Electrical Conductor Sizing above and below ground

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SparksMcgee

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DFW, TX
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EIT
Good morning MHF!

I wanted to ask about conductor sizing regarding if the conductors are going below but also above ground. The NEC has different sizing tables for these scenarios but if the conductors are doing both for some distance which one do we size for? I was thinking the worse case scenario which is for underground which has larger sizes for the same ampacities. However, I wanted to check to make sure I wasn't missing anything or if there was information on this particular situation.

Thanks for your time.
 
1. What is an EIT ??
2. What Tables are you referencing ?? 310.15(B)(16) addresses most above and below ground installations.
 
The tables in the Annex B show lower ampacities for underground duct banks as a result of mutual heating between adjacent ducts, and because the conductors in underground raceways cannot dissipate heat as well as the conductors in above ground open air installations.

As long as all of the loads have been calculated per Article 220, there is no need to use anything other than Table 310.16, but for large engineered commercial and industrial installations, lower ampacities are often used. Typically the ampacities would be from engineering software and not from Annex B.
 
augie47 said:
1. What is an EIT ??
2. What Tables are you referencing ?? 310.15(B)(16) addresses most above and below ground installations.
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1. Engineer in Training
2. I'm referencing those tables in 310 (b) and I should have mentioned that so thank you. Those tables do address above and below ground but what about conductors that travel some distance below ground and then move to some distance above ground in the same run. Would best practice be to change sizing as it transitions or size for the largest potential ampacity?
 
SparksMcgee said:
2. I'm referencing those tables in 310 (b) and I should have mentioned that so thank you. Those tables do address above and below ground but what about conductors that travel some distance below ground and then move to some distance above ground in the same run. Would best practice be to change sizing as it transitions or size for the largest potential ampacity?
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See 310.15(A)(2)
 
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