uderground duct with

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frenchcomp

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NEC 2005 ampacities table for multi-core cables in underground electrical ducts are designed for Three-conductor cable.
Can I use same table when sizing two-conductor cable (120V/240V)?
I can only find "two or three conductors" in B.310.8 annex B Table but this is only for directly buried cables.
 
Re: uderground duct with

Multi-core cable are copper conductors. I just specify "multi-core" in opposition with single core cable which have specific ampacities tables in NEC.
 
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I would say yes you are safe to use that table as there are less heat generators, or conductors if you will, so you are on the safe side in my opinion. But I would use Table 310.16.

Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 through 2000 Volts, 60 deg C through 90 deg C(140deg F through 194 deg F), Not more than three current carrying conductors in raceway, cable , or earth (directly buried) based on ambient temperature of 30 deg C (86 deg F).
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So I think this table fits perfectly for 2 conductor cable, 120/240 V. :D
 
Re: uderground duct with

Table 310.16 does not apply to underground ducts. Table 310.79 has the type of configuration that frenchcomp has described, except that it applies to higher voltage insulation systems.

I think the question is specifically whether Table B.310.6 can be used for 2 conductors with an overall jacket. My answer, like David?s, is ?yes,? for the same reason that David gave.

The concern that might arise is that you could be overly conservative. That table may give you an unnecessarily low value of ampacity, and make you install a larger conductor than is really needed. That would be safe, of course, but costly.

Your other option would be to use (or pay someone else to use) a software package that can perform a Neher McGrath analysis for the specific configuration you have in mind. Is the analysis worth the cost? Hard to say. You?d have to compare the cost to the cost of using a larger conductor (and perhaps conduit).
 
Re: uderground duct with

I agree with analysis but I need to use tables B.310.6 cause I have to take account mutual heating between ducts on ampacities. Table 310.16 do not.
 
Re: uderground duct with

I didn't read charlie's answer before sending last reply. OK about 2 or 3 cores conclusion. ;)
I also believe software calculation is the solution.
The problem I have is my Client wants me to follow its own duct configuration (not in NEC) and to do cable sizing following NEC requirements. :confused:
I'm not sure he'll accept extra cost if I need to proceed with software package(this will induce extra studies engineering time!!!)
Thanks to all.
 
Re: uderground duct with

Hmm...I always thought that electrical duct was a type of raceway. Live and learn. I did not look at table 310.79 because it was medium voltage, but except for the voltage, it does describe the situation more acccurately I agree.

Here's a question...given the same load and conductor size, would say a 480 Volt circuit conductor give off the same heat as say a 4160 V circuit conductor, or say a 12,470 V circuit conductor?

Ok I am looking at tables 310.16 and table 310.77, and I get two different ampacities for the same size conductor on these two tables. So I guess there is a difference in the heat that is released and it is accounted for. So if the spacing is as such in 310.60 for medium voltage and a Rho value is given in the ampacity of tables for medium voltage, then couldn't one infer that table 310.16 ampacities would hold true and account for mutual heating like the medium voltage tables? And why is the duct arrangement for 0-2000 V in an appendix and not in chapter 3 like it is for medium voltage? Man, I am confused! :(
 
Re: uderground duct with

Being confused is a good sign. It indicates that you are thinking clearly about an unclear situation. Let me take on a few points of confusion, and see if I can help you understand.

First, the voltage rating of a conductor (or anything else, for that matter) is based on the capability of the insulation system to prevent leakage current from the current-carrying materials to the outside world. Secondly, the current rating of a conductor is based primarily on its cross-sectional area, and to a lesser degree on the geometry of its construction. Finally, the generation of heat within a conductor is given by I*2 x R. So if you can push the same current through two conductors of the same resistance value, then you will get the same heat generation.

It might seem reasonable, then, that the three classes of insulation systems that you mention should not, by themselves, have an impact on the heat generation rates of the conductors within them. However, each of the three types of conductors will have different values of resistance. Also, at a higher voltage, you would expect lower currents. So the heat generation rates will not be the same, as neither the ?I? nor the ?R? is the same.

But even if you could select a 480 volt cable and a 4160 volt cable with the same resistance, and even if you push the same amount of current through each, the situation will still not be the same. You will get the same internal heat generation, but the two insulation systems will pass that heat to the outside world at different rates. So what you see in the duct, and in the concrete and dirt surrounding the duct, would still not be the same.

Does this help?

p.s. A duct is a type of raceway. You are right about that. It's just that 310.16 doesn't say "underground raceway," but rather "direct buried." That is a cable surrounded by dirt, and does not involve a raceway.
 
Re: uderground duct with

Originally posted by frenchcomp:The problem I have is my Client wants me to follow its own duct configuration (not in NEC) and to do cable sizing following NEC requirements.
Click to expand...
You can have any duct configuration you like (or that your client likes), so long as you maintain spacing between the ducts Determination of the ampacity is the tricky part.
 
Re: uderground duct with

Thanks Charlie! This discussion is indeed helping very much and I appreciate the help. :) It makes sense that the different insulation systems would pass the heat off in different rates, but some questions on your other responses.

However, each of the three types of conductors will have different values of resistance.
Click to expand...
If we are talking the same circular mils for each of our theoretical cables, is this a significant difference in resistance? Or do you mean the resistance of the insulation of each cable to transmit the heat away from the conductor?

Also, at a higher voltage, you would expect lower currents.
Click to expand...
I agree, but in my hypothetical example, both cables are loaded the same.

So in the end, how big of a difference are we talking about? Let me get my room-sized super-computer fired up! ;)

Oh and I interpreted the title of Table 310.16 wrongly, to mean that the raceway or cable was buried underground, when in fact it meant separate situations: cable in a raceway, or a cable that is direct buried...I think. :eek:
 
Re: uderground duct with

Please excuse my ignorance, and the stupidity of this post, but I am dumb founded at this point!
:(
I have been in this trade for a longgg time, but I always learn something new from this forum. But what I am reading in this post is totally different from what I have been taught. The way I have been instructed on 310.15 C Engineering Supervision (which takes us to annex B)was to ignore it, that tables 310.16 thru 310.19 were the means to size conductors for 0-2000V. The heading 310.15 Ampacities for Conductors Rated 0-2000 Volts A states that ampacities for conductors shall be permitted to be determined by tables OR under engineering supervision, as provided in 310.15(B) and C. I have also been instructed that table 310.16 did include underground installations because it stated conductors in raceways, cable, or earth (directly buried), and that 315.15 C was a means for an engineer to utalize ratings other than or different than that listed in the charts. I realities that conductor ampacities must be corrected for more than 3 current carrying conductors in a raceway and for ambient temperature as listed in the correction factors tables.
I am not ashamed to admit an inadequacy because I was taught this way. But I an wondering if anyone else is supprised with this thread, or am I the only one? :roll:
 
Re: uderground duct with

guesseral
I don't think you need to feel ignorant or stupid
about this issue. It is a very complex problem
and has been discussed a number of times on this forum. If you were installing a single run of cable you could get by using table 310.16. When Samuel Rosch developed 310.16 he did not investigate underground circuits and he did not investigate the effects of proximal heating from adjacent conduits, ducts, and duct banks. This heating from adjacent circuit is the reason that 310.16 can not be used to arrive at conductor ampacity.This is not you usual electrical problem. It is a heat transfer problem and requires complex engineering caculations to determine cable ampacity. The NEC is not an engineering manual and is not able to deal with this problem. You can get some additional information about the development of table 310.16
and this site http://www.electrician.com/articles/t31016.htm
 
look that info in this book Rating of Electric Power Cables: Ampacity Computations for Transmission, Distribution, and Industrial Applications (Hardcover)
 
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