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Old 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!

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Last edited by capndave; 01-30-2015 at 11:57 PM.
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Old 01-31-2015, 09:44 AM   #2
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Please double check my derating calculations


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.

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Old 01-31-2015, 10:14 AM   #3
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Please double check my derating calculations


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....
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Old 01-31-2015, 06:02 PM   #4
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Please double check my derating calculations


Ditto, one big ass house IMHO or commercial?
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Old 01-31-2015, 06:45 PM   #5
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Please double check my derating calculations


Quote:
Originally Posted by clw1963 View Post
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)
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Old 01-31-2015, 08:47 PM   #6
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Please double check my derating calculations


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.

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Old 02-01-2015, 11:39 AM   #7
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Please double check my derating calculations


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.
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Old 02-01-2015, 11:48 AM   #8
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LEDs?
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Old 02-01-2015, 12:12 PM   #9
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Please double check my derating calculations


Are you using the 3 watt per square foot from the service sizing rules?
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Old 02-01-2015, 03:18 PM   #10
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Please double check my derating calculations


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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