Rooftop adjustment...

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1793

1793

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Location
Louisville, Kentucky
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Inspector
I have a situation where I need to calculate the adjustments for both temp. and for CCCs.

Using #12 Cu. THHN/THWN 20 amp loads with 6 CCC 4? above roof in EMT.

Calculations:
Table 310.15(B)(2)(c) 4" = 30deg adder
Using a design temp. of 94deg

94 + 30 = 124deg.

Table 310.16
#12 @ 90deg = 30 amp
Correction factor for above calculation @ 90deg is .76

Load = 20 amp / .76 = 26.31 or 26 amp

So: 26 amp * .80, for 6 ccc, = 21 amps

I can still use #12 with a 20 amp load after all corrections?
 
Looks good to me.

I personally start at a 100F and I am further North but I don't think there is a black and white starting point. :)
 
Are you making any terminations to breakers or other devices on the roof ? If you are they would most likely have a 75 degree terminal rating. In that case I would start with the 75 degreeC column or #12 @ 75 degreesC = 25 amps.

Steve
 
Cow said:
30 x .76 x .8 = 18.24 amps??
Click to expand...
Yes... he was "back figuring" and multiplied when he should have divided...

20A ? .76 ? .80 = 32.89 minimum non-adjusted 90?C-rated wire ampacity.
30A ? .76 ? .80 = 18.24 adjusted wire ampacity for #12/90?C (see below*).​

The first is the calculated approach to determining wire size and type. The second is the guesswork method.

*Technically?though there are restrictions?a branch circuit and its wire ampacity on a 20A breaker can be as low as fractionally greater than 15A... as long as the calculated load is equal or less than this value.

15.1 ? .76 ? .80 = 24.84 minimum non-adjusted 90?C-rated wire ampacity.​
 
Cow said:
30 x .76 x .8 = 18.24 amps??
Click to expand...


Smart $ said:
Yes... he was "back figuring" and multiplied when he should have divided...
Click to expand...

Looks like I should have broke out the calculator before saying it looked good.

Sorry folks.
icon11.gif




SG-1 said:
Are you making any terminations to breakers or other devices on the roof ? If you are they would most likely have a 75 degree terminal rating. In that case I would start with the 75 degreeC column or #12 @ 75 degreesC = 25 amps.

Steve
Click to expand...


There would be no need to start with the 75 C rating as long as the finished result is under the 75 C rating.

See the last sentence of 110.14(C)
 
Cow said:
30 x .76 x .8 = 18.24 amps??
Click to expand...

Smart $ said:
Yes... he was "back figuring" and multiplied when he should have divided...
20A ? .76 ? .80 = 32.89 minimum non-adjusted 90?C-rated wire ampacity.
30A ? .76 ? .80 = 18.24 adjusted wire ampacity for #12/90?C (see below*).
​
The first is the calculated approach to determining wire size and type. The second is the guesswork method.

*Technically?though there are restrictions?a branch circuit and its wire ampacity on a 20A breaker can be as low as fractionally greater than 15A... as long as the calculated load is equal or less than this value.
15.1 ? .76 ? .80 = 24.84 minimum non-adjusted 90?C-rated wire ampacity.
​
Click to expand...

iwire said:
Looks like I should have broke out the calculator before saying it looked good.

Sorry folks.
icon11.gif
Click to expand...

I was thinking something did not look right with my original calculations. I was trying to follow the 2008 HB explaining how to figure the temp. adjustment and then add the CCC derating.

So am I correct that the final rating should/would be 18.24A?
 
My head hurts from looking over all of this. I need to back up and rethink all of this.

If the rating of the #12 is dropped to 18.24 then I would have to put this on a 15A breaker instead of a 20A. I'm swimming in circles at this point.
 
1793 said:
My head hurts from looking over all of this. I need to back up and rethink all of this.

If the rating of the #12 is dropped to 18.24 then I would have to put this on a 15A breaker instead of a 20A. I'm swimming in circles at this point.
Click to expand...

For me dumbing down the calculation makes things simpler. Forget division by decimals and just use multiplication. Take the conductor that you think would work and multiply it's ampacity by the two correction factors. Now see if the adjusted ampacity is large enough for the load served, if not try a larger conductor in the calculation. And don't forget about 240.4(B), you may be able to go up to the next standard sized OCPD.
 
iwire said:
...

There would be no need to start with the 75 C rating as long as the finished result is under the 75 C rating.

See the last sentence of 110.14(C)
Click to expand...
I know what you say here is the norm, but I'm thinking, as I have before, the 75?C ampacity also has to be adjusted before the comparison is made. The idea behind the comparison is that we don't exceed the ampacity at which this particular size conductor goes over 75?C... and a 75?C rated conductor would be subjected to the same conditions if it were run instead of a 90?C-rated conductor... so it stands to reason to also adjust the 75?C ampacity.
25A ? .67 ? 80% = 13.40A to reach 75?C under the same conditions.​
Hence this #12 90?C-rated conductor will reach 75?C when conducting only 13.40A.

Food for thought :cool:
 
1793 said:
If the rating of the #12 is dropped to 18.24 then I would have to put this on a 15A breaker instead of a 20A. ...
Click to expand...

If this is not a multi-receptacle circuit, you can keep it on a 20A breaker as long as the calculated load for the circuit does not exceed the 18.24A.
 
Smart $ said:
I know what you say here is the norm, but I'm thinking, as I have before, the 75?C ampacity also has to be adjusted before the comparison is made.
Click to expand...

That is not what the last senstance of 110.14(C) says.

Feel free to do it your way but is not required.
 
iwire said:
That is not what the last senstance of 110.14(C) says.

Feel free to do it your way but is not required.
Click to expand...
I understand it is not required... by convention. The wording itself is neither explicit nor concise enough to say what way it should or shouldn't be done.

...and it wasn't the last sentence of 110.14(C) "general statement" that I am referencing... (see highlighted quote of text below).

General statement does say you cannot exceed the lowest rated temperature.

Subparagraph (1) says to base ampacities on Table 310.16. Is not ambient temperature correction a part of Table 310.16? (a)(2) and (b)(2) do use the word "ampacity" and per Table 310.16 ambient correction is required for conductor ampacity.

110.14(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.
(1) Equipment Provisions. The determination of termination
provisions of equipment shall be based on
110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed
and marked otherwise, conductor ampacities used in determining
equipment termination provisions shall be based on
Table 310.16 as appropriately modified by
310.15(B)(6).
(a) Termination provisions of equipment for circuits
rated 100 amperes or less, or marked for 14 AWG through 1
AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60?C (140?F).
(2) Conductors with higher temperature ratings, provided the
ampacity of such conductors is determined based on the
60?C (140?F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the
equipment is listed and identified for use with such
conductors.
(4) For motors marked with design letters B, C, or D, conductors
having an insulation rating of 75?C (167?F) or
higher shall be permitted to be used, provided the ampacity
of such conductors does not exceed the 75?C
(167?F) ampacity.​
(b) Termination provisions of equipment for circuits
rated over 100 amperes, or marked for conductors larger
than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75?C (167?F)
(2) Conductors with higher temperature ratings, provided
the ampacity of such conductors does not exceed the
75?C (167?F) ampacity of the conductor size used, or
up to their ampacity if the equipment is listed and identified
for use with such conductors​
Click to expand...

However, one must apply adjustments appropriately. Take for example Norb's situation where most of the run is on the roof but both ends of the run(s) drop down into and are terminated in a 30?C living area. Conventional application of using a higher temperature-rated conductor for termination adjustment would be appropriate.

Yet again, let's say we have a similar scenario but the load end(s) terminate on the roof into 75?C-rated mechanical lugs and the supplied devices, equipment, and such has an equivalent or better temperature rating. The first sentence of 110.14(C) says we must select and coordinate the ampacity so as not to exceed the lowest temperature rating of any connected termination, conductor, or device... which is 75?C in this case. So do we not have to correct for ambient temperature the tabled 75?C ampacity to determine the current level at which the conductor exceeds 75?C?

What if there are more than 3 ccc's in the run? Again the key here is by the first sentence of 110.14(C), we have to determine at what current level does the size used exceed the permitted temperature.
 
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Just a thought. Looking at 300.9 in the 2008 I would consider this a wet location since it is installed on the roof. Therefore wouldn't you have to start with the 75 degree column for thwn in table 310.16. That is unless it is
thwn-2.
 
CONDUIT said:
Just a thought. Looking at 300.9 in the 2008 I would consider this a wet location since it is installed on the roof. Therefore wouldn't you have to start with the 75 degree column for thwn in table 310.16. That is unless it is
thwn-2.
Click to expand...
I agree along with the def of wet locations article 100
Location, Wet. Installations underground or in concrete slabs or masonry in direct contact with the earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas; and in unprotected locations exposed to weather.

To use 90 degree would have to be THWN-2
 
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