I was planning to pull three sets of four conductors (3 AWG wire) through 2" EMT conduit for a 30' length that is common to all three sets of wires, and then branch off at a listed metal junction box (10"x10"x4") to three 1 1/4" EMT conduits. I was planning to do this as part of running wiring for three 100-amp subpanels from a 200-amp main.
However, I was doing some re-reading of the NEC (2011 version), and recalled the need to de-rate ampacity of conductors where spacing is not maintained (such as in an EMT raceway) for runs greater than 24" per clause 310.15(B)
Per 310.15(B), it appears that the max ampacity for the wires is now only 92A and that I will not be able to use the single run of 2" conduit I had planned and instead will need to run three completely separate runs of EMT to allow for the three 100A subpanels with the 3 AWG wire I was planning to use -- is this correct or is there something else I am missing that would make my original plans okay?
Calculations and Code References:
- 3 AWG THHN with 90 degree rating = 115 A allowable ampacity per Table 310.15(B)(16)
6 conductors in the same conduit = 80% allowable ampacity per 310.15(B) (relevant sections quoted below)
- +6 conductors => 3 sets of 2 out-of-phase hot wires
+0 conductors => 3 sets of 1 neutral carrying the unbalanced load (per 310.15(B)(5)(a)
310.15(B)(5)(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).
+0 conductors => 3 sets of 1 ground wire (per 310.15(B)(6)
310.15(B)(6) Grounding or Bonding Conductor. A grounding or bonding conductor shall not be counted when applying the provisions of 310.15(B)(3)(a). (b) In a 3-wire
115 A * 80% = 92 A