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01-30-2015, 11:52 PM   #1
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## Please double check my derating calculations

I would appreciate anyone sanity checking my interpretation of NEC 310's
deration tables. I am not a professional, but I like to be a safe DIY'er.

My Project:
I have wired my basement lighting and outlets with six 15a circuits.
There are 90 can lights, which are divided evenly over (5) 15a circuits and 6 switched zones. The cans are typical 75w max fixtures and I am
running 65w incandescent floods in them. I have a (1) 15a circuit for the outlets (and another, plus dedicated outlets to micro and lavatory in another conduit run, but that's not related to this).

I used #14 THHN all around. No shared neutrals. All 1/2" EMT conduit.
Basement will not go above "86 degrees" so using ambient factor 1.0.

My Concern:
I think I am interpreting the tables in NEC section 310 correctly.
I am concerned about derating ampacity for two of my conduit runs.

Run #1) Panel to Junction
Here I have (12) #14g wires. 6 hots, 6 neutrals in 1/2" emt.
The 6 "current carrying conductors" (the 6 hots) derate me to 80%.
So my calculation is:
(Wire temp rating) x (ccc derate) x (ambient temp derate)
25 x .8 x 1 = 17.5a
So, I guess I'm Ok there .. right??

Run #2) Junction to Switch Boxes
Here it gets interesting. Again, I have 12 wires, but there are 4 hots,
6 switched-hots, 2 neutrals. My understanding is that gives me 10 current carrying conductors, right? If so, the NEC chart derates me to 50%.
So my calculation is:
25 x .5 x 1 = 12.5a
Is that right??

For Run #2, at 12.5 derated ampacity, is this a problem with my scenario? To me, since all my lighting circuits are already sized to less than 12 amps (ie, less than 1440w of continuous load, for each 15a light circuit) then it
would seem I am Ok since I am pulling less than the derated ampacity.
If that is a bad interpretation on my part, then please explain where I've misinterpreted.

Any expertise is appreciated. If I've left out any info let me know.

Thank you!

Last edited by capndave; 01-30-2015 at 11:57 PM.

 The Following User Says Thank You to capndave For This Useful Post: clw1963 (01-31-2015)
01-31-2015, 09:44 AM   #2
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Actually, re-reading the code, I need to consider the neutrals as current carrying conductors. So, it looks like that puts me at 50% derating factor / 12.5 amps for
Run 1 and 2. So, I'll have to add another run of EMT and split the circuits up.

 01-31-2015, 10:14 AM #3 Remodel and New Build GC     Join Date: Sep 2010 Location: Colorado @ 7651' Posts: 5,506 Rewards Points: 3,790 Capn.... I'm a GC, not an electrician, and I sub out my conduit work... which I have in very limited instances.... But I wanted to say, your explanation of your situation, is one of the clearest, understandable, I've ever seen on here.... I generally do know that neutrals are current carrying conductors, and I'm interested in some pro electrical guys comments/suggestions / discussion on derateing issues..... Can always learn something... GOOD POST.... __________________ Never stop learning (xcep fer speling en typeing)
 The Following User Says Thank You to MTN REMODEL LLC For This Useful Post: clw1963 (01-31-2015)

 01-31-2015, 06:02 PM #4 DIYer/PoleMonkey     Join Date: Jul 2014 Location: So Calif Posts: 1,280 Rewards Points: 262 Ditto, one big ass house IMHO or commercial?
 The Following User Says Thank You to clw1963 For This Useful Post: nap (01-31-2015)
01-31-2015, 06:45 PM   #5
Remodel and New Build GC

Join Date: Sep 2010
Location: Colorado @ 7651'
Posts: 5,506
Rewards Points: 3,790

Quote:
 Originally Posted by clw1963 Ditto, one big ass house IMHO or commercial?
Must be a brother of mine in Colorado with a grow house.....(except his thinking and writting is pretty darn clear.... must be cheap weed)
__________________
Never stop learning (xcep fer speling en typeing)

Last edited by MTN REMODEL LLC; 01-31-2015 at 06:47 PM.

 01-31-2015, 08:47 PM #6 Member     Join Date: Sep 2012 Posts: 187 Rewards Points: 77 Do you realize, if you have all 90 can lights on you will be using 5800 watts of electricity. If your electric rate is .10/kWh, it will cost you .58 per hr just to see what your doing? Why are you running EMT? It would be a lot harder for a non electrician to wire with EMT as opposed to Romex. In a 3 way switch you don't count both travelers as current carrying, just count 1 traveler per 3 way circuit. Last edited by deverson; 01-31-2015 at 08:51 PM.
 02-01-2015, 11:39 AM #7 Newbie   Join Date: Feb 2008 Posts: 27 Rewards Points: 38 It's our basement. 1,700 sq/f liveable, 750 for utilities and crawl space. 80 in the liveable, 10 in the utilities and crawl. So, at the "3w per sq/f" rule of thumb, 90 is about right. I agree. I need to get cfl's in there if we end up using it a lot.
 02-01-2015, 11:48 AM #8 DIYer/PoleMonkey     Join Date: Jul 2014 Location: So Calif Posts: 1,280 Rewards Points: 262 LEDs?
 02-01-2015, 12:12 PM #9 Licensed electrician   Join Date: Sep 2007 Location: Maryland Posts: 10,869 Rewards Points: 1,892 Are you using the 3 watt per square foot from the service sizing rules?
02-01-2015, 03:18 PM   #10

Join Date: Dec 2014
Posts: 17
Rewards Points: 24

Don't have a code book in front of me (also I'm in Canada) but 12 wires in 1/2" EMT sounds like way over pipe fill. And yes, neutrals are current carrying conductors, especially if they're not shared. Same current returns on the neutral as went out on the hot.

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