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Old 06-07-2011, 08:35 PM   #1
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derating and logistics woes


I am in the process of upgrading my electrical service and I'm running into a few issues. Advice? Thanks!

Note: I am doing the work and I am learning a huge amount. I am going to have all work checked and service moved by a licensed electrician before inspection. I'm in the spirit of DIY but not at the expense of safety.

Background
There are two large junction boxes attic connected to the sub distribution panel with 3x3/4" conduit runs. I'm using #12 as I was given quite a bit of wire even though all but one circuit will be 15a.

My problem:
I have 12 circuits currently, 4 as existing 'boats' leaving me with 22 conductors. -2 for the multiwire circuit grounded conductors. I need to figure out a way to run as little conduit as possible into the attic.

Primary Issues:
1. Derating:
In addition to the number of conductors, I have to derate according to ambient attic temp. I have a fan in the attic but it is a good 90 degrees on most warm days.
2. Adding Circuits
I will need to add circuits for a kitchen remodel and a bathroom.
I'm thinking at least 10 circuits plus a few for contingency totaling 40 conductors to fanangle into the attic.
3. Sub Panel Positioning
The panel is a NEMA 3R and I'm semi flush mounting it. Knockouts are only through the bottom and there is a fireplace behind it. I have to go through two studs and up through a 2x top plate to get into the attic. I have three 3/4 runs so far. I can swiss cheese the studs but its not ideal. I also only have three more knockouts that will be behind the stucco after restuccoing.

This is the sub panel that will be the distribution panel for the house. Try not to laugh at the poor placement. I actually was given only one choice of placement by code.

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Old 06-07-2011, 08:48 PM   #2
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derating and logistics woes


You might want to consider this regarding mounting the panel semi-flush.

312.2 Damp, Wet, or Hazardous (Classi
fied) Locations.
(A) Damp and Wet Locations.
In damp or wet locations,
surface-type enclosures within the scope of this article shall
be placed or equipped so as to prevent moisture or water
from entering and accumulating within the cabinet or cutout
box, and shall be mounted so there is at least 6 mm (
14

in.) airspace between the enclosure and the wall or other
supporting surface. Enclosures installed in wet locations
shall be weatherproof. For enclosures in wet locations,
raceways or cables entering above live parts shall use
fittings

listed for wet locations.

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Old 06-07-2011, 08:58 PM   #3
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crap... thank you, i wish someone told me that previously.

Strange though. The inspector came by and gave it a preliminary when it was installed as it is and never said anything. Do I take my chances or correct my plunder?

I just have to say that beware of taking advice even from electricians. I know two electricians who actually recommended I semi flush mount instead of surface mounting before I started this project.
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Old 06-08-2011, 03:47 AM   #4
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Is that panel mounted outside of the house?
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Old 06-08-2011, 07:33 AM   #5
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Outdoor
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Old 06-08-2011, 07:55 AM   #6
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I suggest contacting the inspector now and tell him about the current semi-flush mounting and section 312.2 that Jim pointed out. Be up front with him now about it. If he won’t accept it later when the wall is finished, it’s a lot easier to fix it now.
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Old 06-08-2011, 09:36 AM   #7
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Quote:
Originally Posted by SD515 View Post
I suggest contacting the inspector now and tell him about the current semi-flush mounting and section 312.2 that Jim pointed out. Be up front with him now about it. If he wonít accept it later when the wall is finished, itís a lot easier to fix it now.
That sounds like the best course of action. In the case he wants it taken out and surface mounted, how would you run the conduit to the attic from the box in the least ugly way?
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Old 06-10-2011, 04:33 PM   #8
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Inspector Okayed the semi recessed installation. .
The old panel is actually semi recessed and doesn't look like it has had an issue in its 60 years of service.

I'm looking at adding 3 more 3/4FMC runs. Do the two studs on the left of the panel count as double? Meaning: can I drill a hole up to 60% of the stud width instead of the normal 40%? I'd like to put in 1" with a 1.5" hole.

Are these derating calc correct?

#12 90c derating calc:
30a x .70 (fill 6-9 wires) x .70 (135F Derating for the attic) = 14.7A rounds to 15A breaker allowable per 240.3(B)

#10 90c derating calc:
40a x .70 (fill 6-9 wires) x .70 (135F Derating for the attic) = 19.6 rounds to 20A breaker allowable per 240.3(B)

My rounding is allowable, correct?
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Old 06-10-2011, 04:50 PM   #9
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I believe 230.4(B)(1) would prohibit your round up.

(1) The conductors being protected are not part of a multioutlet
branch circuit supplying receptacles for cord and-
plug-connected portable loads.
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Old 06-10-2011, 07:19 PM   #10
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I could easily use 45C derating at .82 factor. forget about the rounding. A .82 would give 17.22A and 22.96A respectively. What about the hole sizes and is the way I did the calculations correct?

I sincerely appreciate the help. Its so nice that professionals like yourself take time to answer questions.

If there was a structural concrete section I would help there!
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Old 01-15-2012, 10:36 AM   #11
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Just an update for other's reference.

Inspection went great and everything passed. He laughed that I had put together a spreadsheet of junction box fill and derating calculations.

He did make me put my 10' ground rod 1 foot above the ground. I originally had installed the rod with the top below ground with a burial rated clamp but left it exposed for inspection. I believe that NEC 250.53 allowed my original installation. I did bring up the NEC section but in the end it was easier just to pull the rod up with a come along than to get into an argument with someone more knowledgeable than me.

I agree that the recessed Nema 3r distribution panel installations is technically against code but the inspector said that this type of installation is common place and has not been a problem in his experience.

Where I think it would have made me feel a lot better to know that it was by the book, the installation looks extremely clean and I have no doubts about it's longevity.

Thanks to everyone for the great help.

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