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
Best Answer
Your final wire size selection for the ungrounded conductors was exactly right at #2 copper. Your neutrals, however, could have been much smaller. You are allowed to calculate the total unbalanced load and in turn size the neutral accordingly. You still must apply the ambient temperature derating factor and you are required to at least match the size of the grounding conductor. For example, if your total calculated unbalanced load on the neutral was 25 amps and the ambient temperature was negligible, the ampacity table would allow for only a #10 AWG wire to carryn the neutral current, but because the neutral cannot be smaller than the grounding conductor you would still be required to use a #6 AWG. I have the tables underlined in my codebook but don't have it on hand , sorry. The minimum size grounding conductor chart chart is in article 250. Just found this on Mike Holt's website: { The grounded (neutral) service conductor must be sized to carry the maximum unbalanced load in accordance with 220.22 and must not be sized smaller than required by 250.24(B).}