New Subpanel Installation - Conduit question

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  #1  
Old 11-28-13, 04:01 AM
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New Subpanel Installation - Conduit question

So I will be installing a new flush mounted panel in a wall for an attached garage and laundry room addition. It is a 100A panel with a 90A breaker in it. I plan to feed it from the main panel with 2-2-2-8 Cu NM by routing it through the crawlspace (the main panel is outside and the back already has a knockout into the crawlspace), then into the laundry room where the new subpanel will be located. All walls are 2x6 in the laundry room and garage for reference. I want to use a 2.5" PVC conduit from the crawlspace up inside the wall to the bottom of the panel (the main breaker will be mounted at the bottom of the panel). And then I wanted to use a 2.5" or 3" conduit from the top of the panel to the attic above for the branch circuits. I plan to use mostly 12/2 NM for the branch circuits.

My head is spinning trying to figure out Ch 9 Table 4 and 8 as to how many wires I can run within the conduits.

Can anyone answer that question?
 
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Old 11-28-13, 06:52 AM
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Welcome to the forums!

First off you do not need a main breaker in your sub panel is it if an attached garage. A Main Lug Only (MLO) would be just fine, unless you want a main in the garage. That will save you $40.

The breaker feeding the cable going to the garage will dictate the size of the wire going to the garage. If you want to use a 90 amp breaker you can use #3 copper. If you want a full 100 amps, you will need #2 copper.

If you are using NM cable why would you want to fight it in conduit, let alone 2.5". Neither is needed and overkill if it is installed in a wall/joist space. IF you want to use conduit, then run it the entire way and use THHN wire. You will also then be able to reduce your wire size by 1 size. Make sure to have no more than 360 degree of bends between pull points.

Same is true for the branch circuits. Unless the conduit coming out of the top is less than 24" long, you will have to derate the wires/cables if you have more then 9 current carrying conductors. ( 4 - 12/2 cables)

As far a calculation the fill of the conduit (not really needed as I pointed out above) look at Chapter 9 note #9. You need to take the cross-sectional area of the actual cable you are using and the info in tables 4 for the conduit you are using.
 
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Old 11-28-13, 06:58 AM
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I want to start by addressing your plan for the feeder and subpanel. First of all, 90 amps is enough for an entire house. Do you have some unusually large loads that will be powered from the subpanel? Even for 90 amps, you have everything substantially oversized. You could easily use 1 1/4" conduit to feed the subpanel with 3 - #3s and 1 - #8 ground (all copper THHN/THWN) and change to a 100 amp breaker. There is no need to have a main breaker in this subpanel since this is part of the house, a main lug panel would be fine, but you can have a main breaker if you wish. You'll need a main breaker hold-down kit if you are backfeeding a main breaker plugged into the bus.

If this were my project, I'd run the feeder with 1" conduit, a 60 amp breaker and 3 - #6s and 1 - #10 ground (all copper THHN/THWN).
 
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Old 11-28-13, 07:09 AM
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Thanks for the replies. I am all about overkill I already have the wire, panel, and breakers, so I don't plan to change to something else unless code requires it and/or something is unsafe. I know the main breaker in the subpanel is not required, but personal preference to shut off the panel without having to walk outside and around the house to shut it off.

Also this will be feeding the laundry room and a very large 2 story garage. So some things power off it will be an electric dryer, a 2 post lift, several tools including a large air compressor, and a mini-split heat pump unit. I believe the 90A is the proper size for this (assuming the dryer and heat pump could easily be running at the same time, and then possibly the lift or air compressor could turn on).

The conduit going from the bottom of the panel to the sill plate would only be about 48", that way I don't have to worry about mounting it inside the wall every X inches. I know I have to mount it X inches from the end of the conduit. It should in fact make things easier, especially if anything needs to be changed in the future. I wouldn't have to open up the sheetrock. Same goes for the conduit going from the top of the panel to the top plate in the wall, that would be about 24" long or so.

Quick calculations gives me the following:

type area qty total
12-2 NMB 0.132 12 1.584 Sq. inches
10-3 NMB 0.1399 4 0.5596 Sq. inches
2.1436 Sq. inches
percent 30%

diameter max area
conduit 3.042 40% 7.2642 Sq. inches
max fill 2.9057 Sq. inches

I could be off on that by a little, but it is close.

Also I don't think the wire is oversized. Southwire says that the wire is rated for 95A.
http://www.southwire.com/ProductCata...=prodcatsheet6
NM - 2/3 - 95A Max

I don't see anything about derating multiple NM cables in a conduit in Ch 9.
http://records.jonesboro.org/Technic...ode/Chap_9.pdf
 
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Old 11-28-13, 07:23 AM
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I am all about overkill I already have the wire, panel, and breakers, so I don't plan to change to something else unless code requires it and/or something is unsafe.
OK, but I'll make a couple suggestions. 1.) Check to be sure your 90 amp breaker's lugs will accept the #2 wire. It should be in fine print on the label on the breaker or check the manufacturer's catalog. 2.) Be sure to get a main breaker hold down kit for the backfed main breaker you'll be installing. 3.) Since you are using NM cable for the feeder, drop the conduit for a professional looking installation. You'll wish you had before you get done and it'll just look amateurish putting NM cable in conduit.

Also I don't think the wire is oversized. Southwire says that the wire is rated for 95A.
That is correct, it's in the 60 degree column that you must use for NM cable. I was suggesting using Type THHN/THWN which would be good for 100 amps in the 75 degree column in table 310.16.
 
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Old 11-28-13, 07:33 AM
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OK, but I'll make a couple suggestions. 1.) Check to be sure your 90 amp breaker's lugs will accept the #2 wire. It should be in fine print on the label on the breaker or check the manufacturer's catalog. 2.) Be sure to get a main breaker hold down kit for the backfed main breaker you'll be installing.

Read more: http://www.doityourself.com/forum/el...#ixzz2lxF4LN6u
The 90A Main breaker and 90A breaker in the main panel both support up to #2/0, so I am well within what it supports.

I'm not sure I understand your other suggestion. I thought a hold down kit was only for when you are doing something strange like backfeeding a regular breaker. I am feeding the main breaker in the panel. The main breaker is a Square D QOM90VH. Best photo I could find:
http://www.pacificcoastbreaker.com/Q...40&resizew=240
 
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Old 11-28-13, 07:45 AM
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I am not familiar with that breaker, how does it mount? Does it simply plug onto the bus or is held in by screws? A 90 amp main breaker would be non-standard. Are you removing a standard main breaker and replacing it with the 90 amp main breaker? I'll see if I can find some info on it.
 
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Old 11-28-13, 07:59 AM
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The main breaker is factory installed? Is it a 90 or 100 amp?

Also I don't think the wire is oversized. Southwire says that the wire is rated for 95A.
The wire is over sized. NM-b ampacity is listed in the 60 degree column. #2 is rated for 95 amps. There is no 95 amp breaker you you can go up to the next size, 100 amps. Or if you really want to use the 90 amp breaker, you could have used #3 which is rated for 85 amps. Same holds true so you could go up to 90 amp breaker.

I don't see anything about derating multiple NM cables in a conduit in Ch 9.
Look in 310.15(B)(2) (2008)
 
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Old 11-28-13, 09:22 AM
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The Panel is a Square-D HOM24M100
Image Library: HOM24M100 HOMELINE load center


It comes with a 100A main breaker which I replaced with a 90A, therefore derating the panel down to 90A based on Southwire stating that the wire is rated at 95A. I was always under the impression that you should keep your wire rating above the breaker rating. At least that is how we always do it in the data center world. But then again we are usually dealing with 250+ kcmill cables and buildings built for 2-20 MW.

Look in 310.15(B)(2) (2008)
I will read over that now. Thanks.
 
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Old 11-28-13, 09:31 AM
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Look in 310.15(B)(2) (2008)
So after reading over that, is that stating that if you run 2 x 12/3 cables in a 3" conduit, that each wire is now derated to 80%? That sounds pretty crazy. In fact I know in my house there is a house sub-panel that has a conduit from the original construction (not but 15 years ago) that goes from the bottom of the panel to the crawl space. In that conduit is the main 90A feed for the panel (NM cable) along with at least 3 other 14/2 NM cables. That would make at least 9 conductors (unless neutrals don't count?). And therefore be derated down to 70%?
 
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Old 11-28-13, 09:39 AM
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That is correct. NM-b cables have 90 degree wire in them (THHN) so you start derating at the 90 degree column. #12 THHN is rated at 30 amps. 30 x 70% is 21 amps, Still above the 20 amp breaker you are required to use under 240.4(D). However, if you go over 9 wires you have to derate to 50% which knocks you down to 15 amps. This is why it is never a good idea to install a very large conduit to run a lot of wires through unless that conduit is less than 24". Then the conduit is a nipple and derating factors do not apply.

(unless neutrals don't count?)
Neutral do not count in a multi wire circuit where there is two hots and the neutral only carries the imbalance current.

It comes with a 100A main breaker which I replaced with a 90A, therefore derating the panel down to 90A
You would not have needed to do that as the main breaker in the sub panel is only a disconnect. The 90 amp breaker in the main panel is what would be protecting the feeder and the sub panel.
 
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Old 11-28-13, 10:40 AM
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So basically if I keep the conduit going from the top of the box to the top plate under 24", I am fine. The lower conduit going from the bottom of the box to the sill plate will not really matter on length since it should only have the feeder cable (and maybe one more for the dryer).

Thanks
 
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Old 11-28-13, 10:56 AM
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Yes. If it was me, and I had to use conduit, I would just stand up multipul smaller conduits rather then one big 3". 3" is crazy large and you will have a tough time knocking out a hold for the connector. I think the biggest KO that is factory installed in a load center panel is 2" or 2 1/2".
 
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Old 11-28-13, 11:03 AM
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My load center has a 3" KO on top and bottom. I knocked out the top one in about 30 seconds. Not hard with the right tools if you know what you are doing.

It also means I only have to drill one 3" hole in my top plate, rather than a bunch of small ones, which need to be spaced at certain distances.
 
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Old 11-28-13, 07:47 PM
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My load center has a 3" KO on top and bottom.
That would be very unusual to have a 3" K.O. on any loadcenter. Are you measuring the diamter as 3" (2 1/2" K.O. is 3" Dia)? A 3" K.O. should be approximately 3 1/2" in diameter, the same as the O.D. of 3" rigid conduit.
 
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Old 11-28-13, 08:05 PM
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Using the sleeve into the top of the panel is only allowed if the panel is surface mounted. The code does not allow the sleeve into the bottom of the panel. There are also other conditions that limit the sleeve out of the top.
 
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Old 11-29-13, 05:50 AM
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That would be very unusual to have a 3" K.O. on any loadcenter. Are you measuring the diamter as 3" (2 1/2" K.O. is 3" Dia)? A 3" K.O. should be approximately 3 1/2" in diameter, the same as the O.D. of 3" rigid conduit.
I think you are correct there, it is a 3" diameter knockout, which I guess would be used for 2.5" conduit.

Using the sleeve into the top of the panel is only allowed if the panel is surface mounted. The code does not allow the sleeve into the bottom of the panel. There are also other conditions that limit the sleeve out of the top.
By sleeve, do you mean knockout? I am not sure what you are getting at exactly. I went over the NEC and could not find anything regarding using top and/or bottom knockouts only on surface mounted panels. I know for a fact that my other sub panel from when the house was built has a conduit on top and bottom. And I am sure I have seen that before as well. Can you point me to specific code please?
 
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Old 11-29-13, 07:19 AM
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From Article 312.5

(C) Cables. Where cable is used, each cable shall be secured
to the cabinet, cutout box, or meter socket enclosure.
Exception: Cables with entirely nonmetallic sheaths shall
be permitted to enter the top of a surface-mounted enclosure
through one or more nonflexible raceways not less
than 450 mm (18 in.) and not more than 3.0 m (10 ft) in
length, provided all of the following conditions are met:

(a) Each cable is fastened within 300 mm (12 in.),
measured along the sheath, of the outer end of the raceway.
(b) The raceway extends directly above the enclosure
and does not penetrate a structural ceiling.
(c) A fitting is provided on each end of the raceway to
protect the cable(s) from abrasion and the fittings remain
accessible after installation.
(d) The raceway is sealed or plugged at the outer end
using approved means so as to prevent access to the enclosure
through the raceway.
(e) The cable sheath is continuous through the raceway
and extends into the enclosure beyond the fitting not
less than 6 mm (1⁄4 in.).
(f) The raceway is fastened at its outer end and at
other points in accordance with the applicable article.
(g) Where installed as conduit or tubing, the allowable
cable fill does not exceed that permitted for complete conduit
or tubing systems by Table 1 of Chapter 9 of this Code
and all applicable notes thereto.
 
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Old 11-30-13, 05:54 AM
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By sleeve, do you mean knockout?
I think pcboss was referring to the conduit sleeve, not the knockout.
 
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Old 11-30-13, 07:00 AM
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I was referring to the restrictive conditions that allow the sleeve and showing that a bottom,sleeve is not allowed.
 
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Old 11-30-13, 07:12 AM
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PC boss, I had to approve your post #18.. It should be viewable now....Dont know why this site rejects some posts. Maybe all the parenthesis?? Spam o matic at work.
 
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Old 11-30-13, 07:25 AM
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I was referring to the restrictive conditions that allow the sleeve and showing that a bottom,sleeve is not allowed.

Read more: http://www.doityourself.com/forum/el...#ixzz2m8u3kiS6
I thought the NEC was a restrictive document, meaning that if it does not restrict something, it is allowed.

It does not mention flush mounted panels at all there. So does that mean you cannot use conduits at all for flush mounted panels? I have seen dozens of homes that have at least one conduit coming into the panel.
 
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Old 11-30-13, 07:57 AM
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PC boss, I had to approve your post #18.. It should be viewable now....Dont know why this site rejects some posts. Maybe all the parenthesis?? Spam o matic at work.
Mike, I had that happen a couple times in the last month or two. On one post about a week ago, the post never did show up.
 
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Old 11-30-13, 09:05 AM
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The exception is only for surface mounted panels. If the panel is flush mounted with a conduit sleeve it would be a violation.
 
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