Double Derating

[Dennis Alwon]

I remember a few threads that discussed this issue. If I had to derate for number of conductors in a conduit and also for temperature then I only needed to do the one with the largest deduction. Am I going nuts?? I remember a thread with Smart $ and a member who wrote for an electrical mag. I can't remember his name

Anyway if this is true where is the section that allows this, I have looked and done searches until I was blue in the face.

 

[Strathead]

I don't believe that is true, and it doesn't really make sense.

 

[HEYDOG]

I remember a few threads that discussed this issue. If I had to derate for number of conductors in a conduit and also for temperature then I only needed to do the one with the largest deduction. Am I going nuts?? I remember a thread with Smart $ and a member who wrote for an electrical mag. I can't remember his name

Anyway if this is true where is the section that allows this, I have looked and done searches until I was blue in the face.

Not true!

 

[augie47]

That was my thinking way back when my tool belt actually fit around my waist. I don't know if I was mistaken or there was a Code change.
(Likely the former, but I have no Code that go back to the '60s)

 

[winnie]

With respect to code requirements, you must apply adjustments for both temperature and number of conductors.

I've not done the math, but I bet if you looked closely you would find that the math for one type of derating actually changes in the context of the other type, but code doesn't take this into consideration.

-Jon

 

[kwired]

Here is how I see it, for the most commonly used applications 310.16 base ampacity is figured for a 30C ambient. You make adjustments for number of current carrying conductors in raceway when needed.

Have an ambient of other than 30C, you need to correct the table value for your "base" then apply adjustments if they are required. If ambient is less than 30C you can actually increase the base ampacity.

 

[Dennis Alwon]

I know that every test and every example always shows us doing a double derating. I remember a thread with a question where I did a double derate and got the wrong answer while Smart $ did not do a double. I think the rationale was once you derate to the lowest ampacity why would there be a need to go further? I also think there was a code change to clarify this issue. I may have the entire story wrong but I think there was something to it. I had bookmarked the thread but when we changed the software for the forum it got lost.

 

[Dennis Alwon]

I am only half crazy but I think this is what I was thinking about

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors shall
be sized to carry not less than the larger of 210.19(A)(1)(a)
or (b).
(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.
(b) The minimum branch-circuit conductor size shall
have an allowable ampacity not less than the maximum load to
be served after the application of any adjustment or correction
factors.

 

[electrofelon]

I am only half crazy

Just keep telling yourself that Dennis......

 

[Dennis Alwon]

Just keep telling yourself that Dennis......

I have to hold on to something....

 

[Dennis Alwon]

In this section if you had a 40 amp motor and a ambient temp factor of .7 then you wouldn't do both calculations

40 x 1.25 = 50 amp
OR
40/.7 = 57 amps
That means our conductor would have to be good for 57 amps and NOT

40 x1.25 and then divide by .7 That would be 71 amps

 

[infinity]

Let's make this more confusing what is the definition of ambient temperature?

 

[kwired]

I am only half crazy but I think this is what I was thinking about

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors shall
be sized to carry not less than the larger of 210.19(A)(1)(a)
or (b).
(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.
(b) The minimum branch-circuit conductor size shall
have an allowable ampacity not less than the maximum load to
be served after the application of any adjustment or correction
factors.

All that is saying is you need to use the larger of a or b.

Most the time b will be larger if any adjustments/corrections are needed. b can be lesser if the ambient is lower than 30C though, in that situation you still need to be no smaller than a with your conductor ampacity.

 

[Dennis Alwon]

Let's make this more confusing what is the definition of ambient temperature?

And ambient temperature changes throughout the year. Ambient is really referring to the immediate surroundings. It is a terrible use of the word

 

[Strathead]

And ambient temperature changes throughout the year. Ambient is really referring to the immediate surroundings. It is a terrible use of the word

But it says maximum ambient temperature

 

[Dennis Alwon]

But it says maximum ambient temperature

So if 2 days out of the year the outside temp reaches 100 degrees you would base the calculation on that? There are so many variables. I wire a house that is under trees so the temp in the attic is not very hot but then some trees get removed.... I think they need to come up with a better term or at least a good way to determine ambient temp. I think there is some chart somewhere for this...

 

[HEYDOG]

So if 2 days out of the year the outside temp reaches 100 degrees you would base the calculation on that? There are so many variables. I wire a house that is under trees so the temp in the attic is not very hot but then some trees get removed.... I think they need to come up with a better term or at least a good way to determine ambient temp. I think there is some chart somewhere for this...

Here is a link of some studies done for rooftop temperatures and conductors in raceways! https://copper.org/applications/electrical/building/pdf/rooftop.pdf

 

[HEYDOG]

Here is another link about adjustment factors for conductors!

Article 310, Conductors for General Wiring - IAEI Magazine

NEC Article 310 deals with conductors. Conductors are used on every electrical installation, so naturally we have a variety of installations

iaeimagazine.org

 

[infinity]

You can post links to articles.

 

[HEYDOG]

You can post links to articles.

Okay! Thank you!