Ambient Temperature :)

From what I've read on other posts, I know that the definition of "ambient temperature" is not clear in the code book, but I'm going to throw this out there anyway.

I have a customer that tests generators. He has a open bottom ladder style cable tray feeder with (6) tri-tied sets of #500 MCM CT rated copper conductors protected by a 2000 amp breaker (100% rated) and feeding a load bank with a max load of 1972 amps (I've debated that the load will get that high. This is figured at 240 volts and I know they don't produce 240V 3-phase generators, but anyway). Another EC has told him that his cables are undersized.

The tray runs outside from the load bank and into a building, through an air conditioned space, and into a "test cell". It does get warm in the test cell, but it's only the last 10' of the cable tray run.

Per the last sentence of 392.80 (a)(2)(d) where it states "supported on a messenger" I'm using table 310.15(b)(20) that has #500 MCM rated for 496 amps at 75 degrees. When I de-rate for continuous load I'm using the 90 degree column that has 500's rated at 580....this gives me 580x6x.8 = 2,784 amps. Even if I use the 496 in the 75 degree column and derate that for continuous load I get 2380 amps.

Am I missing something? Do I need to derate for ambient temperature due to the last 10 or 20' of the run being in a warm test cell? If so can I use the 90 degree column?
 

GoldDigger

Moderator
Staff member
Do I need to derate for ambient temperature due to the last 10 or 20' of the run being in a warm test cell?
See 310.15(A)(2) Exception [2011]. If the total length of the run is 100' then you could get away with 10' in the test cell.
But even then you would need to derate based on ambient for any temperature limited terminations which are inside the test cell.
 

Smart $

Esteemed Member
Location
Ohio
Do I need to derate for ambient temperature due to the last 10 or 20' of the run being in a warm test cell? If so can I use the 90 degree column?
Yes, you need to apply adjustments for ambient temperature... but see 310.15(A)(2) Exception. If the 10' through the test cell is equal or less than 10% of the circuit length, you get to ignore it. And provided the conductors are 90?C-rated, you can use that column's value as the basis for adjustment.
 
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Smart $

Esteemed Member
Location
Ohio
See 310.15(A)(2) Exception [2011]. If the total length of the run is 100' then you could get away with 10' in the test cell.
But even then you would need to derate based on ambient for any temperature limited terminations which are inside the test cell.
Please elaborate.
 
So 580 (90 deg) x 6 sets x .8 (for continuous load) x .87 (if ambient is determined at 105-113) = 2422 amps.....right?

Thanks for the help guys.
 

Smart $

Esteemed Member
Location
Ohio
So 580 (90 deg) x 6 sets x .8 (for continuous load) x .87 (if ambient is determined at 105-113) = 2422 amps.....right?

Thanks for the help guys.
Perhaps I'm misunderstanding, but if you have a 100%-rated breaker, why are you figuring 125% for continuous load???
 

Smart $

Esteemed Member
Location
Ohio
See 310.15(A)(2) Exception [2011]. If the total length of the run is 100' then you could get away with 10' in the test cell.
But even then you would need to derate based on ambient for any temperature limited terminations which are inside the test cell.
Please elaborate.
So 580 (90 deg) x 6 sets x .8 (for continuous load) x .87 (if ambient is determined at 105-113) = 2422 amps.....right?

Thanks for the help guys.
Also, I believe what GoldDigger is referring to is if your terminations are rated 75?C, your final ampacity after derating cannot exceed the 75?C column value of 310.15(B)(16).
 
That was a point of debate. You can add that to my list of questions:

1. Do the conductors need to be sized for 125% of the load? Someone smarter than I told me that the do.

2. Do I need to "double de-rate"? Meaning derate for continuous load and then for ambient? Or just worst case scenario?

3. When I'm done derating, if the value is higher than what I get in the 75 deg column, do I use the 75 deg rating? Edit: You just answered this, so disregard.
 

Smart $

Esteemed Member
Location
Ohio
That was a point of debate. You can add that to my list of questions:

1. Do the conductors need to be sized for 125% of the load? Someone smarter than I told me that the do.
With a 100%-rated breaker, you figure continuous loads at 100%... not 125%


2. Do I need to "double de-rate"? Meaning derate for continuous load and then for ambient? Or just worst case scenario?
This question is rendered moot per the preceding answer... but we can discuss this later if you want.

3. When I'm done derating, if the value is higher than what I get in the 75 deg column, do I use the 75 deg rating?
If your terminations are 75?C-rated, you have no choice but to use the 75?C column value of Table 310.15(B)(16) as the maximum ampacity.

380A x 6 = 2280A
 

GoldDigger

Moderator
Staff member
If your terminations are 75?C-rated, you have no choice but to use the 75?C column value of Table 310.15(B)(16) as the maximum ampacity.

380A x 6 = 2280A
That is one basic hard and fast limitation regardless of ambient. But if the terminations are in an ambient of 60?C, then the ampacity correction because of ambient will have to be applied to that value, just as it would have to be factored in along with the ampacity adjustment in the other branch of the calculation where you are allowed to start at 90?C. The short distance exception may allow you to escape the ambient derating of the conductors, but will not, IMHO, allow you to ignore the ambient effect at the termination.
The latter point is arguable, since the code language does not explicitly say that, but if that is not the case, the whole issue of termination temperature could be bypassed just by sticking a 6" pigtail in line using a higher temperature rated butt connector.

PS: By that I mean a same-size pigtail. If you increase the wire size in the pigtail it should legitimately ease the termination temperature restriction. :)
 
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It looks like no matter which way you slice it, the conductors are not undersized.

But......why would I go back to 310.15(b)(16) when 392.80(a)(2)(d) says that I can use 310.15(b)(20)?
 

Smart $

Esteemed Member
Location
Ohio
It looks like no matter which way you slice it, the conductors are not undersized.

But......why would I go back to 310.15(b)(16) when 392.80(a)(2)(d) says that I can use 310.15(b)(20)?
Because you can use 310.15(B)(20) for conductor ampacity. But 110.14(C) says you have to coordinate the conductor ampacity with the termination temperature limitations... which isn't really an ampacity, but rather a minimum permitted conductor size at the termination, and that is determined per 310.15(B)(16). The 75?C column value of that size conductor is the maximum permitted current allowed through the conductor, even if its ampacity is greater.
 

Smart $

Esteemed Member
Location
Ohio
whether the breaker is 100% rated or not has no effect on the conductor ampacity required.
Well first off I have to say I oversimplified my statement. It has to be a 100%-rated assembly, not just a 100%-rated breaker.

Now that I got that out of the way, please refer to 210.19(A)(1) Exception and/or 215.2(A)(1) Exception No. 1.

Then come back and explain what you mean...
 

Smart $

Esteemed Member
Location
Ohio
That is one basic hard and fast limitation regardless of ambient. But if the terminations are in an ambient of 60?C, then the ampacity correction because of ambient will have to be applied to that value, just as it would have to be factored in along with the ampacity adjustment in the other branch of the calculation where you are allowed to start at 90?C. The short distance exception may allow you to escape the ambient derating of the conductors, but will not, IMHO, allow you to ignore the ambient effect at the termination.
The latter point is arguable, since the code language does not explicitly say that, but if that is not the case, the whole issue of termination temperature could be bypassed just by sticking a 6" pigtail in line using a higher temperature rated butt connector.

PS: By that I mean a same-size pigtail. If you increase the wire size in the pigtail it should legitimately ease the termination temperature restriction. :)
Your assessment is logical and should be required, but as noted it is not. I had discussed collaborating on a proposal with Bob Alexander on the subject, but he never contacted me during the proposal submission period.
 

GoldDigger

Moderator
Staff member
Well first off I have to say I oversimplified my statement. It has to be a 100%-rated assembly, not just a 100%-rated breaker.

Now that I got that out of the way, please refer to 210.19(A)(1) Exception and/or 215.2(A)(1) Exception No. 1.

Then come back and explain what you mean...
Tell me what the heck "the assembly" refers to (maybe the OCPD and the enclosure it is part of?) and I will be happy to try to explain what those exceptions mean!
(Except that I then have a problem with just how the "before the application of any adjustment or correction factors" fits in.
The best I can get out of that part is that in addition to having to meet the ampacity requirements after derating, the conductors also have to meet the requirements before derating. That does not seem to be a particularly useful limitation. ):blink:
 

Smart $

Esteemed Member
Location
Ohio
Tell me what the heck "the assembly" refers to (maybe the OCPD and the enclosure it is part of?) and I will be happy to try to explain what those exceptions mean!
(Except that I then have a problem with just how the "before the application of any adjustment or correction factors" fits in.
The best I can get out of that part is that in addition to having to meet the ampacity requirements after derating, the conductors also have to meet the requirements before derating. That does not seem to be a particularly useful limitation. ):blink:
Assembly means for example the enclosure (usually ventilated IIRC), bussing, and the breaker itself.

Regarding "before the application of any adjustment or correction factors", I wish those sections were revised as such...

Feeder/Branch-circuit conductors shall have an ampacity not
less than required to supply the load after the application of adjustment and
correction factors... The minimum feeder/branch-circuit
conductor size shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Well first off I have to say I oversimplified my statement. It has to be a 100%-rated assembly, not just a 100%-rated breaker.

Now that I got that out of the way, please refer to 210.19(A)(1) Exception and/or 215.2(A)(1) Exception No. 1.

Then come back and explain what you mean...
I'll be darned.
 
Because you can use 310.15(B)(20) for conductor ampacity. But 110.14(C) says you have to coordinate the conductor ampacity with the termination temperature limitations... which isn't really an ampacity, but rather a minimum permitted conductor size at the termination, and that is determined per 310.15(B)(16). The 75?C column value of that size conductor is the maximum permitted current allowed through the conductor, even if its ampacity is greater.
Doesn't anyone see this as being odd? Triangular stacked cables with proper spacing in an open bottom/top ladder tray are limited to the same ampacity as if they are installed in a conduit? I would suggest that 392.80(a)(2)(d) allows us to use the 75 degree column "in accordance with 310.15(B)."

But what do I know...
 
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