at less than rated AIC, using the "let-through" of a fuse', since UL489 specifically tests:
cf
quogueelectric
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NFPA 70e 2009 has different methods of calculations for available fault current. Some engineers use open air some use blast in a box IE panel/trough. There is also IEEE which seems to be the prefered method so each is subject to interpretation.
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UL series ratings are determined through actual testing of fuse-breaker combinations, they are not determined by calculations involving "let-through" current.
Cold Fusion
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mpd said:
Anybody run any of the numbers on this? Here is a bunch of wild guesses:
It's a service, so the xfm belongs to the utility.
70KA at the pad xfm:
A 500kva, 208V, 2% is about 60kA (infinite primary). FLA would be 1400A.
If it's 480, is gets even weirder. To get 70ka, we are looking at a 4MVA xfm, 7%, FLA ~ 4800A
Feeder from the service is 600A. The panel is an MLO so the 600A fuses are in the service disconnect.
None of this matches up. I'm pretty sure I don't understand the system.
Anybody got any ideas?
cf
like i said how he determined the fault current is his responsibility, the bottom line is the engineer wants to use let thru current at the branch breakers and call it a fully rated system, i disagree and cited 240.86, it is not an existing installation, that is the question i am asking, is this code compliant
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