let thru current

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mpd said:
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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I agree with your actions based on these facts.
 
quogueelectric said:
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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You are listing methods of calculating Arc Flash Incident Energy, not methods to calculate short circuit (fault) current.
 
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
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mpd said:
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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Emphasis is mine - cf
You have already answered your own question. You disagree. You failed it. No need to go past that. You are not going to change your mind just because someone points out the inconsistancies in your reasoning.

As for technical justification, I don't know how anyone could give you a good answer. Your information about the system is all over the place. Some of it would have to be built from unobtainium to come up with those numbers. I'm thinking we are only getting part of the story.

From your responses, I can't tell if the engineer mentioned in passing there were current limiting fuses in the service disconnect, or if his calculation shows that the calculated let through of the CLF is needed to reduce the SSC at the panel. By your own words, you didn't get into the analysis enough to understand.

For all I can tell the current limiting fuses may be in there to protect the service/feeder cables. At 50kA, 500kcmil gets into cable damage within a few cycles. Maybe there is enough impedance the SSC is down for faults after the panel - I can't tell any different from your responses.

You are looking for a response to justify your decision. Okay.

Given:
1. The system description is incomplete and inconsistent.
2. The inspector either does not understand or did not take time to understand the fault current analysis.
3. The engineer either mentioned in passing there are current limiting fuses in the system, or stated in his calculation the CLF are needed to reduce the fault current at the panel to below the CB rating.
4. The engineer stated the panel, (and CBs), are fully rated. The inspector disagreed.

Based on items 1 ? 4, I agree with your actions.
cf
 
cold fusion

did you understand the question, all i am asking is it code compliant to design a brand new service using let thru current of current limiting fuses at the service disconnect to protect the branch breakers about 30 ft away, and call it a fully rated system, i cited 240.86 tell me where i am wrong, i am just looking for info, so how about getting off your high horse
 
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cold fusion

i am still waiting for your expert answer, is it code compliant to use the let thru current of current limiting fuses to protect downstream branch breakers and call it a fully rated system?
if you can put your ego aside, or your problem with an inspector questioning an engineer i would like to hear your answer, i asked a question that you have had plenty of arrogant responses but no answer, like i said i cited 240.86, tell me where i am wrong, this post was not meant offend anybody i am interested in learning for the future if the situation arises again, thank you in advance again
 
mpd said:
cold fusion

i am still waiting for your expert answer, is it code compliant to use the let thru current of current limiting fuses to protect downstream branch breakers and call it a fully rated system?
if you can put your ego aside, or your problem with an inspector questioning an engineer i would like to hear your answer, i asked a question that you have had plenty of arrogant responses but no answer, like i said i cited 240.86, tell me where i am wrong, this post was not meant offend anybody i am interested in learning for the future if the situation arises again, thank you in advance again
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Sigh -
I'm not ignoring you, just was working on a decent answer. As you said (paraphrase), I was mouthy enough that you deserve some payback - Say like a good answer. And I've been working on that even before your last post.

So, I was a 1/2 hour into a response and screen blinks and the response is gone. So I figure someone is trying to tell me something and I had better listen - Not sure what to do, I truly thought I had all the stuff in blue covered.

So I start a second response and fat finger the keyboard at a very inopportune time. There is no way I can edit in the time alowed, so I delete. Hummm, I'm still thinking my Higher Power is trying to tell me my response is wrong.

So, I'll try this and see if this one makes it through:
Answer to Q in red: No
If there is not a communication problem, you are dealing with one screwed up engineer - In either case, I'm sympathetic.

Apparently I had to leave out anything to do with technical issues to get this to go through - If it does infact post.

cf
 
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Cold Fusion said:
So, I'll try this and see if this one makes it through:
Answer to Q in red: No
If there is not a communication problem, you are dealing with one screwed up engineer - In either case, I'm sympathetic.

Apparently I had to leave out anything to do with technical issues to get this to go through - If it does infact post.

cf
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thank you very much, I apologize for sarcastic remarks, this situation is just very interesting to me, i have no problem with the engineers design, just if it is code compliant and like i said it has never come up before, and i was also thinking that if it is a fully rated system it would not require any field labeling like a series system would, but if you have anymore more info that applies to this please post
 
mpd said:
---, but if you have anymore more info that applies to this please post
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Just some comments on the technical end. Most of this has been covered and will be repetitive from previous posts. And I had understood you to say you weren't really interested in the technical aspect. But perhaps I mis-translated you - I'm capable of doing that.

So:
There just isn't enough information to understand the system. For example we don't even know if it is 480 or 208. You haven't even said if it is three phase - I guess it could even be single phase 120/240. Considering my lack of system information, anything I'm saying is a wild guess.

A service with 70kA available SSC feeding a 600A panel is really a strange duck.

It's possible, just nothing I have ever worked with (on, around, seen). Maybe it is one of those high density commercial grid connected things with cable limiters that was being discussed on another thread - which is an example of, "nothing I have ever worked on".

You mentioned the feeder to the panel is 350kcmil: per ICEA, at 70KA, cable damage starts around 4 cycles; at 50kA around 8 cycles. Even if the panels and CB can stand the fault current, one would still want to get the circuit open really quick. As I said earlier that could be a reason why there are CLF in the design.

This is the kind of stuff that doesn't make sense that makes me wonder if the main issue is not mis-communication.

cf
 
cold fusion

the system is a 120/208 3 phase system, the fault current at the utility pad mount trans was supplied by the utility, the engineer calculated the fault current at the service disconnect about 15ft away and the fault current at the 600 amp MLO panel about 30ft away from the service disconnect, the pad mount does not just feed this future tenant space it also feeds part of the shopping center, the problem is the engineer is trying to make this work because they cannot find a series system that will work with the panelboard, why it is already installed is another story, i was really interested in the use of the CLF's to protect the branch breakers and calling it fully rated
 
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