Use of Annex B tables for LV Ductbanks

P

philly

Senior Member
When evaluating ampacity of underground ductbanks for 480V I understand that the code allows for "engineering supervision" (calcs etc..) or provides tables in Annex B as informative information to use/consider for these ampacity calculations. A couple of questions here I was hoping to get some refreshment guidance on:

1) When considering ampacity of underground ductbanks that match one of the arrangements in figure B.2(1) in Annex B do most use the tables in Annex B when access to other engineering evaluation (calculations, software, etc...) is not readily available?

2) For an application where I have (3) single conductor cables in a conduit in underground ductbank Table B.2(7) appears to be the table that would be applicable. When I look at the cable ampacities for a single electrical duct arrangment I notice that the conductor ampacities are actually greater than those listed in Table 310.16. I would expect the tables in Annex B for underground cable to have lesser ampacities given the need to cable to dissipate heat through earth etc...?

3) For Table B.2(7) it appears to have the correction factor table built into the overall table itself as opposed to referencing 310.15(B)(1). I'm assuming that when using these tables you are to use the built in correction factors? Is there a reason why they are listed separately for these underground tables?

4) Although not applicable to my application I notice that Annex B also has tables for cables in free air. I'm curious why these free air tables are listed as part of his Annex if free air values are given in Table 310.17? Are there additional considerations that are included as part of these free air cables in Annex B?

Thanks
 
Unless recently changed, for LV cables, the NEC does not *require* derating for mutual heating effect in underground duct banks. The standard ampacity tables can be used, along with derating for more than three CC conductors, etc. The underground duct bank derating due to mutual heating is only required for MV cable.

In my experience, for 480 V circuits in duct bank, the table values are used. It doesn't seem logical, but in the real world, I've never heard of a problem of cable failures in these situations. I think this is due to the extremely conservative load calculations and cable sizing required as a result by the NEC.

Decades ago - mid 1980s?, the NEC was changed to required derating of LV cables in duct banks. This lasted only a few months (IIRC) and the general outcry was so loud that they backed down and issued a change.
 
David Castor said:
Unless recently changed, for LV cables, the NEC does not *require* derating for mutual heating effect in underground duct banks. The standard ampacity tables can be used, along with derating for more than three CC conductors, etc. The underground duct bank derating due to mutual heating is only required for MV cable.

In my experience, for 480 V circuits in duct bank, the table values are used. It doesn't seem logical, but in the real world, I've never heard of a problem of cable failures in these situations. I think this is due to the extremely conservative load calculations and cable sizing required as a result by the NEC.

Decades ago - mid 1980s?, the NEC was changed to required derating of LV cables in duct banks. This lasted only a few months (IIRC) and the general outcry was so loud that they backed down and issued a change.
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It was never actually changed...it was an accepted proposal, but that proposal was rejected at the comment stage and it never was part of the code. Many of the comments cited the conservative load calculations in Article 220. However other comments cited actual LV duct bank failures where the load calculations were done by engineering methods and not Article 220.
 
David Castor said:
In my experience, for 480 V circuits in duct bank, the table values are used. It doesn't seem logical, but in the real world, I've never heard of a problem of cable failures in these situations. I think this is due to the extremely conservative load calculations and cable sizing required as a result by the NEC.
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Are you referring to the use of Tables in 310.16?

I was curious why if the tables in Annex B are intended to be more conservative to account for underground heat dissipation and mutual heating why Table B.2(7) had higher ampacity values then the standard table (at least for the single duct configuration). Upon further review it appears that these table values are based off of 20degC as opposed to 30degC in standard tables and thus take advantage of a cooler earth temperature for heat dissipation?
 
Perform engineering calculations and make sure you utilize the correct Rho. it is amazing how many duct banks out there are not designed correctly and should be either current limited or design should have been modified to account for mutual heating.
 
If using table 310.16 for underground is there still a need to adjust for ambient temperature from 310.15(B)(1) if temperature of earth is typically considered less?
 
philly said:
If using table 310.16 for underground is there still a need to adjust for ambient temperature from 310.15(B)(1) if temperature of earth is typically considered less?
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Your ampacity is limited by the section of raceway with the highest ambient temperature. For me, there is almost always a stretch of conduit above ground where the ambient is higher.
 
In Table 310.16 there is not any indication of installation [for Earth
(Directly Buried) ] then could be the worst-deeper, closer no distance], rho=120 or more, Earth Temperature of 30°C
In Table B2(7) Earth Temperature of 20°C , RHO of Earth=90, for concrete 55, 7.5" center line distance.
 
We always use Annex B when our application falls within the prescriptive conditions of those tables. When our ductbank application is not found in the prescriptive tables, we use Ampcalc. If your application has high load factors, you use the derating of Annex B for multi-ducts to avoid failure.
I have heard of ductbank failure in data center applications due to high load factors.
 
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