Derating...is it taken seriously?

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peter d

peter d

Senior Member
Location
New England
In this area, especially in the large chain retail stores, it's common to use one or several large pipes (1.25" or 2") for ALL of the lighting home runs, without upsizing the conductors, at least to a code compliant size. They are then run into crammed wireways or 4 11/16" boxes that are ready to pop open.

The inspectors in the area usually just let it fly. :roll:

It seems this is an issue that is not easily understood, or just not taken seriously. :?
 
Basically I was taught the derating rules in class's and as soon as I expected to implement them in the field I was looked at like I was crazy. :roll:

Luckily at this point it is up to me. I try very hard to comply. I won't say that I have never violated 310.15(B)(2).
 
Re: Derating...is it taken seriously?

it is entirely possible that not all of the conductors are considered current carrying. no current = no heat generated = no derating required.
 
It has almost become a "culture" thing in our industry - maybe in our area - that anything that requires a calculation in the field gets ignored, except by a few who actually know how to make the calculation.

How many inspectors do you see with a calculator?
 
iwire said:
Basically I was taught the derating rules in class's and as soon as I expected to implement them in the field I was looked at like I was crazy. :roll:
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Same here. I walked off one job a couple of months ago because of pipe fill and derating being completely ignored.
 
tonyi said:
iwire said:
Basically I was taught the derating rules in class's and as soon as I expected to implement them in the field I was looked at like I was crazy. :roll:
Click to expand...

Same here. I walked off one job a couple of months ago because of pipe fill and derating being completely ignored.
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years ago a piece of equipment I designed the controls for was installed by a contractor who took a fixed price bid on a major rework of a plant prior to knowing just what equipment he would need to even install. I have no clue how he came up with a price. you would recognize the contractors name if I emntioned it.

you cannot imagine how many wires they packed in conduits. one conduit was so jammed they put two seals in parallel in that conduit because they could not get all the wires through one seal.
 
What I run into is getting a derating flag from inspectors who are incapable of calculations or learning. I get told I need to derate 5 #12 thhn in a 3/4" emt to 15 amp breakers. When I try to explain that I have de-rated them and then put them on a 20 amp breaker, out comes the flag.
 
Tonyi,
That is quite admirable. :!:

Here in Massachusetts 310.15(B)(2) is revised stating
Mass. NEC

4 though 6 @ 80 percent 4 though 6 @ 80 percent
7 though 24 @ 70 percent 7 though 9 @ 70 percent
25 though 42 @ 60 percent 10 though 20 @ 50 percent

42 10 Awg conductors at 90 degrees Celsius would be good for 24 amps a piece (10 Awg @ 40 amps). I have seen many a pipe filled to the max. Few were NEC compliant.

How is it that Massachusetts is contrary to the NEC?


Justin W.
 
Most common around here is "you can't do that."

As in, the result of derating is somehow going to be "zero". :roll:

Very rare to be called on this (bundling, in my case.)
 
I can't stand full boxes or pipe. When I see one, I count the conductors. And I like to pride myself & not use a calculator :) I will also write the violation on the inspection card.
 
Don't I get to start in the 90? column when I start my derating for #12 THHN? It's not until I reach 10 current carrying conductors that I have to worry about derating. That's 3 full boats, plus an extra circuit, 15 wires total (I like to pull a ground). I'll rack up another conduit before I pull that many wires in one raceway.
 
I enforce it. I give the electrician a heads up on rough. I get a lot of responses like - "but isn't 2 inch EMT is good for XX amount of #12's???"

Anyway, it is not a violation I can point any failures at.

Does anyone know of any failures due to NOT derating???
 
paul said:
Don't I get to start in the 90? column when I start my derating for #12 THHN? It's not until I reach 10 current carrying conductors that I have to worry about derating.
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Very true. The reference is 110.14(C).
 
Re: Derating...is it taken seriously?

petersonra said:
it is entirely possible that not all of the conductors are considered current carrying. no current = no heat generated = no derating required.
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I agree wholeheartedly. I never do any derating on my spares either. :twisted: :twisted:
 
More to the point, if you have a switched circuit with several conductors, but have it arranged such that not all of the conductors can have current at the same time (depending on the position of the switch), then you don't have to derate for the total number of conductors in the conduit. You only derate for the number of conductors that can be carrying current at the same time.
 
charlie b said:
More to the point, if you have a switched circuit with several conductors, but have it arranged such that not all of the conductors can have current at the same time (depending on the position of the switch), then you don't have to derate for the total number of conductors in the conduit. You only derate for the number of conductors that can be carrying current at the same time.
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Charlie, although I agree that this is a common sense approach to derating, is it specified anywhere in the NEC that CCC have to being carrying current simultaneously?
 
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