I had a contractor tell me that the ampacity of 5 sets 4-500kcmil in a duct bank was determined from table 310.16. I told him that one must (should) use table B310.7. He says that Annexes are not mandatory (90.3). So, I say what the heck are they there for if not to use under special heat transfer conditions of concrete-encased duct banks; also, tables 310.77-86. My argument is in 310.15(A)(1). I have the handbook that has all kinds of nifty explanations that are not in the code. So, aside from the fact that I'm the engineer & determine from my training that I want to use B310.7, what's my recourse?
underground duct banks
- Thread starter stanrat
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Re: underground duct banks
My guess is that it is a 1600A feeder and maybe copper conductors. The maximum load is probably less and even from annex B, the system is conservative. If there is a real potential of overheating the cables, were the plans yours and did you specify larger wire? Who's problem is it if they burn up over time? Who buys?
My guess is that it is a 1600A feeder and maybe copper conductors. The maximum load is probably less and even from annex B, the system is conservative. If there is a real potential of overheating the cables, were the plans yours and did you specify larger wire? Who's problem is it if they burn up over time? Who buys?
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- Lockport, IL
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- Retired Electrical Engineer
Re: underground duct banks
I hope that I am misinterpreting this statement, or that you have made in jest. No one in our business, not an engineer, nor an electrician, nor an inspector, has any business asking that question. We are all professionals; we do not design or install something that we know is wrong, and atempt to justify it by saying that we will not have to pay for the repairs.
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- Lockport, IL
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- Retired Electrical Engineer
Re: underground duct banks
But first, let?s talk code. B310.7 is not a code requirement. 310.77-86 apply to conductors rated over 2000 volts. So where in the enforceable sections of the code does it address the possibility that a 600 volt conductor might be in need of a de-rating due to the heat imposed upon it by nearby conductors in the same ductbank? Where? Nowhere, that?s where.
Here is how I resolve the issue in my own mind. If you calculate the load per 220, and if you look in 310.16 for a set of conductors that can handle the calculated load, you are OK. If you determine the existing load at an existing facility from measurement, and if you wish to determine if you have room to add more load, and if you look in 310.16 to see if the existing set of conductors can handle the ?measured plus newly added? load, you are NOT OK.
The difference comes from an (admittedly) weak argument, as follows:
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Back to your question: What recourse do you have? You already know the answer to this question. You have a duty to the owner and to the future occupants of the facility, to create a design that is safe. If you think you are being pushed into an unsafe direction, your first recourse is to appeal to the owner, and your next recourse is to report the situation to the Authority Having Jurisdiction.
I?m an engineer, too. This topic has come up several times in this forum. You can do a word search and find several related discussions. I have often stated that I essentially agree with the position you are now advocating.
But first, let?s talk code. B310.7 is not a code requirement. 310.77-86 apply to conductors rated over 2000 volts. So where in the enforceable sections of the code does it address the possibility that a 600 volt conductor might be in need of a de-rating due to the heat imposed upon it by nearby conductors in the same ductbank? Where? Nowhere, that?s where.
Here is how I resolve the issue in my own mind. If you calculate the load per 220, and if you look in 310.16 for a set of conductors that can handle the calculated load, you are OK. If you determine the existing load at an existing facility from measurement, and if you wish to determine if you have room to add more load, and if you look in 310.16 to see if the existing set of conductors can handle the ?measured plus newly added? load, you are NOT OK.
The difference comes from an (admittedly) weak argument, as follows:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Premise A: The NEC 220 calculated load is higher than the true load that the facility will see in real life. This is a step toward conservatism, and helps us.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Premise B: The NEC 310.16 ampacities are lower than they should be, for use in underground ducts. This is a step away from conservatism, and works against us.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Conclusion: The two steps cancel each other out.</font>
Back to your question: What recourse do you have? You already know the answer to this question. You have a duty to the owner and to the future occupants of the facility, to create a design that is safe. If you think you are being pushed into an unsafe direction, your first recourse is to appeal to the owner, and your next recourse is to report the situation to the Authority Having Jurisdiction.
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- Mission Viejo, CA
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- Professional Electrical Engineer
Re: underground duct banks
Charlie B,
I think this is the first time I fully understood your distinction between "calc'ed" and "measured" loads. It was an epiphany
Historically, many of the duct banks we dealt with were so large and complex, we routinely did Neher-McGrath calcs, even on the simple ones. If we used 220.35 to determine feeder loads we still did Neher-McGrath conforming calcs to determine ampacities.
Before we had easy access to Neher-McGrath software, we commonly derated conductors in duct banks to 30%. For refineries this wasn't a significant cost. Often, the feeders and branch circuits were so long, voltage drop was the determining factor anyway or close enough that the derating made little difference. [Edit Add:] Typically, our biggest problem was finding space for the duct banks - not derating the conductors
As for the rest of your replies, I hope it doesn't surprise you that I agree wholeheartedly. I would also add, if you are a PE you are an (if not the) AHJ; i.e., you are still "...an individual responsible for approving equipment, materials, an installation, or a procedure." In most jurisdictions, a PE is obligated to notify the "legal" AHJ, if a known safety hazard is unresolved.
[ August 23, 2004, 02:30 PM: Message edited by: rbalex ]
Charlie B,
I think this is the first time I fully understood your distinction between "calc'ed" and "measured" loads. It was an epiphany
Historically, many of the duct banks we dealt with were so large and complex, we routinely did Neher-McGrath calcs, even on the simple ones. If we used 220.35 to determine feeder loads we still did Neher-McGrath conforming calcs to determine ampacities.
Before we had easy access to Neher-McGrath software, we commonly derated conductors in duct banks to 30%. For refineries this wasn't a significant cost. Often, the feeders and branch circuits were so long, voltage drop was the determining factor anyway or close enough that the derating made little difference. [Edit Add:] Typically, our biggest problem was finding space for the duct banks - not derating the conductors
As for the rest of your replies, I hope it doesn't surprise you that I agree wholeheartedly. I would also add, if you are a PE you are an (if not the) AHJ; i.e., you are still "...an individual responsible for approving equipment, materials, an installation, or a procedure." In most jurisdictions, a PE is obligated to notify the "legal" AHJ, if a known safety hazard is unresolved.
[ August 23, 2004, 02:30 PM: Message edited by: rbalex ]
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