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#168595 - 09/09/07 11:19 PM How many #12 thhn conductors can you run in 3/4''  
controler  Offline
New Member
Joined: Sep 2007
Posts: 4
Oregon, U.S
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".



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#168596 - 09/09/07 11:30 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: controler]  
leland  Offline
Member
Joined: Aug 2007
Posts: 853
Lowell area, Ma. USA
I think that I know the answer.

Just to help a little. Does a neutral carry current?

Yes.
Grounds do not. And you need 1 ground sized to the largest current carrying conductor (per conduit).

So keep going your on the path.


#168597 - 09/09/07 11:35 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: leland]  
controler  Offline
New Member
Joined: Sep 2007
Posts: 4
Oregon, U.S
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?


#168598 - 09/10/07 12:31 AM Re: How many #12 thhn conductors can you run in 3/4'' [Re: controler]  
JSterling  Offline
New Member
Joined: Sep 2007
Posts: 1
Arkansas
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.

Feel free to correct me if I am wrong

James


#168613 - 09/10/07 11:20 AM Re: How many #12 thhn conductors can you run in 3/4'' [Re: JSterling]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,057
Estero,Fl,usa
310.15(B)(4)
Quote
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.


Greg Fretwell

#168625 - 09/10/07 07:39 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: gfretwell]  
leland  Offline
Member
Joined: Aug 2007
Posts: 853
Lowell area, Ma. USA
James, Don't worry about toes.

I responded as a way to stimulate more thought. Controler was on the right track.
I personaly know I have learned more when answerd with a question.

And 5Am comes quick.


#168627 - 09/10/07 09:53 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: leland]  
controler  Offline
New Member
Joined: Sep 2007
Posts: 4
Oregon, U.S
Regarding 310.15b4.

I understand not b and c but note a what the original question that was posted, revolves around.

If a neutral carries unbalanced current from other conductors of the same circuit shall not be required to be counted when applying provisions of 310.15b2a.

If the circuits were balanced then there would not be any load on the neutral conductor, but the code says unbalanced current which means there is current present on the neutral.

The questing is why is the neutral not required to be counted as a current carrying conductor in note a?

In this case it seems like you could pull 3 full boats each with a neutral in a 3/4" emt., and each would be on a 20A circuit.

Hey everyone thanks for your input, it rally gives more food for thought.


#168628 - 09/10/07 09:55 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: controler]  
controler  Offline
New Member
Joined: Sep 2007
Posts: 4
Oregon, U.S
Sorry, I meant (note) and (not) in the first paragraph. I better check my spelling more often.


#168629 - 09/10/07 10:24 PM Re: How many #12 thhn conductors can you run in 3/4'' [Re: controler]  
Active 1  Offline
Member
Joined: Feb 2003
Posts: 687
Grayslake IL, USA
#12's on 20A CB longer than a nipple:

I think the answer is 9 for 1/2"-4"

With an EG then you could then the answer is 10 for 3/4" & up.

It's too bad there is no adjustment for raceway size when it comes to derating.


#168635 - 09/11/07 06:27 AM Re: How many #12 thhn conductors can you run in 3/4'' [Re: Active 1]  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
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.

Roger



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