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-   -   Conduit Fill and Derating (http://www.diychatroom.com/f18/conduit-fill-derating-31811/)

jamiedolan 11-13-2008 07:41 PM

Conduit Fill and Derating
 
I am going to start moving circuits off my old panel. There is very little legenth to the wires, so I am going to have to put a junction box up in the joists and junction to extend them. I am just going to connect them to THHN and run that in EMT to the new panel. Nice and clean and simple.

Inphase taught me the calculations for conduit fill and detating the wires a couple weeks ago. I want to verify that I understood it correctly and I am doing the math correctly.

3/4" EMT @ 40% = .213 / .0133 (for 12 gage- heaviest wire I am going to junction) = 16 wires per 3/4" conduit.

My plan is to run 5 120V circits in each junction box. That would put me at 15 total wires (hot neutral ground for each circuit). For derating, I would use the 4-6 current carring conductor value of 80%. So for my 12 gage, I would derate from 30A to 24A. and for my 14 gage I would derate from 25A to 20A.

My conclusion is that I am fine running 5 circuits into a junction box and them connecting them to my pannel with a 3/4" piece of EMT with THHN.

Thanks again for all the help and suggestions.

Jamie

chris75 11-13-2008 07:48 PM

Quote:

Originally Posted by jamiedolan (Post 184493)

My plan is to run 5 120V circits in each junction box. That would put me at 15 total wires (hot neutral ground for each circuit). For derating, I would use the 4-6 current carring conductor value of 80%. So for my 12 gage, I would derate from 30A to 24A. and for my 14 gage I would derate from 25A to 20A.

.

Jamie


I'm only gonna help with the derating part for now, first off, you would only need 1 ground wire per conduit, 2nd, if you pull all MWBC ( multiwire branch circuits) then you dont count the grounded (aka : neutral) conductor as current carrying. So, if you ran 3 sets of blk, red, wht, and one ground wire, (10 conductors total, but only 6 are current carrying) Just some usefull INFO thats all.

jamiedolan 11-13-2008 07:54 PM

Quote:

Originally Posted by chris75 (Post 184494)
I'm only gonna help with the derating part for now, first off, you would only need 1 ground wire per conduit, 2nd, if you pull all MWBC ( multiwire branch circuits) then you dont count the grounded (aka : neutral) conductor as current carrying. So, if you ran 3 sets of blk, red, wht, and one ground wire, you would end up with 6 ckts, and only 6 current carring conductors. Just some usefull INFO thats all.

Thanks. I forget to research the grounding issue. I was wondering with EMT if I even need to run a ground wire at all or if as long as I am bonded to the box if the EMT serves as the ground path? It's not a big deal to run the ground wire, I am just curious if the EMT is a proper ground.

Thanks
Jamie

chris75 11-13-2008 07:54 PM

Quote:

Originally Posted by jamiedolan (Post 184497)
Thanks. I forget to research the grounding issue. I was wondering with EMT if I even need to run a ground wire at all or if as long as I am bonded to the box if the EMT serves as the ground path? It's not a big deal to run the ground wire, I am just curious if the EMT is a proper ground.

Thanks
Jamie

Yes, EMT is allowed to be used as the grounding conductor.

Stubbie 11-13-2008 09:11 PM

Why are you using 4-6 current carrying conductors if you have 5 120 volt circuits and none are MWBC?

I'm also not understanding how your using the emt. It sounds like you are using a "piece" of emt. How long is it?

jamiedolan 11-13-2008 10:30 PM

Quote:

Originally Posted by Stubbie (Post 184563)
Why are you using 4-6 current carrying conductors if you have 5 120 volt circuits and none are MWBC?

I'm also not understanding how your using the emt. It sounds like you are using a "piece" of emt. How long is it?

HI;

I have about 18 pieces of NM that need to be junctioned and connected to a panel. They are up in the joist area.

My plan is to install metal 4" boxes, clamp 4-5 pieces of NM to the box, then connect the NM wires to THHN and run the THHN about 5-7 feet through EMT to the electrical panel.

I am using 3/4" EMT with runs that are all less than 10' (I have not done it yet, so I am not positive how long they will end up being, but I am sure none will be over 10'). It is longer than the 24" piece of conduit I have read about in the NEC that has some expemptions.

5 seemed like a reasonable number to junction in a 4" box, and accourding to my understanding of derating cables, 5 circuits in 1 conduit will allow me to maintain 15A with 14 gage and 20A with 12gage.


Thank You
Jamie

Gigs 11-13-2008 11:27 PM

Is this another go at aesthetic improvement?

Needlessly terminating the NM in junction boxes above the panel just for looks is just going to make your life that much harder down the road. You won't be able to figure out what wire goes to what breaker without doing electrical tests, normally you can just see where it's going.

jamiedolan 11-13-2008 11:32 PM

Quote:

Originally Posted by Gigs (Post 184646)
Is this another go at aesthetic improvement?

Needlessly terminating the NM in junction boxes above the panel just for looks is just going to make your life that much harder down the road. You won't be able to figure out what wire goes to what breaker without doing electrical tests, normally you can just see where it's going.

The wires need to be extended one way or another to get them out of the old panel and into the new one. So either I patch on about 18 pieces of NM and run them into the new panel or I do it with THHN and conduit.

They currently go into my old pushmatic panel, I installed the new panel a couple feet away and wired it up as a sub panel. Now you going to ask why I didn't just put the new panel in place of the old panel and not have to splice any wire. Well even if I put it in the same place I would have to splice all of the neutrals because they are all cut short and attached at the top of the pushmatic box. I really don't want to have all thoses splices in the panel. So that leaves me with extending the wires some place or another, and I would rather not have it be in the breker box.

Jamie

Gigs 11-13-2008 11:33 PM

Quote:

Originally Posted by jamiedolan (Post 184649)
The wires need to be extended one way or another to get them out of the old panel and into the new one. So either I patch on about 18 pieces of NM and run them into the new panel or I do it with THHN and conduit.

They currently go into my old pushmatic panel, I installed the new panel a couple feet away and wired it up as a sub panel. Now you going to ask why I didn't just put the new panel in place of the old panel and not have to splice any wire. Well even if I put it in the same place I would have to splice all of the neutrals because they are all cut short and attached at the top of the pushmatic box. I really don't want to have all thoses splices in the panel. So that leaves me with extending the wires some place or another, and I would rather not have it be in the breker box.

Jamie

Ah, OK, carry on then. :)

Stubbie 11-14-2008 12:04 AM

5 120 volt branch circuits that are not MWBC's is 10 current carrying wires. You would need to derate at 50%.

I don't like your plan....:)

jamiedolan 11-14-2008 12:39 AM

Quote:

Originally Posted by Stubbie (Post 184663)
5 120 volt branch circuits that are not MWBC's is 10 current carrying wires. You would need to derate at 50%.

I don't like your plan....:)


I might have understood this wrong, does this only apply to the neutral leg of a 240v circuit?

NEC 310.15:

(4) 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 3l0.l5(B)(2)(a).

lf so, then, I will only run 4 circuits per conduit and then I only have to derate 70% and can use the wires at 15 and 20 (14# and 12#).

Thanks
Jamie

Stubbie 11-14-2008 12:52 AM

Quote:

I might have understood this wrong, does this only apply to the neutral leg of a 240v circuit?

Correct... shared neutrals of MWBC as an example.

120 volt 2 wire circuits require both the hot and neutral to carry the same amps when loaded... so both must be counted.

jamiedolan 11-14-2008 02:03 AM

Quote:

Originally Posted by Stubbie (Post 184678)
Correct... shared neutrals of MWBC as an example.

120 volt 2 wire circuits require both the hot and neutral to carry the same amps when loaded... so both must be counted.

Ok. So I will count the neutrals. Then I will stick with 4 circuits per conduit, derating 70% which will leave my 12# at 20A and my 14# at 15A.

Sound like a better plan. :-)

Thanks Much
Jamie

frenchelectrican 11-14-2008 02:36 AM

The magic number for most conductor derating is typically 9 so you have 4 circuits which it will have 8 ccc { current carry conductors } so you are one wire below of magic zone.

If you gone with 5 circuit then you will have to use the derating on that conduit that can affect depending on the conductor size.

Merci,Marc

InPhase277 11-14-2008 10:34 AM

Jamie, don't forget about box fill when you are putting all these circuits in. You have more or less got the gist of the pipe fill and derating, but you must keep in mind the volume of the boxes too. Since you are going with the pipe method, you might consider a 24 x 6 x 6 wireway as your junction. The only draw back is you have to drill your own holes in it, but it will likely have more than enough volume. Just keep good track of the circuits and don't mix things up.


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