NEC 2011 Table 310.15 question sizing DC PV conductors

[Andrew445]

I understand how to size per NEC 690, but I have some questions regarding the 310 tables.

First question:

In trying to size 3 PV DC feeders (6 conductors) run in cable tray, which table is applicable? All tables I would normally use in NEC 2008 have been changed and it now seems the tables are limited to 3 current carrying conductors.

Second question:

The cable tray feeders transition to underground conduit. All solar design guides I have found instruct me to use tables 310.15. My question is should I be using the Annex B tables and interpolation because the conduits are run underground? Generally solar is not "on" all the time and I don't know if this would affect the "load factor," but I know that the thermal resistivity value (rho) is going to affect the ampacity.

Currently I am modeling based off Table 310.15(B)(16) and verifying thermal requirements with an ETAP simulation.

There are whole new issues opened up when deciding how to meet 392 cable tray ampacity requirements but one problem at a time

I am fairly new to this and would much appreciate some direction.

 

[OTT2]

For conductors in cable tray refer to 392.80, 2011 NEC

For your conductors within a raceway refer to 310.15(B)(16), ampacity adjustments for more than 3 current carrying conductors within a raceway refer to 310.15(B)(3), see also 310.15(B)(3)(c) for those conductors that may need ampacity adjustment due to rooftop installations.

Annex B is informative 90.3

Good luck

 

[Smart $]

Being the feeders transition to underground PVC, their ampacity in the cable tray is pretty much rendered moot, unless the PVC run is less than 10'. Refer to 310.15(A)(2) and its exception. I say this because typically (with emphasis) the ampacity of a conductor is higher in cable tray than in PVC conduit (reminder: cable tray is not a raceway)... but unique and extenuating conditions introduce the possibility of my assessment being in error.

Another factor is temperature limitation determination under 110.14(C), which limits conductor ampacity to Table 75?C column value when using 90?C or higher rated conductors... and typically (again with emphasis and for same reason) this limitation is less than a conductors ampacity determined otherwise. Also, the table used is always 2011 NEC Table 310.15(B)(16) or its former equal.

As to what table to use to determine cable tray ampacity, I typically use 310.15(B)(17) or (B)(19).

 

[don_resqcapt19]

I... All tables I would normally use in NEC 2008 have been changed and it now seems the tables are limited to 3 current carrying conductors. ...

The "3 current carrying" conductors is not a change. There were a few minor changes in the actual ampacity of some conductors, and the table numbers have all been changed.