Number of cables in conduit?

Reply

  #1  
Old 09-15-05, 09:05 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Question Number of cables in conduit?

I have a ten foot length of 3/4" conduit that I need to run cables through (From outside circuit box into the crawlspace, terminating at a metal junction box.) The plan is to use individual wires w/o the jacket in the conduit and then use regular romex from the junction box to the outlets. I want to put in wires w/o the jacket in order to fit as many as I can in the conduits. This is because I want to avoid having 15+ conduits on the outside of the house.

I know that because of the length of the conduit, I need to derate the cable, so I was planning on using 10ga THWN (White, Black, Green). I was going to try to get three circuits (9 10ga wires total) in the conduit. These circuits are going to be 20 AMP circuits which normally calls for 12ga (the romex will be 12/2). Is it safe to put three 20amp circuits on 10ga wires going through ten feet of 3/4" conduit?
 
Sponsored Links
  #2  
Old 09-15-05, 09:17 AM
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Received 0 Votes on 0 Posts
For three circuits you don't need 9 cables. You only need 7 cables. You can eliminate two of the ground wires. You are fine with 3/4 inch conduit.
 
  #3  
Old 09-15-05, 09:46 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
Received 42 Votes on 40 Posts
I think that #10 is overkill for this situation. If the conduit was 100 feet instead of 10 feet, then I would consider using the #10. For just 10 feet however, you can use #12 and de-rating is not an issue with 3 circuits of #12 in a conduit because you only need 7 wires as racraft said.

Using the same gauge of wire throughout the entire circuit has many benefits like avoiding confusion of breaker sizes down the road and only needing to purchase one size of wirenuts.
 
  #4  
Old 09-15-05, 09:47 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Talking

Originally Posted by racraft
For three circuits you don't need 9 cables. You only need 7 cables. You can eliminate two of the ground wires. You are fine with 3/4 inch conduit.
:: Ears perk up ::

Just to make sure I'm not misunderstanding you.

Circuit Panel through conduit to junction box:

3 Hot (connected to 3 20amp breakers), 3 neutral (connected to neutral bar), and 1 ground (connected to ground bar) all wires 10ga.


IN junction box:

3 romex (12/2) from diff house circuits coming in. Obviously connect the hots and neutrals together 1-to-1, but all of the grounds coming in can be connected to the one 10 ga ground coming from the circuit panel?

If that's the case, there may be (physical) room for one more set of hot & neutral wires making for four circuits total. Would this be allowable?

Thanks!!!!!!!!!
 
  #5  
Old 09-15-05, 09:51 AM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Received 0 Votes on 0 Posts
You can route three 20 amp B-C's with five #12 THHN conductors---- 2 Blacks, 2 Whites, 1 Red. I would only "de-rate" the #12 conductors if the raceway was 1/2". The area of a 3/4" raceway with five #12 conductor will allow forsufficient circulation of air which diminishes as the # of condutors in the racewat increases.
 
  #6  
Old 09-15-05, 09:52 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
Received 42 Votes on 40 Posts
Originally Posted by Bromius76
If that's the case, there may be (physical) room for one more set of hot & neutral wires making for four circuits total. Would this be allowable?
Yes. You are okay with 4 circuits on conduit fill and on de-rating for either #10 or #12 wire. As mentioned earlier, I recommend #12. The de-rating rule for #12 wire is unique allowing up to 9 conductors before the 20A ampacity must be reduced.

If you wanted to mess with multi-wire circuits, you can get more circuits out of the wires. That would complicate the installation quite a bit though.
 
  #7  
Old 09-15-05, 10:00 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Originally Posted by ibpooks
I think that #10 is overkill for this situation. If the conduit was 100 feet instead of 10 feet, then I would consider using the #10. For just 10 feet however, you can use #12 and de-rating is not an issue with 3 circuits of #12 in a conduit because you only need 7 wires as racraft said.

Using the same gauge of wire throughout the entire circuit has many benefits like avoiding confusion of breaker sizes down the road and only needing to purchase one size of wirenuts.

ibpooks, I had been told (on this board I believe) than if in a conduit of more than 2 or 3 feet (can't remember the exact distance) if there was more than a certain number of current carrying conductors that you would have to derate the cable.
 
  #8  
Old 09-15-05, 10:06 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Originally Posted by PATTBAA
You can route three 20 amp B-C's with five #12 THHN conductors---- 2 Blacks, 2 Whites, 1 Red. I would only "de-rate" the #12 conductors if the raceway was 1/2". The area of a 3/4" raceway with five #12 conductor will allow forsufficient circulation of air which diminishes as the # of condutors in the racewat increases.

Pattbaa, I think I understand what you're saying.

The red wire would need to be on the opposite phase as one of the blacks. Then inside the junction box, the red wire and the black that was on the opposite phase would need to share the same neutral. Correct?

This does make things a little tricky in the circuit panel because that needs to be remembered in case someone moves the circuit breakers inside the box. Correct?

And lastly, you can run the three circuits in the five wires, but you still need a ground wire, correct? So six wires in the conduit (2 black, 1 red, 2 white, 1 ground), and three romex going to each of the different circuits.

This thread has been amazingly informative. Thanks all!!!!
 
  #9  
Old 09-15-05, 10:10 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
And finally

And finally, just to confuse things one last time... given all the info on this so far, here's what I'm considering:

running 6 circuits in 3/4" conduit.

3 black, 3 red, 3 neutral, 1 ground. All 12 gague. This adds up to 9 current carrying cables. Is nine the limit for derating, or is it _after_ nine?

I'm confident in my ability to remember which wires need to be on different phases if the need ever arises to move a circuit box.



And to my understanding, you can have a black and red share the same neutral (with all wires being the same gauge) because even if both the black and red are using close to their max (say 17 amps and 16 amps), they cancel each other out on the neutral is only carrying 1 amp, no where _near_ it's rated 20 amps, correct?

I'm seriously digging this thread :-D
 
  #10  
Old 09-15-05, 10:13 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Received 0 Votes on 0 Posts
Actually, in your specific case, you could run up to 4 separate 20A circuits using 12ga THHN wire in a single conduit.

The derating requirements for conductors sharing a conduit are in table 310.15(B)(2)(a):
http://www.houwire.com/catalog/techn...icle310_15.asp

And the raw ampacity ratings are given in table 310.16:
http://www.houwire.com/catalog/techn...icle310_16.asp

12ga THHN wire has a 90C ampacity of _30A_. However for reasons of short circuit protection, you may only protect 12ga wire with a 20A breaker (NEC 240.4). So in a sense 12ga wire has some 'extra capacity' that can be used up by derating that doesn't directly alter its utility.

If you place 3 separate circuits in the conduit, you will have 6 current carrying conductors. This forces derating to 80% of the original ampacity, meaning that 12ga wire in this case would have an ampacity of 24A, fine on a 20A breaker.

Note that this doesn't take into account thermal derating, which must be calculated prior to the 'more than 3 conductors' derating. If this conduit just happened to pass through a 120F attic, then the thermal derating would be 82% of original capacity...combine this with the derating of 80% for 6 current carrying conductors, and you get a required derating of (0.80 * 0.82) 65.6%, or an ampacity for 12ga wire of 19.6A.

-Jon
 
  #11  
Old 09-15-05, 10:20 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Question

Originally Posted by winnie
12ga THHN wire has a 90C ampacity of _30A_. However for reasons of short circuit protection, you may only protect 12ga wire with a 20A breaker (NEC 240.4). So in a sense 12ga wire has some 'extra capacity' that can be used up by derating that doesn't directly alter its utility.

If you place 3 separate circuits in the conduit, you will have 6 current carrying conductors. This forces derating to 80% of the original ampacity, meaning that 12ga wire in this case would have an ampacity of 24A, fine on a 20A breaker.

Note that this doesn't take into account thermal derating, which must be calculated prior to the 'more than 3 conductors' derating. If this conduit just happened to pass through a 120F attic, then the thermal derating would be 82% of original capacity...combine this with the derating of 80% for 6 current carrying conductors, and you get a required derating of (0.80 * 0.82) 65.6%, or an ampacity for 12ga wire of 19.6A.

-Jon
Jon, in my above proposed example, I'd be running 9 current carrying conductors, 12 ga, THWN. From the first table, they're rated for 30 amps. From the second table I need to derate down to 70%. 30 * .7 = 21.0 amps. The conduit runs 3 feed down the side of the house and seven feet into the crawlspace. Nothing in the attic. So am I still okay?
 
  #12  
Old 09-15-05, 10:37 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Received 0 Votes on 0 Posts
I'd strongly recommend keeping it simple. I recommend that you pull only 4 circuits per conduit, and keep all of the circuits separate (no shared neutral circuits). If the temperature is high, then pull 3 circuits per conduit.

Yes, you _can_ do what you describe. In fact, could run _8_ 20A circuits; using a total of 3 wires for each pair of circuits, and a single ground. You would have a total of 13 wires in the conduit, but only 8 of them would 'count' as current carrying conductors. _But I strongly recommend against this_

The reason is that you will then have to deal with all of these separate circuits at the junction box at the end of the conduit, and it will be a monster. You would need to keep 13 splices in order, including a monster ground splice with 9 wires in it. If any of your shared neutral splices opened, then you might put 240V across some of your circuits. There are a bunch of other details that can jump up and bite you, and these increase exponentially as your install gets more complex.

Read the appropriate code sections on derating! You have to deal with both the number of conductors in the conduit _and_ the ambient temperature. Both more conductors or higher temperature could force you to use 10ga wire. The 9 conductor threshold happens because the normal ampacity of 12ga wire is _30_ amps, and 9 conductors forces a 70% derating, giving you a derated capacity of 21A. But there is also an 80% derating requirement for 4-6 conductors; you just don't see this because it takes the ampacity of 12ga wire to 24A. But this other threshold is important if you need to deal with thermal deration on tome of conduit fill.

You should also read up on 'box fill' to calculate the size of junction box required, and then after you figure this out, I'd recommend using a larger junction box anyway. You always want more room to work.

Finally, one of those little details: you mention THWN wire. And I went back to your original post and see that the conduit is outside. This means that you have to use wet rated wire. However THWN wire is only rated at _75C_, and 12ga THWN is only rated for 25A. If you need 90C conductors, then you will need to get THWN-2 or XHHW conductors.

Just to recap: running 6 shared neutral circuits using 12ga THWN-2 conductors with 20A breakers is fine up to an ambient temperature of 113F, however I recommend running only 3 unshared circuits, 4 unshared circuits if the ambient around the conduit will not get above 35C.

-Jon
 
  #13  
Old 09-15-05, 11:03 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Originally Posted by winnie
I'd strongly recommend keeping it simple. I recommend that you pull only 4 circuits per conduit, and keep all of the circuits separate (no shared neutral circuits). If the temperature is high, then pull 3 circuits per conduit.

Yes, you _can_ do what you describe. In fact, could run _8_ 20A circuits; using a total of 3 wires for each pair of circuits, and a single ground. You would have a total of 13 wires in the conduit, but only 8 of them would 'count' as current carrying conductors. _But I strongly recommend against this_
Agreed, that may be overkill, and I'm rethinking my 6 circuit attempt.


Originally Posted by winnie
The reason is that you will then have to deal with all of these separate circuits at the junction box at the end of the conduit, and it will be a monster. You would need to keep 13 splices in order, including a monster ground splice with 9 wires in it.
True.... that would be a bear.

Originally Posted by winnie
If any of your shared neutral splices opened, then you might put 240V across some of your circuits.
This one I'm proud of... I've already been reading up on the 240V danger at this site. They also talk about shared neutrals needing thier own terminal in the panel. None of my terminals are rated for multiple conduits, but apparently they're out there. And I'm assuming the requirement for it's own terminal is to prevent you from trying to remove the other wire in the terminal and momentarily disconnecting the shared neutral.


Originally Posted by winnie
There are a bunch of other details that can jump up and bite you, and these increase exponentially as your install gets more complex.

Read the appropriate code sections on derating! You have to deal with both the number of conductors in the conduit _and_ the ambient temperature. Both more conductors or higher temperature could force you to use 10ga wire. The 9 conductor threshold happens because the normal ampacity of 12ga wire is _30_ amps, and 9 conductors forces a 70% derating, giving you a derated capacity of 21A. But there is also an 80% derating requirement for 4-6 conductors; you just don't see this because it takes the ampacity of 12ga wire to 24A. But this other threshold is important if you need to deal with thermal deration on tome of conduit fill.
definitely planning a trip to the library to read up on code on my way home.

Originally Posted by winnie
You should also read up on 'box fill' to calculate the size of junction box required, and then after you figure this out, I'd recommend using a larger junction box anyway. You always want more room to work.
Didn't think about box fill, thank you for mentioning it.


Originally Posted by winnie
Finally, one of those little details: you mention THWN wire. And I went back to your original post and see that the conduit is outside. This means that you have to use wet rated wire. However THWN wire is only rated at _75C_, and 12ga THWN is only rated for 25A. If you need 90C conductors, then you will need to get THWN-2 or XHHW conductors.
Good catch, I'm operating from memory as to the cable, but I know it's wet rated. I need to get more anyway if I want to do more than 2 circuits, so I'll be sure to pick up something rated for 90C

Originally Posted by winnie
Just to recap: running 6 shared neutral circuits using 12ga THWN-2 conductors with 20A breakers is fine up to an ambient temperature of 113F, however I recommend running only 3 unshared circuits, 4 unshared circuits if the ambient around the conduit will not get above 35C.

-Jon
You have been amazingly helpful, and I appreciate you taking your time to share all of this information with me.

Because this install is in Charlotte, NC... I'm going to go with 90C cable, and I'm going to use a .82 therman derating... that will get me up to 122(f). .87 would get me to 113(f) which _should_ be enough... but I'm thinking just in case... that monster heat wave that hits once every five years.

Given that termal derating, if I want to go with more than 2 circuits (w/o shared neutrals) I'll need to go 10ga.

So now I need to decide between 10ga, 4 black (hot), 4 white (neutral), 1 ground (no shared neutrals) or 10 ga, 3 black (hot 1), 3 red (hot 2), 3 white (neutral), 1 ground (shared neutrals).
 
  #14  
Old 09-15-05, 11:08 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Actually, I just did the math, and I should be okay with THWN 10ga that I have.

THWN 10 ga, rated for 35 amps.

up to 122(f) = .82 thermal derating

up to 9 current carrying cables = .7 derating

35 * .82 * .7 = 20.09. :-D So I've got .09 to spare, right? :-D
 
  #15  
Old 09-15-05, 11:40 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Received 0 Votes on 0 Posts
Close but not quite. The 0.82 for thermal derating at 122F is for the 90C wire column. In the 75C column, you need to use 0.75.

So if you have 75C wire, the answer is:
35 * 0.75 * 0.7 = 18.375A
and if you have 90C wire, you get:
40 * 0.82 * 0.7 = 22.96A

-Jon
 
  #16  
Old 09-15-05, 11:58 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Originally Posted by winnie
Close but not quite. The 0.82 for thermal derating at 122F is for the 90C wire column. In the 75C column, you need to use 0.75.

So if you have 75C wire, the answer is:
35 * 0.75 * 0.7 = 18.375A
and if you have 90C wire, you get:
40 * 0.82 * 0.7 = 22.96A

-Jon
DOH... so close! But I definitely understand it a lot better than I did before. And I got what you meant as soon as you said it. :grin: THWN-2 it is :-D And I'll probably go the 4 circuit unshared.
 
  #17  
Old 09-15-05, 12:08 PM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Received 0 Votes on 0 Posts
There are 2 advantages with multi (3-wire) B-C's applicable to this installation;

(1) One Neutral for 2 circuits, which minimizes the "fill" of the raceway.

(2) The Neutral will conduct only the "difference" in the current-load of the 2 circuits. Ex: 15 amps Load "A" minus 10 amps Load "B" = a Neutral current of 5 amps.This would minimize the heating effect of conductor curents.
If this was effected with two 2-wire circuits, 2 conductors would conduct 15 amps, and 2 would conduct 10 amps.

With the exception of 20 amp appliance circuits, and 20 amps circuits with heater and motor loads, you have the option of protecting circuits wired with #12 wire with 15 amp C-B's, presuming a 15 amps is sufficient for the connected load.
 
  #18  
Old 09-15-05, 01:19 PM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
:-\ Okay... now the problem is <i>finding</i> THWN-2 cable. Tried lowe's & HD even though I knew they wouldn't have it. Tried the local graybar and a couple electric stores... no go.

Looks like I may go with 3 circuits.

3 black, 3 white, 1 green, 10ga THWN.

10ga THWN = 35 amps

6 current carrying conductors = .8 derating.

Ambient up to 122(f) = .75 (for THWN)

35 * .8 * .75 = 21 amps.


I can always swap out for THWN-2 later if I'm able to find any. The romex will be able to stay where it is, it will just be the short 10' length of conduit that needs to be re-pulled.

Thanks for your help everyone!
 
  #19  
Old 09-15-05, 10:07 PM
solipsist9
Visiting Guest
Posts: n/a
if ever there was a mountain made out of a molehill, this is it. i see a lot of overkill here, and it's not in the wires.
john
 
  #20  
Old 09-15-05, 10:18 PM
Member
Join Date: Jan 2003
Posts: 111
Received 0 Votes on 0 Posts
All of the THHN at any of the box stores Iíve seen is dual rated THHN/THWN-2, which has the 90deg C rating.

What type of conduit are you using?
The maximum number of 10AWG conductors in ĺ Schedule 80 PVC is 7.
 
  #21  
Old 09-15-05, 10:31 PM
solipsist9
Visiting Guest
Posts: n/a
Because this install is in Charlotte, NC... I'm going to go with 90C cable, and I'm going to use a .82 therman derating... that will get me up to 122(f). .87 would get me to 113(f) which _should_ be enough... but I'm thinking just in case... that monster heat wave that hits once every five years.

Given that termal derating, if I want to go with more than 2 circuits (w/o shared neutrals) I'll need to go 10ga.

QUOTE]

it's in a crawlspace. why are you correcting for ambient temp of 122 degrees?
you only need #12 thhn/thwn for this project. check the wire markings and you'll see that (in most cases) the wire is both types, thereby covering wet locations, which is not the same as outside. i seem to remember a manufacturer (or maybe more) that had a thhn/thwn/thwn-2 type, but i'm not sure and that might be an older product. but again, all this is, practically speaking, irrelevant.
as far as thwn being 75degree rated, that's true. but for derating conductor amperage, you can start at the 90degree column, regardless of wire type. for ambient temp correction, use the type temp. the type designation is about the insulation, not the copper. in any case, you cannot put #12 on anything larger than a 20 amp cb, so keep it at/below 9 current-carrying conductors, share neutrals if you want to, skip all the math, and you're fine.
john
 
  #22  
Old 09-16-05, 05:19 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Received 0 Votes on 0 Posts
Umm, thermal derating must be done from the temperature rating of the insulation as used. If you have a conductor that is rated THHN/THWN, and it is used in a wet environment, then the temperature rating of that conductor is _75C_, and you use the 75C column for ampacity and derating.

Many conductors are rated THHN/THWN/THWN-2, but not all, possibly not even most. I picked up some spools that I have in my lab, and they are THHN/THWN.

The conductors that the original poster happens to have are THHN/THWN, thus the 75C column is the correct one to use.

Good catch on the crawlspace; for some reason I'd envisioned this conduit going _up_ to the attic, not _down_ to the crawlspace. Going up into the attic temperatures would tend to be higher than normal high temperatures. Note that if this is a conduit on the outside exposed to direct sunlight, you really should consider the fact that the sun will heat the conduit.

If in doubt, the correct answer is to call the electrical inspector and _ask_ what ambient temperature should be used for thermal derating; the local inspector will be aware of the expected temperatures in your region.

Now a small rant: Just because a particular aspect of the code is often ignored, does not mean that it is making a mountain out of a molehill to properly explain its application and suggest that people actually follow the code. There are loads of situations where people ignore code and the system 'works', but is not as safe as it really should be. In particular regarding thermal effects on insulation are a real problem, causing the insulation to get brittle, to crack, and then to fail. Have you ever looked at the wires in a light fixture where the lamp wattage limits have been exceeded?

I agree that using 12ga THWN-2 conductors would be suitable in this installation. But 12ga THWN conductors will have their ratings exceeded, so the choice is 12ga THWN-2 or 10ga THWN.

-Jon
 
  #23  
Old 09-16-05, 11:22 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Originally Posted by winnie
I agree that using 12ga THWN-2 conductors would be suitable in this installation. But 12ga THWN conductors will have their ratings exceeded, so the choice is 12ga THWN-2 or 10ga THWN.

-Jon
And 10ga THWN it was :-D Got the install (mostly) done last night. All I have to do now is put in the old work gang boxes, and wire up the outlets. Thanks again to everyon here, not only for the help--but more importantly the education.
 
  #24  
Old 09-16-05, 11:33 PM
solipsist9
Visiting Guest
Posts: n/a
the way it's been described, this is not a wet location.
 
  #25  
Old 09-17-05, 05:53 AM
Member
Thread Starter
Join Date: May 2005
Posts: 35
Received 0 Votes on 0 Posts
Er? To clarify: Main panel is outside on the side of the house (under the eaves). EMT Conduit runs from the bottom of the panel, down three feet, makes a ninety degree turn into the crawlspace. The point at which it makes the turn is approximately six inches off the ground. Is this considered a wet application?
 
  #26  
Old 09-17-05, 06:01 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Received 0 Votes on 0 Posts
Many consider _any_ outside conduit to be a wet location. Clearly not all outside conduit will actually be wet, but conduit that cross temperature boundaries (say inside to outside) will get wet from condensation, even if they are protected from rain.

-Jon
 
  #27  
Old 09-17-05, 02:51 PM
solipsist9
Visiting Guest
Posts: n/a
Originally Posted by winnie
Many consider _any_ outside conduit to be a wet location. Clearly not all outside conduit will actually be wet, but conduit that cross temperature boundaries (say inside to outside) will get wet from condensation, even if they are protected from rain.

-Jon
many might consider any outside conduit to be in a wet location, but that does not make it correct.

now that it's been clarified, it is definitely not a wet location. this is a damp location, which allows use of thhn and the 90degree column (at least for calc purposes). of course, since most terminations are 75 degrees, the final calculated amperage will likely not be allowed to exceed the 75 degree colmun.
john
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: