conductor ampacity

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electricalperson

electricalperson

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massachusetts
the nec says we have to base conductor ampacity off of the terminal ratings. if we have 75 degree terminals we use the 75 degree column for thhn for example. now thhn has an insulation tempature rating of 90 degrees c.

im having a hard time understanding how to correct the ampacity based of ambient tempature and terminal ratings. if i correct the tempature of the 90 degree column, whats the point in doing that if we have to use the 60 or 75 degree column because of the terminals?

should i base the tempature rating off of the 75 degree column when i know that its going to land on terminals?

i hope this makes sense since it doesnt to me and the code doesnt really say anything neither does the mike holt nec prep book.
 
because the terminals have a 60 or 75 degree rating the smallest conductor you can use has to be selected from the 60 or 75 degree column in 310.16.

Now lets assume that you have 10 current carrying conductors in the raceway that will contain this conductor, you will have to derate this conductor (as well as the 9 others).

When you start the deration calculations you can start with the 90 degree ampacity value if it is a 90 degree conductor. The same also applies if you have to derate because of a high ambient temperature.

If after derations your minimum 90 degree conductor size is smaller than the 60 or 75 degree conductor (whatever the terminals are) you still have to use the 60 or 75 rating as the smallest conductor allowed.
 
A better way to size conductors that I was once taught was to do it in a reverse way from what most people usually do to calculate conductor size. Instead of determining your load, selecting a conductor size, then derating, possibly derating again, then see if your conductor is still large enough a person should determine ampacity needed for load, uprate ampacity where needed then the result of your calculations is your minimum ampacity. I will run through a five step method that I was taught how to do this.

Step 1

Calculate the load at 125% of continuous load, and 100% of non continuous load. (If an assembly rated 100% use 100% for both continuous and non continuous)

Step 2

Based on equipment rating select a conductor from 60 or 75 degree column for the load calculated in step 1.

If deration is not required you are done this is your minimum conductor size.

The remaing steps deal with deration and are a new calculation. Start over using 100% of load and select conductor at its maximum operating temperature (90 degrees for THHN)

Step 3

If more than three current carrying conductors in raceway or cable adjust according to 310.15(B)(2)(a)
(to possibly make less steps, for 4-6 conductors for example - instead of derating a conductor by 80% uprate the load by the reciprocal of 80% which is 125%)

Step 4

If ambient temperature correction is required adjust again according to correction factors at bottom of 310.16.
(again instead of derating a conductor by a percentage uprate the load by the recriprocal of the applicable deration factor)

Step 5

Use the load you now have calculated after steps 3 and 4 and and select a conductor from appropriate column in 310.16 (90 degree for THHN).

Compare this conductor with the one selected in steps 1 and 2.
The larger of the two is your minimum conductor size.

By using recriprocals of derations and applying them to load values you save the possibility of selecting a conductor and then derating it only to find out it is too small and starting the calculations over with a larger conductor.
 
so when you derate the conductors in the 90 degree column, if that number is smaller than the 75 degree column you have to use the 75 degree column based on terminal ratings?

so if i have a 100 amp continuous load

100 x 1.25 = 125

ambient tempature is 55 degrees c,

a conductor that can hold 125 amps in the 90 degree column is a number 2

number 2 is rated for 130 amps

130 x .76 = 98.8

the next size up is 1/0 and after derating it comes out to 129.2 amps

in the75 degree column 1/0 is rated for 150 degrees

150 x .67 = 100.5

next size up is 2/0

175 x .67 = 117.25

3/0
200 x .67 = 134 amps

so the conductor id have to use for a 100 amp continuous load in an area with an ambient tempature of 51-55 degrees is 3/0?
 
hardworkingstiff said:
Not from what you wrote.

Here is how I would have done it.

100-amp continuous load * 125% = 125-amp load.

125 / .76 (90C derating for 55C ambient temp) = 164.5 or 165.

using the 90C column (THHN) of table 310.16 I see that #1/0 is the smallest conductor that would be allowed for the conditions listed.
Click to expand...

i get that part but what about the terminals it lands on? how do we factor in terminal tempature after we derate the wire?
 
i dont get ampacity adjustment. if we use the 90 degree column to adjust the ampacity of the thhn conductor, the code says we have to use the 75 degree column if the terminals are rated for 75 degrees. i understand how to adjust the ampacity im just confused if i have to adjust it then terminate them. what if the ampacity of the conductor is higher than the 75 degree ampacity? isnt that a code violation?
 
electricalperson said:
i dont get ampacity adjustment. if we use the 90 degree column to adjust the ampacity of the thhn conductor, the code says we have to use the 75 degree column if the terminals are rated for 75 degrees. i understand how to adjust the ampacity im just confused if i have to adjust it then terminate them. what if the ampacity of the conductor is higher than the 75 degree ampacity? isnt that a code violation?
Click to expand...

I assume you are referencing 110.14(C). As far as I understand, all that is telling you is that whatever conductor you use, it must be rated for the load at the appropriate column (60 or 75) of table 310.16. If you are derating for ambient temperature, then your wire size will usually be larger than the 110.14(C) minimum requirements. You don't apply derating to the terminals.
 
electricalperson said:
so when you derate the conductors in the 90 degree column, if that number is smaller than the 75 degree column you have to use the 75 degree column based on terminal ratings?

so if i have a 100 amp continuous load

100 x 1.25 = 125

ambient tempature is 55 degrees c,

a conductor that can hold 125 amps in the 90 degree column is a number 2

number 2 is rated for 130 amps

130 x .76 = 98.8

the next size up is 1/0 and after derating it comes out to 129.2 amps

in the75 degree column 1/0 is rated for 150 degrees

150 x .67 = 100.5

next size up is 2/0

175 x .67 = 117.25

3/0
200 x .67 = 134 amps

so the conductor id have to use for a 100 amp continuous load in an area with an ambient tempature of 51-55 degrees is 3/0?
Click to expand...

hardworkingstiff said:
Not from what you wrote.

Here is how I would have done it.

100-amp continuous load * 125% = 125-amp load.

125 / .76 (90C derating for 55C ambient temp) = 164.5 or 165.

using the 90C column (THHN) of table 310.16 I see that #1/0 is the smallest conductor that would be allowed for the conditions listed.
Click to expand...


Using the 5 step method I described in Post #3 I come up with the following:

Step 1 (125% of continuous load plus 100 percent of noncontinuous load)

you had a continuous load of 100 amps X 1.25 = 125

Step 2 (select conductor based on terminal temperature rating)
Assuming your 100 amp device has 75 deg terminals 310.16 75deg copper > or = #1AWG

Derating: Start calculations over using 90 degree conductor values @ 100% of load

Step 3 derate for more than 3 current carrying conductors in a raceway or cable.

there was not more than 3 conductors no deration still leaves you needing a 100 amp 90 degree conductor.

Step 4 ambient temperature correction is required for temperature above 30 deg C Appropriate deration factor for 90 deg conductor @55 deg C = 76%.

instead of selecting a conductor at this point and then derating it only to possibly find out you need a larger conductor and then derating again to make sure it is large enough use the recriprocal of the deration factor on the required ampacity value.
100 amps X (recriprocal of .76 = 1.31) = 131 amps

Step 5 select conductor from 310.16 90 degree column
131 amps > or = #1AWG

select larger of step 2 or step 5 = #1AWG is the minimum size conductor required for your application.
 
kwired said:
Using the 5 step method ...
...
...
...#1AWG is the minimum size conductor required for your application.[/b]
Click to expand...
Hmmm.... can't agree. You have to use 125% continuous load in your step 3 as you are determining conductor ampacity.
Code: 
	0		100% non-continuous load
+	[U]125[/U]		125% continuous load
	125		Minimum Circuit Ampacity
?	[U]0.76[/U]		Ambient factor for 75/90? conductor
	164		
?	[U]100%[/U]		More than 3 CCC derating factor
	164		Minimum 75/90?C Table Ampacity
Looking at 1/0, 90?C-rated @ 170A here...
 
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electricalperson said:
the nec says we have to base conductor ampacity off of the terminal ratings. if we have 75 degree terminals we use the 75 degree column for thhn for example. now thhn has an insulation tempature rating of 90 degrees c.

im having a hard time understanding how to correct the ampacity based of ambient tempature and terminal ratings. if i correct the tempature of the 90 degree column, whats the point in doing that if we have to use the 60 or 75 degree column because of the terminals?

should i base the tempature rating off of the 75 degree column when i know that its going to land on terminals?

i hope this makes sense since it doesnt to me and the code doesnt really say anything neither does the mike holt nec prep book.
Click to expand...

It may help if before you follow the rest of the directions in this thread you change your mindset a bit. It certainly worked for me. :)

These columns have many headings. Temperature is just one of them.

When you deal with terminal ratings it is fine to think about "temperature" 60*, 75*, 90*. That's how they are rated and reading the table for temperature is intuitive.

However when I want to derate a wire I am not derating based on the conductor I am derating it based on the insulation. Here's how this makes it simpler for me.

Let's say I have THW insulated wire that I want to derate. When I get to Table 310.16 I blow right past the above mentioned numbers' since they don't really even apply to what I'm here for, and find the column titled "THW".

You should keep in mind the good advice others have said here and the articles involved but you must make a distinction between terminal temperature rating and insulation rating if you want to avoid confusion. :)



*=degrees
Smart$, how did you get a degree symbol?
 
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electricalperson said:
i dont get ampacity adjustment. if we use the 90 degree column to adjust the ampacity of the thhn conductor, the code says we have to use the 75 degree column if the terminals are rated for 75 degrees. i understand how to adjust the ampacity im just confused if i have to adjust it then terminate them. what if the ampacity of the conductor is higher than the 75 degree ampacity? isnt that a code violation?
Click to expand...
The wording of 110.14(C)(1) does lack clarity of implementation. You can find a little clarification in Example D3(a) under the subheading Ungrounded Feeder Conductors. The gist is both of the following statements must be true:
  1. Minimum circuit ampacity ≤ temp. corrected and derated conductor ampacity at rated temperature.
  2. Minimum circuit ampacity ≤ tabled 75?C value for size of conductor used.
There is no requirement to temp. correct the 75?C value, as the code is currently written, and likely will not change in the 2011 NEC. It stands to reason there should be some correction if the termination is located in an other than 30?C environment. There is a fair chance a proposal will be made for such for the 2014 cycle.
 
Dave58er said:
...

*=degrees
Smart$, how did you get a degree symbol?
Click to expand...

Hold down the Alt key while you key 0 - 1 - 7 - 6 on the number pad then release the Alt key. The other option is to copy it from Character Map, where you can find some other Alt+4 key codes in the lower right corner. You can also look them all up online. There are also some Alt+3 key codes but I don't remember them.
 
In Connecticut we are repeating this topic in the CEU classes. I taught 6 classes last year and this topic was misuderstood by most of each class.

We are covering it again and it's going alot better this year. :)
 
I start with the total load = 100 x 1.25 = 125 amps

next I divide load/muliplier= 125/.76= 164.474

90 deg column = 1/0

Now I confirm that the 75 deg column is not smaller than my load (125 A).

So 1/0 THHN is my final answer.
 
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I've been giving this example sheet to students. It is sized to fit on page 171 as noted on the sheet. I made it up in Visio and pasted it into word because I couldn't attach it as a visio doc. I hope it prints out ok.

For appretices I have them write it in but the CEU guys can insert or tape.

Let me know if I need to make any changes. I hope it works.
 
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