Conductor Adjustment Factors

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iwire

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Massachusetts
I am looking for some thoughts on how the figures for Table 310.15(B)(2)(a) were arrived at.

I ask because the table we use in MA is much different.

Code:
NEC Table 310.15(B)(2)(a)
 
 4?6   conductors 80%
 
 7?9   conductors 70%
 
 10?20 conductors 50%
 
 21?30 conductors 45%
 
 31?40 conductors 40%
 
 41 and above     35%
Code:
Now from the MEC
 
 4?6   conductors 80%
 
 7?24  conductors 70%
 
 25?42 conductors 60%
 
 43 and above     50%
So what are your thoughts are the NEC figures overly cautious?

Are the MEC figures to lenient?

I install to the MEC and have seen no problems.

I am not looking to change anything just further my understanding of this.

I will say in general I think his makes us MA electricians less worried about derating when working in states under the NEC. :(
 

don_resqcapt19

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Illinois
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retired electrician
Re: Conductor Adjustment Factors

Bob,
That is the old NEC ampacity adjustment factor table. You can still find that table in the NEC in Annex B (Table B.310.11). The difference is that in the Annex B table, for conductor fills greater than 9, the table assumes a "load diversity of 50%". If you use the MA table and all of the conductors are fully loaded you will have a problem. One saving factor is that the Article 220 calculations often result in an artificially high load. Look at how the utility sizes their service equipment compared to how we size ours. Another problem with using "load diversity" is that no one can agree on exactly what "load diversity" means and how you calculate it.
Don
 

crash

Member
Re: Conductor Adjustment Factors

That's interesting information Don. Out of curiosity over I-wire's enquiry, I looked up the figures in the CEC and found them identical to the old NEC figures. When did the NEC change those figures?
 

don_resqcapt19

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Location
Illinois
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retired electrician
Re: Conductor Adjustment Factors

In the '87 code, Note 8 to Tables 310-16 through 310-31 had the old derating factors, but for more than 9 conductors there was a * note that said: "These factors include the effects of a load diversity of 50 percent". I didn't find my '84 code, but the '81 code did not have the "load diversity" note. In the '90 code there was two columns in the table.
Column A
4-6 80%
7-9 70%
10-24* 70%
25-42* 60%
43+* 50%
Column B
4-6 80%
7-9 70%
10-20 50%
21-30 45%
31-40 40%
40-60 35%

In the '93 code there was only one column and the "load diversity" note was gone. This is the same table that is in the '02 code.
Don
 

iwire

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Location
Massachusetts
Re: Conductor Adjustment Factors

Don Thank you for that information. :)

Do you or anyone have some information on what prompted this change.

Was there a history of failures?

Bob
 

charlie b

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Location
Lockport, IL
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Re: Conductor Adjustment Factors

Originally posted by don_resqcapt19: I didn't find my '84 code, but the '81 code did not have the "load diversity" note.
I looked in the 84 version, and the ?load diversity? was not there. Apparently, it was added in 1987.
 

don_resqcapt19

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Illinois
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retired electrician
Re: Conductor Adjustment Factors

Bob,
I have the TCRs(ROPs) and TCDs(ROCs) for these changes and will try to do some research after Christmas.
Don
 

iwire

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Location
Massachusetts
Re: Conductor Adjustment Factors

Originally posted by don_resqcapt19:
Bob,
I have the TCRs(ROPs) and TCDs(ROCs) for these changes and will try to do some research after Christmas.
Don
Thanks I do appreciate it and of course take your time. :)

Happy Holidays, Bob
 

don_resqcapt19

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Illinois
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retired electrician
Re: Conductor Adjustment Factors

Proposal 6-89 submitted by CMP 6 for the 1987 code cycle resulted in the "50% diversity factor" note being added to the ampacity adjustment factor table in Note 8 to the ampacity tables. The substantiation pointed out that the ampacities in Article 310 were extracted from IPCEA (Insulated Power Cable Engineers Association) document #P-46-426. This document was originally published in 1962. The document is now published by IEEE and is standard #835-1994. The original document included the ampacity adjustment factors as shown in the 1984 and previous codes, however the IPCEA document included the "50% diversity factor" wording for wire fills greater than 9. The substantiation did not include reports of field problems caused by the omission of the "diversity factor" rule. The panel was just making the code technically correct based on the original document. Calculations were shown that showed cables could overheat when there were more than 9 current carrying conductors in a raceway without the "50% diversity factor". This same proposal also added an additional adjustment factor table that was to be 8(b) for derating when horizontal banks of conduit were installed with a conduit spacing less than the conduit diameter. This table would have required additional adjustment factors of up to 68% for closely spaced conduit. The requirement for derating based on conduit spacing is also found in the original IPCEA document. In the comment stage, panel action on comment 6-98 deleted the conduit spacing adjustment factor and in its place inserted new wording that read: "8(b) Spacing between conduits, tubing or raceways shall be maintained". The code has never given any clear guidance as to what spacing is required. The IPCEA document requires additional derating whenever the conduit spacing is less than one conduit diameter, but the code does not require this spacing. There were proposals submitted for the '90, '93 and '99 code cycles to clarify the spacing that is required between conduits, but they were all rejected. The panel did state that the spacing is required to dissipate the heat, but would not specify the amount of spacing that is required.

Proposal 6-98 for the 1990 code resulted in a second ampacity adjustment factor table being placed into Note 8. This second table was for raceway fills where the conductors did not have a "50% diversity factor". This second table in the 1990 code is the same as the current ampacity adjustment factor table that is now Table 310.15.(B)(2)(a) with one exception. In the '90 code the 35% factor applied for wire fills of 41 to 60 conductors, but in the current table the 35% factor applies to all wire fills above 41. The substantiation for this new table said that this change is required: "... due to the missunderstanding of many inspectors and contractors on the use of Note 8(a) and the load diversity of 50% for 10 or more conductors ... ".

In the '93 code cycle, CMP 6 submitted proposal 6-110a which moved the ampacity adjustment factor table that included the "50% diversity" to Appendix B. The panel stated that this move was made to clear up the confusion that had resulted from the addition of the "50% diversity factor" wording to Note 8(a). In the panel comment on comment 6-51, the panel said: "... the panel reaffirms its decision to remove the 50% diversity column from Article 310 because of the misapplication in the field".

Don

[ December 26, 2003, 10:08 PM: Message edited by: don_resqcapt19 ]
 

iwire

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Location
Massachusetts
Re: Conductor Adjustment Factors

Thank you so much for that Don. :)

I am surprised by what seems to me to be a lot of changes without substantiation.

Change it this way change, it that way, make it match the original document, but then again no when it came to conduit. ;)

Thanks again, Bob
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Re: Conductor Adjustment Factors

Bob,
I am surprised by what seems to me to be a lot of changes without substantiation.
The original research document was the technical substantiation for the changes. I think this is just another example of where the artificially high loads that result from the Article 220 calculations prevent us from seeing the problems that can occur when conductors are fully loaded.
Don
 
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