110.14(C) question

[andrew grant]

The 2002-110.14(C) seems to require that the ampacity adjustments be made prior to checking for the terminal provisions. This used to be a separate test.

Example in Handbook, 210.20 seems to do it the new way, but example at 220.10 looks like the old way.

Which is it?

 

[charlie b]

Re: 110.14(C) question

Example 220.10 does a slight injustice to the reader. It leaves out what I believe to be an important ?failed attempt.? It does show that a 500 MCM THWN will not work. I think it should also show why a 500 MCM THHN would not have worked either. I think this would have clarified the issue.

So here, using their process, is why a 500 MCM THHN would not have worked either:
Table 310.16, 90C column, 500 MCM THHN is 430 amps. 80% of 430 is 344 amps. We note that a cable de-rated to 344 amps would require an OCPD no higher than 350 amps, so that a 400 amp OCPD would not be acceptable. Therefore, the 500 MCM THHN is not acceptable. When you next try the 600 MCM THHN, it will work, as is shown in the example.

So the bottom line is this: If you know you will need to de-rate (for temperature or for number of conductors) you start with the 90C table (assuming that you conductor is rated for 90C). You then apply the de-rating factor(s). You use as the final ampacity the lower of this number and the number shown in the 75C column.

 

[andrew grant]

Re: 110.14(C) question

The wording of 110.14(C)(1) seems to imply that Table 310.16 must be modified by the ampacity adjustments of 310.15(B)(1)thru(6)prior to picking a size. Is this a correct interpretation?

Example in the 2002 Handbook, 210.20 uses this procedure. The same example in the 1999 Handbook shows a different method - the independent tests that we are all used to using.

The additional code lanuage in 110.14 must mean something - what?

 

[charlie b]

Re: 110.14(C) question

Originally posted by andrew grant:The wording of 110.14(C)(1) seems to imply that Table 310.16 must be modified by the ampacity adjustments of 310.15(B)(1)thru(6)prior to picking a size. Is this a correct interpretation?

I would have stated it differently, but we might be saying the same thing. My interpretation is based on the fact that this is often a ?trial and error? process. You have to know your load first. Next you ?pick a size.? That is, you select a conductor that you think might be adequate. You take the ampacity value from Table 310.16, 90C column, and apply the appropriate de-rating factors. You compare against your required ampacity (i.e., your load). If the cable isn?t big enough, go back and try a bigger cable. Is this what you are asking?

By the way, no one in my office seems to have the 1999 handbook at their desk. So I can?t address this part of your question.

 

[andrew grant]

Re: 110.14(C) question

In the past, the test for terminal provisions was a separate activity from the test for "conditions of use" - grouping, ambient, etc.

The test for conditions of use was as you describe, applying the adjustments, using the 90C column, etc.

The test for terminal provisions (110.14C) was done by looking up the computed load in the 60C or 75C column directly, without any of the conditions of use adjustments.

There was, then, a comparison and the larger conductor size selected.

Now, it looks like the language of 110.14C1 seems to imply that the conditions of use adjustments need to be applied to the Table prior to selecting a size for terminal provisions. This will result in larger conductors than the previous method.

This is not a good thing and I am trying to confirm that this is indeed the case. I'm getting mixed opinions.

 

[kevin]

Re: 110.14(C) question

The change to 110.14(c), according to the May, 2001 NEC Report on Proposals, was submitted by Jim Pauley, Square D Co. His substantiation for the change, in essence, was solely to assure that designers and installers did not utilize the wrong ampacity table, i.e. 310.17. UL General Information Directory clearly spells out that Table 310.16 is the basis for rating terminals, but that information is generally not known to most of us who use the Code. So Jim Pauley proposed including the info from UL in the body of the NEC. I do not believe that this change in wording to 110.14(c) goes beyond that point.

Insofar as the calculations are concerned, if the minimum allowable conductor ampacity is calculated to be 375 amps, and 4 conductors are in the raceway, a simpler approach it seems here is (1)Divide 375 by 80 % (equals 469 amps), (2)Find in the 90C column of Table 310.16 that 600 kcmil Cu is the minimum acceptable size (475 amps), (3)Observe that 600kcmil will not reach 75C until it experiences 420 amps of electric current- well above the 375 amps that will result (4)Protect the circuit with the next higher rated OCPD above 375 (400 amps). Is there a reason that escapes me why a "trial-and error" method is required, especially one that seems as tedious as the 5 Steps in the Handbook for 220.10?

 

[andrew grant]

Re: 110.14(C) question

Thanks so much for helping me on this one. I appreciate all the input and learning the proposal information was very useful. Just one more question before I move on.

If the intent of the change was to focus the attention on Table 310.16 only, why the added phrase "...as appropriately modified by 310.15(B)(1) through (6)." This seems unrelated.

It (still) looks to me as though the ampacity adjustments must be applied prior to selection. This is, in fact, the process undertaken by the example in 210.20 Handbook. Using your example, Kevin, the adjusted value of 375A would have to be looked up in the 75C, not the 90C column, resulting in a selection of 750 kcmil, not 600.

I did not like this change. But I did not question it until I ran across the example in the handbook, 220.10, that is doing the old process and that brought up the question - which way is okay? What is required?

And, here we are. Thanks again for your help. I think the best thing for me is to look up the history of the change and see what its intent was and why the extra wording about modifications.

 

[kevin]

Re: 110.14(C) question

Andrew,
"There is more than one way to skin a cat." Whatever methodical steps you take to select the conductors for the given conditions is fine as long as the correct results occur. The post which I used to illustrate what works best for me was, in retrospect, not clearly enough presented. At the risk of repetition, my suggested approach is as follows:
DO THE TERMINATIONS [110.14(c)] LAST!!!
Step 1. Calculate minimum conductor ampacity for the calculated load per 210.19(A)(1). This value must be not less than 100% of the non-continuous plus 125% of the continuous load. Call this MCLA, for minimum conductor load amps.
Step 2. Adjust MCLA for (a) ambient temperature by DIVIDING BY the appropriate correction factor in Table 310.16 and (b) for more than three current-carrying conductors by DIVIDING BY the appropriate adjustment factor from Table 310.15(B)(2)(a).
Step 3. From the appropriate column in Table 310.16 select a conductor size whose allowable ampacity in the table is NOT LESS THAN calculated in Step 2.
Step 4. Lastly, check to make sure that the terminations will not be exposed- from MCLA- to more current than is allowed by Table 310.16. This is easily done visually by looking at the values stated in the table. If MCLA exceeds the maximum allowable, go up one or more size conductors until you satisfy the termination temperature restrictions.

ILLUSTRATION FROM HANDBOOK 210.20.
1. MCLA = 31.25.
2. 31.25 divided by 0.8 = 39.06.
3. Minimum 90C copper conductor size is 10AWG.
4. 10AWG won't work because it will cause the 60C terminal to reach its limit at 30 amps, but MCLA (31.25) is greater than 30 amps. Go up one size to 8AWG which will not cause the terminal to reach its maximum allowable 60C rating until 40 amps- well above MCLA of 31.25.

ILLUSTRATION FROM HANDBOOK 220.10.
1. MCLA = 375.
2. 375 divided by 0.8 = 469.
3. Minimum 90C copper conductor size is 600kcmil.
4. 600kcmil will work, because it won't cause a 75C terminal to reach its limit until 420 amps- a value higher than the MCLA of 375.

In Step 2, for the examples given, no information was given that it was necessary to do an additional adjustment for ambient temperature. Had such been the case, then the answer in that Step 2 would need to have been increased by dividing by the appropriate correction factor from Table 310.16. Steps 3 and 4, then would be conducted based on the higher value calculated thereby. For example, if the ambient temperature was given to be 100 deg F, the values in Step 2 in the above ILLUSTRATIONS would need to have been divided by .91 before proceeding to Step 3.

End result: Whatever works!

 

[andrew grant]

Re: 110.14(C) question

Thanks, Kevin. I think I will proceed as you are advising. This is the way I'm accustomed to and it seems reasonable.

I found an article written by James Pauley and he had a similar approach, so my confusion looks to be unwarranted.

However, I read the ROP about this and I found something interesting. Maybe you could talk to me about it. 1-227-(110-14c) was accepted and has been the subject of our discussion.

The one right before it, 1-226-(110.14c) by Bill Whitlow was rejected. I found this one very interesting in light of all our discussions. He looked to be trying to get words in the code to clarify that the adjustments do not apply for 110-14c. He was rejected because the panel members said he was incorrect in saying that adjustments are not to be applied.

What do you make of that?

 

[kevin]

Re: 110.14(C) question

Bill Whitlow's proposal called for deletion of a sentence which makes it clear that derating is based on the temperature rating of the conductor's insulation and not on the temperature rating of the terminals. In this it seems he misunderstood the intent of 110.14(c). I'm not sure that the Panel Statement correctly stated Whitlow's position, but they had at that time, in hand, a proposal from Pauley, who sits on the Technical Correlating Committee, that was acceptable. The rejection of Whitlow's proposal was, in my view, correct, but the stated reason for rejecting it was a bit off target.