Overhead to underground service

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MarkBurggraaf

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Location
Montrose, CO
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Professional Engineer
Maybe someone can point me to the applicable code section -
I'm designing a service entrance which would have an overhead section (75+ feet) from a pole mounted xfmr to a second pole and then an underground section (125+ feet) to the building service (400 amp, single phase). Underground wire sizing would be two runs of 250 MCM AL. Assuming that I'll just have to carry that overhead as well? Or is there a way to back off to the 2/0 AL from Table 310.15(B)(17) on each run once I get overhead?

Haven't checked the voltage drop yet, but that would just end up bumping the wire size on both if needed.

Thanks
 
I see no problem using 310.15(B)(17) from an ampacity standpoint. Depending on your actual load and ignoring VD you could reduce it more.
That said, all the installations I have seen like that used a non-NEC rated ACSR cable such as "snail" due to the steel messenger.:) )
 
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My concern is in changing the conductor size - effectively in the middle of the run. I've never done that before - most of what I do is straight underground. It works by the numbers, but do inspectors want to see the same conductors all the way to the xfrmr? It occurs to me that since this is before the meter, its actually on the utility side of the installation. I'll call my utility engineer and see what he says.
 
You're correct. (particularly on this one) - the utility indicated that its 100% their call. I've got some smaller municipal / local utilities that rely on the electricians to install / pull the conductors and they just make the final connections to the transformers. In those cases, they tend to rely on the NEC (I suspect that provides them with a level of conservatism).

Thanks for the responses.
 
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