How many #12 thhn conductors can be installed in a 3/4" emt?
According to 310.16b2a 4 to 6 current carrying conductors has to be derated to 80%, 6 to 9 conductors be derated 70%. I guess one of the questions I am not quite clear on is in which case is a neutral considered a current carrying conductor? For example in a multi-wire circuit. If three phases are sharing a neutral would the neutral be considered a current carrying conductor? Also another thing to clear up is 12 awg thhn, according to 310.16 is good for 30amps. I am not quite clear whether 75degree terminations overide this insulation rating.
I understand that I have touched a couple of issues in this question, so back to my original question how many #12 awg thhn conductors can use legally run in a 3/4" EMT? I think that I know the answer. I would appreciate anymore input on this questions. Thanks for reading my "story".
thanks for the reply, leland. I do realize that the neutral carries current. The only reason why I would like more input is because I have heard otherwise from other electricians. I wonder if there are any cases when a neutral is not a current carrying conductor. I understand that if two phases are sharing a neutral and each phase is carrying the same load in amps theoretically you would not have any load on the neutral right?
A neutral carries current in the (very) likely event of an unbalanced multiwire circuit. So, yes, the neutral counts unless you know for sure that it will have no current on it.
I hate to step on toes leland but 358.60 (2005) says that EMT shall be permitted to be an equipment grounding conductor. So no equipment grounding conductor needed (but I recommend it).
To answer the original question. The termination ratings have nothing to do with the derating value of the conductors, only the overcurrent device used. So you would use the 90C (thhn) column of T310.16 for derating as necassary for T310.15(B)(2)(A).
My Ugly's says 16 #12s in a 3/4" EMT. If you want to keep a 20 amp breaker on those #12s then you would be limited to 9 current carrying (thhn) conductors (30x70%). This includes your neutrals (unless you know otherwise). So, 6 circuits (with 3 neutrals) in a single phase system, or 6 circuits (with 2 neutrals) in a three phase system. The grounding conductor (if present) does not count in the calculations.
Neutral Conductor. (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)(2)(a). (b) In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a). (c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
Let's take a purely resistive "full boat" and say each phase conductor is loaded to 20 amps, if we tally these conductors currents we will have 60 amps flowing and the neutral would be at zero.
Now take one of the phases and reduce it to 15 amps, when we tally the conductors again we will have 55 amps flowing and the neutral will have 5 unbalanced amps flowing so once again a total of 60 amps when all is added, so the neutral does not add to total current unlike a grounded conductor does in a two wire circuits.
Note; In heavy non-linear loads this would not necessarily be true.