Underground Ampacity

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reko19

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We have multiple runs where a portion of the run is under ground in concrete encased ductbank and then it transitions to the overhead within the facility. Only about 20% of the run is under ground, the rest is overhead. Right now I am basing an entire run on the ampacity requirements for the underground, are there any references in the NEC other than 310.15(A)(2) that would allow me to use a free air ampacity if there is only a certain % of the run is installed under ground?

Thank you.
 

iwire

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Location
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are there any references in the NEC other than 310.15(A)(2) that would allow me to use a free air ampacity if there is only a certain % of the run is installed under ground?

Even if you could use the free air ratings of the conductors you would be limited to the 75 C rating by the terminations.

Besides, voltage drop would likely become problematic.
 

reko19

Member
I am just trying to see if I could use table 310.16 based on the fact that underground portion is a small % of the run length
 

don_resqcapt19

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I am just trying to see if I could use table 310.16 based on the fact that underground portion is a small % of the run length
I don't understand. The ampacities in Table 310.16 apply to both above and below grade installations.
 

Smart $

Esteemed Member
Location
Ohio
I am just trying to see if I could use table 310.16 based on the fact that underground portion is a small % of the run length
What is the voltage rating of the cable or conductors?

If over 2000 volts, it would put your ampacity determinations under the 310.60 tables. However, as iwire mentioned, you would still likely be limited to the 75?C rating under Table 310.16 due to termination ratings. So at best you could use the other tables' ampacities for temperature correction and derating.

No matter which way you go about this, where there exists more than one ampacity rating in a run, the run's ampacity is determined by the lowest ampacity rating, and only excepted where the lowest under 310 is ten feet or 10%, whichever is less.

With your runs being 20-80% of total length, you could only use the exception if the total run length is 50 feet or less (20% of 50' is the 10' max) and this shorter section is the lower of two adjacent ampacities. If the shorter section is the higher ampacity the exception is rendered moot.
 

raider1

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Take a look at Appendix B, depending on the Rho and burial depth you might end up with 20-30% more copper underground as compared to 310.16.

Annex B provides application information for ampacities calculated under engineering supervision and don't apply to the Tables in 310.15(B).

Chris
 

reko19

Member
Bottom line is, in order to size conductors properly for the underground installation a calculation has to be made (Neher-McGrath is one of them) unless your soil Rho, Concrete Rho. burial depth, ductbank configuration matches exactly to what is shown in the NEC.
 

david luchini

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Bottom line is, in order to size conductors properly for the underground installation a calculation has to be made (Neher-McGrath is one of them) unless your soil Rho, Concrete Rho. burial depth, ductbank configuration matches exactly to what is shown in the NEC.

And what is the soil Rho, concrete Rho, burial depth and ductbank configuration for conductors in raceway below grade in Table 310.16?
 

reko19

Member
take a look at tables 310.77-310.86, it tells you in the header of the table

table 310.16 can not be used for the underground installations

don't forget ambient earth temperature and load factor
 

david luchini

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take a look at tables 310.77-310.86, it tells you in the header of the table

I'm confused. In post #10, you reference 310.16 (0-2000V). Yet here you reference 310.77-310.86 (2001-35,000V.)

Is this a low voltage or a medium voltage installation?
 

reko19

Member
Perhaps my reference to 310.77-310.86 was not a good idea because of a different voltage class. Regardless of the voltage, ampacities have to be adjusted for the underground installations. For low voltage application take a look at table B.310.5, etc. Heat dissipation for underground installations has to be taken into consideration. We had instances where conductors were melting when sized based on 310.16 and installed 6' under in the concrete encased ductbank.
 

david luchini

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Location
Connecticut
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Perhaps my reference to 310.77-310.86 was not a good idea because of a different voltage class. Regardless of the voltage, ampacities have to be adjusted for the underground installations. For low voltage application take a look at table B.310.5, etc. Heat dissipation for underground installations has to be taken into consideration. We had instances where conductors were melting when sized based on 310.16 and installed 6' under in the concrete encased ductbank.

No, your statement is not correct. Read 310.15(A)(1). Ampacties shall be permitted by tables as provided in 310.15(B) OR under engineering supervision, as provided in 310.15(C).

Also read the first line of Annex B. "This annex is NOT a part of the requirements of this NPFA document...."

You are permitted by the NEC to use T310.16 for conductors in raceway (or cable) both above and below ground.
 
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RB1

Senior Member
Reko,

The derating factors for duct banks in Annex B are for informational purposes (that is why they were moved after the 1987 Code). Typically the values found in Table 310.16 are suitable for loads calculated in accordance with Article 220 as stated by the FPN following 310.15(B). If your loads are not diversified it may be wise to use the duct bank figures from Annex B. In any case the length of the run underground is going to dictate the ampacity of the conductors. As was stated previously, where more than one ampacity occurs on a circuit the lower value must be used.
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Take a look at Appendix B, depending on the Rho and burial depth you might end up with 20-30% more copper underground as compared to 310.16.
The reason those tables and that information is in the appendix is because when you size your conductors based on the very very conservative load calculations found in Article 220, you can get away with using the values found in T310.16.
If you are sizing feeders where the loads have been calculated by more realistic methods, you will need to use the reduced ampacities found the Appendix or risk burning the underground conductors up.

Note that the ampacities found in the appendix were the code for one cycle (1987). They were removed to the non-binding appendix in the next code cycle.
 
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