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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)

    NFPA-70:2011 Table 310.15(B)(16)

  • 6 conductors in the same conduit = 80% allowable ampacity per 310.15(B) (relevant sections quoted below)

    NFPA-70:2011 Table 310.15(B)(3)(a)

    • +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

statueuphemism
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    Looks like you probably have it right that you'll need 3 runs. A word to the wise - you may not want to stick with the minimum size that passes code (40% fill) - it can be rather a pain to do the pulling at that fill, and a bit more room is often not terribly expensive. For one thing, use the 2" for one run if you have it in place already. – Ecnerwal Jun 02 '15 at 21:58
  • What are the temperature ratings for the terminals of the breaker in the main panel, and the lugs in the second panel? Seems like your conductors may have been undersized to begin with. – Tester101 Jun 03 '15 at 02:39
  • @Tester You got me, the terminals in both the main and subpanels are 75 degrees C. So, I should be derating from the 75 degree column (100 A) to 80 A. 3 separate conduit runs should still be fine at 30 degree C ambient, no? – statueuphemism Jun 03 '15 at 03:40
  • @statueuphemism No, you should be sizing from the 75°C column. If the conductors are rated at 90°C, then you can derate using the 90°C column. In either case, you'll end up using #2 CU conductors I think. – Tester101 Jun 03 '15 at 03:45
  • @Tester101 Why would the wire size differ from your recommendation here: http://diy.stackexchange.com/a/29077/36011 – statueuphemism Jun 03 '15 at 04:10
  • @statueuphemism That answer recommends #3 CU, when using 75°C terminals and 90°C conductors. It does not, however, take into account the derating of the conductors that you must apply in your situation. – Tester101 Jun 03 '15 at 09:43
  • @Tester101 I am likely misunderstanding you comments, but for some reason your comments here seem to indicate 3 AWG copper wire runs in three separate sets of EMT is still undersized to carry 100 A on each different set of wires. Which derating factor am I missing that you are including which would necessitate 2 AWG copper wire to carry 100 A in three separate runs? – statueuphemism Jun 03 '15 at 11:18
  • Couple of questions. 1. Do plan on having three 100 amp breakers in the 200 amp main? 2. What is the total imbalanced load for the one neutral? – Kris Jun 03 '15 at 11:26
  • @Kris 1. Yes - three 100 amp breakers off of a single 200 amp main. The load calculation for my home and additional wiring needs come out to 112 amps. 2. I haven't done any exact calculations for the unbalanced load, but I plan to keep the circuits reasonably balanced. The maximum unbalanced load (if I happened to use only circuits fed from one half of the 220) is theoretically 100 A, but I expect it will more likely be on the order of 20 A or less in normal use. Is there something else I should be consider to account for an unbalanced load? – statueuphemism Jun 03 '15 at 12:59
  • @statuephemism, to calculate the total unbalanced load you'd take the total VA (watts) of all three panels. For example, sub-panel A has 10kW, sub-panel B has 9kW, and sub-panel C has 15kW, A+B+C / 240 = 142 Amps. I'm concerned the neutral may be undersized. – Kris Jun 03 '15 at 13:46
  • @Kris I see what you mean, if I was sharing a single neutral, I would definitely need to size it appropriately. I am planning to run 3 separate neutrals all at 3 AWG. – statueuphemism Jun 03 '15 at 14:28
  • May I ask WHY you are running conduit and not just SER cable? Considering this is a home SER would be MUCH easier, and considering the installation it is HIGHLY unlikely that each panel will see anywhere near 100A, so using #2AL, or #4CU, SER cable on a 90A breaker would seem perfectly fine. – Speedy Petey Jun 07 '15 at 13:03
  • @SpeedyPetey I am running the wire through an accessible attic which requires wiring methods to protect from physical damage (NEC:2011 320.23). I extrapolated that protection from physical damage is equally necessary for SER cable and opted to use individual conductors and continuous lengths of EMT based on these sections: 338.12(A) Service-Entrance Cable. Service-entrance cable (SE) shall not be used under the following conditions or in the following locations: (1) Where subject to physical damage unless protected in accordance with 230.50(B) – statueuphemism Jun 08 '15 at 16:22
  • *(continued from previous comment)* and: 230.50(B) All other service-entrance conductors, other than underground service entrance conductors, shall be protected against physical damage as specified in 230.50(B)(1) or (B)(2). (1) Service-Entrance Cables. Service-entrance cables, where subject to physical damage, shall be protected by any of the following: (1) Rigid metal conduit (2) Intermediate metal conduit (3) Schedule 80 PVC conduit **(4) Electrical metallic tubing** (5) Reinforced thermosetting resin conduit (RTRC) (6) Other approved means – statueuphemism Jun 08 '15 at 16:23
  • @SpeedyPetey With regards to 100A subpanels. You are right, I'll probably never use all 100 A on each subpanel, but I want to keep my options for future expansion open. – statueuphemism Jun 08 '15 at 16:26
  • Also, for anyone who is interested in my end solution: I ended up going with 2" EMT for the majority of my longest run to one subpanel to make it super easy to pull and 1 1/4" EMT for the other two subpanels. I also switched to 2 AWG Cu with 90 degree C insulation for the three current carrying wires in each subpanel run (to account for some ampacity adjustment due to temperature in my attic) and 8 AWG Cu for each ground wire per Table 250.66 (NEC:2011). – statueuphemism Jun 08 '15 at 16:34

2 Answers2

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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).}

fritz
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What you're missing is you get to round up on breaker size

92A. They don't make a 92A breaker.

95A. Oh, they don't make one of those either.

OK, 100A breaker. Done.

What remains is my stream-of-consciousness thoughts on sizing wire.

It's an interesting puzzler, to be sure.

Indeed, when computing the derate under 310.15b3a, you disregard neutrals. But why? Because there is no configuration where the 3 conductors will be worse than 2 conductors - usually they'll be better. It's all about heat, and heat is the square of current. Say 100% usage makes 10,000 heat units (100^2).

  • If power is split 100% on L1 and L2 and 0% neutral, that's 10,000 heat units x2 for 20,000.
  • If power is split 100% on L1 and neutral, and 0% on L2, that's 10,000 heat units x2 for 20,000.
  • If power is split 100% on L1 and 50% on L2 and neutral, that's 10,000 plus 2x 2,500 heat units, for 15,000.

In a similar vein, since your supply is only 200A, your worst case load is actually four conductors carrying 100A and all others carrying 0. There's a case to make to your AHJ to let you rate for 4 conductors instead of 6. Of course this doesn't help here, but it would if you added a fourth or fifth 100A circuit.

Now with the ratings, it's important not to get tangled up. The 310.15b3a multi-wire derate always comes off the highest thermal rating of the wire type in the ampacity chart, even if you have 60C terminations. Then separately, the wire/termination temperature columns come into play. The lowest number applies. And then even separate from that, is any upsizing of the wire for long distances. And separate from that any statutory limits like "20A on 12AWG". So suppose you have 12AWG wire going 200' but traveling with 1 other circuit in a conduit:

  • 24A based on 80% derate due to 4 active conductors off the 30A in the 90C column
  • 25A based on wire and terminations are all good For 75C
  • 15A recommended to limit voltage drop given the long length
  • 20A as a hard rule since it is 12 AWG wire

The final answer is 20A mandatory, with 15A recommended to limit voltage drop.

Now if you're using EMT as a wiring method rather than just a damage shield, you don't need to wire grounds. EMT is the ground. I mean it doesn't hurt, belt and suspenders, but it doesn't help either.

Harper - Reinstate Monica
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  • @statueuphemism de nada. But you are only thinking about the utility for you... and it's not for you. It's for others with the same problem. That is SE's format here, less a "custom for you" Q&A and more an encyclopedic reference that aims to answer similar questions too. – Harper - Reinstate Monica Nov 12 '17 at 17:27
  • @statueuphemism this forum is not competitive like that. I'm way past caring about rep, I answer questions for fun and to improve my own thinking. I wish I was better at searching but *the question you linked is not that similar*. I think you are equating questions and answers with large overlap in the shards of info they provide. This disregards structure and the brainpower needed to organize structure. For instance I had never thought of structuring the idea in the "maximum of" bullet list you see above. – Harper - Reinstate Monica Nov 14 '17 at 20:57