Derating underground wiring

[mwm1752]

6 pvc conduits- 3 phase 120/208 service laterals - 7.5" average center to center for electrical ducts - 100' total lenght - parallel installation for a 1600 amp service - each conduit has 3 phases & grounded conductor
Is derating of conductors required per 310.15 ? and why?

 

[ron]

I would suggest 310.15(B)(2)

You can't really know the ambient condition around each of your raceway individually because they are in a "bank" of (6) and will get mutual heating from adjacent raceway.

Annex B of the NEC gives advice. For your situation, I often use Table B.310.15(B)(2)(7) or I use Ampcalc to do the Neher McGrath Calculations

 

[Smart $]

...

Annex B of the NEC gives advice. For your situation, I often use Table B.310.15(B)(2)(7) or I use Ampcalc to do the Neher McGrath Calculations

310.15(C) permits engineering supervision to use other than Table methods to determine conductor ampacities. Without engineering supervision, using the Table(s) is the only method permitted.

 

[mwm1752]

I've been given arguement that with only 3 current carrying conductors no adjustment is required. The handbook example show 3 current carrying conductors in the illustration. Where is the code wording requiring underground ducts with 3 current carrying conductors needing to have derating adjustments. Everything I see says more than 3 current carrying conductors. Of course I am refering to the initial example.

 

[GoldDigger]

I've been given arguement that with only 3 current carrying conductors no adjustment is required. The handbook example show 3 current carrying conductors in the illustration. Where is the code wording requiring underground ducts with 3 current carrying conductors needing to have derating adjustments. Everything I see says more than 3 current carrying conductors. Of course I am refering to the initial example.

You are seeing here a good reason that Mike Holt in his materials is careful to distinguish, as the Code itself does, between a "correction" and an "adjustment".
The adjustment factor deals with more than three current carrying conductors in a raceway. If you have only three, no adjustment is required.
But although you can therefore get the base ampacities for the conductors from the appropriate table, you still have to apply the separate correction for ambient temperature. Except in the case of circular raceways on or above a roof, the Code does not provide tables for determining the ambient temperature to be used for correction based on changes from the basic earth or air ambient temperature.
So for a duct bank with spacing close enough that the change in ambient caused by other ducts will be significant (and other than "spacing must be maintained" the Code does not tell you what that threshold is), you cannot get the necessary ambient temperature factor for correction from a table. The difficulty in getting it does not absolve you of the requirement to make the temperature correction.
That type of ambient correction would apply equally well to nine direct burial cables spaced in a ditch as it would to nine single conductors in nine ducts spaced in a bank.

 

[mwm1752]

GoldDigger,
As I try to collect my premise does this line of thought work --
310.15(B)(3) refers to the adjustments needed for 0 - 2000v conductor ampacities for based upon the sub sections of 310.15(B)(3)(a), 310.15(B)(3)(b), and 310.15(B)(3)(c).
310.15(B)(3)(a) basically deals with more than three conductors in raceways and cable separations with 5 additional specific conditions.
310.15(B)(3)(b) refers to conduit/raceway separation without any conditions on number of conductors in either free air or underground duct configurations.
310.15(B)(3)(c) defines it's self with raceways exposed to sunlight on a roof condition.
There are tables to refer to for adjustments within the 310.15(B)(3)(a), & 310.15(B)(3)(c) requirements.
Information regarding 310.15(B)(3)(b) was first inserted in the NEC 1984 code to be applicable 1/1/87. This table moved to Annex B in the 1990 NEC edition and is in the current edition. Tables in Annex B sections purpose is informative and factual for parallel type installations. A Neher-MGgrath calculation is an option to the Annex B information in order to be precise on the adjustments for in those tables.
The Annex B tables and the Neher-McGrath calculation are both reliable and code applicable information regarding a parallel installation per NEC 310.15(B)(3)(b).

 

[GoldDigger]

GoldDigger,
As I try to collect my premise does this line of thought work --
310.15(B)(3) refers to the adjustments needed for 0 - 2000v conductor ampacities for based upon the sub sections of 310.15(B)(3)(a), 310.15(B)(3)(b), and 310.15(B)(3)(c).
310.15(B)(3)(a) basically deals with more than three conductors in raceways and cable separations with 5 additional specific conditions.
310.15(B)(3)(b) refers to conduit/raceway separation without any conditions on number of conductors in either free air or underground duct configurations.
310.15(B)(3)(c) defines it's self with raceways exposed to sunlight on a roof condition.
There are tables to refer to for adjustments within the 310.15(B)(3)(a), & 310.15(B)(3)(c) requirements.
Information regarding 310.15(B)(3)(b) was first inserted in the NEC 1984 code to be applicable 1/1/87. This table moved to Annex B in the 1990 NEC edition and is in the current edition. Tables in Annex B sections purpose is informative and factual for parallel type installations. A Neher-MGgrath calculation is an option to the Annex B information in order to be precise on the adjustments for in those tables.
The Annex B tables and the Neher-McGrath calculation are both reliable and code applicable information regarding a parallel installation per NEC 310.15(B)(3)(b).

Looks like a well organized line of thought, but nothing proceeds in a straight line in the real world.

So here are some digressive comments to round out the picture (in no particular order, but numbered just the same):

1. The calculated adjustments for more than three conductors in a raceway found in Annex B build in a diversity factor of 50%. The main body of the Code provides tables of adjustment factors which DO NOT allow for diversity. To use the Annex B tables for adjustment you would need to justify that diversity factor, under engineering supervision.

2. The ampacity tables in Annex B give ampacities directly, rather than as an adjustment factor. To apply these in combination with other derating factors you must either use these ampacities as your starting values and then apply the other factors or convert these values to factors and apply the various factors in any order.

3. There is a fundamental difference between the temperature correction table in the body of the code for circular raceways on or above roofs and the duct tables in Annex B: The table in the main body does not require engineering supervision, while any use of the Annex B information is only applicable as part of engineering supervision, not as a replacement for it.
I am not sure what you mean by "factual" in "informative and factual". It certainly does not allow you to bypass engineering supervision, although the engineer involved can rely on them as he will, based on his informed judgement.

4. The Annex values for duct banks do not take into consideration that the ambient correction for each of the different ducts within one Detail # configuration will be different. Instead it assumes that parallel conductors are involved and so, given that the conductors must be identical, the worst case duct in the bank will determine the necessary correction.

5. Just for fun, consider what would happen if the provision for paralleling cables, 310.10(H)(2), required the parallel wires in one group to have the same calculated (adjusted and corrected) ampacity instead of the same physical characteristics. :blink:

 

[mwm1752]

I appreciate the conversation

1. The calculated adjustments for more than three conductors in a raceway found in Annex B build in a diversity factor of 50%. The main body of the Code provides tables of adjustment factors which DO NOT allow for diversity. To use the Annex B tables for adjustment you would need to justify that diversity factor, under engineering supervision.
My thoughts about Annex B is that the tables & duct configuration lead to the justification of diversity factor.
2. The ampacity tables in Annex B give ampacities directly, rather than as an adjustment factor. To apply these in combination with other derating factors you must either use these ampacities as your starting values and then apply the other factors or convert these values to factors and apply the various factors in any order.
With less than 3 current carrying conductors are there any more adjustments -- still using my original example
3. There is a fundamental difference between the temperature correction table in the body of the code for circular raceways on or above roofs and the duct tables in Annex B: The table in the main body does not require engineering supervision, while any use of the Annex B information is only applicable as part of engineering supervision, not as a replacement for it.
code commentary:The basis for determining the ampacities of conductors for Tables 310.15(B)(18) and 310.15(B)(19) and the ampacity tables in Informative Annex B was the Neher?McGrath method.
I am not sure what you mean by "factual" in "informative and factual". It certainly does not allow you to bypass engineering supervision, although the engineer involved can rely on them as he will, based on his informed judgement.
Tables do outline a duct configuration and if indeed they are based upon the Neher/McGrath calculation seems they should be factual to at least start the process..

4. The Annex values for duct banks do not take into consideration that the ambient correction for each of the different ducts within one Detail # configuration will be different. Instead it assumes that parallel conductors are involved and so, given that the conductors must be identical, the worst case duct in the bank will determine the necessary correction.
True, Table B. 310.15(B)(2)(7) does not specify the type of duct in which an accurate determination can be obtained.
5. Just for fun, consider what would happen if the provision for paralleling cables, 310.10(H)(2), required the parallel wires in one group to have the same calculated (adjusted and corrected) ampacity instead of the same physical characteristics. :blink:[/QUOTE]
Stiil racking my brain on this one not sure I'd call it fun.

Seriously, Your responses are helpful and my questions are getting fewer.

 

[GoldDigger]

I appreciate the conversation

1. My thoughts about Annex B is that the tables & duct configuration lead to the justification of diversity factor.

2. With less than 3 current carrying conductors are there any more adjustments -- still using my original example
3. code commentary:The basis for determining the ampacities of conductors for Tables 310.15(B)(18) and 310.15(B)(19) and the ampacity tables in Informative Annex B was the Neher?McGrath method.
I am not sure what you mean by "factual" in "informative and factual". It certainly does not allow you to bypass engineering supervision, although the engineer involved can rely on them as he will, based on his informed judgement.
Tables do outline a duct configuration and if indeed they are based upon the Neher/McGrath calculation seems they should be factual to at least start the process..

4. True, Table B. 310.15(B)(2)(7) does not specify the type of duct in which an accurate determination can be obtained.
5. Just for fun, consider what would happen if the provision for paralleling cables, 310.10(H)(2), required the parallel wires in one group to have the same calculated (adjusted and corrected) ampacity instead of the same physical characteristics. :blink:
Stiil racking my brain on this one not sure I'd call it fun.

Seriously, Your responses are helpful and my questions are getting fewer.[/QUOTE]

Then, hopefully my answers will get fewer too.

1. There is nothing about the conductors sharing a single duct in a duct bank that speaks to diversity factor in any way. Either you use the tables in 310 with no diversity factor (and no allowance for fudging one in based on either estimated or known diversity: for example, using the 310 tables you arguably have to count both travelers of a three-way circuit as current carrying even though you know that at most one will be.)
If you think that you need to escape from the constraints of the table, you have to go to a ground-up engineering evaluation similar to what derived the figures in the annex.

2. No adjustments at all, only the corrections.

3. I agree that they are a good starting point (or reality check) for the process, as part of an engineering evaluation. I just do not see how factual gets involved at all. There are assumptions made in all such calculations, and part of the assessment is verifying that they are met.

4. I was not referring to type of duct, just to the fact that the theoretical correction factor for the center duct of a 3 x 3 will be higher than the correction for the four side ducts, which in turn will be higher than the correction for the four corner ducts, which will still be greater than 1.0.

5. To me it would only be fun if I were not the one stuck doing it. Fortunately the code does not take that approach.

 

[mwm1752]

Does this read more proper---
Information regarding 310.15(B)(3)(b) was first inserted in the NEC 1984 code to be applicable 1/1/87. This table moved to Annex B in the 1990 NEC edition and is in the current edition. Tables in the Annex B section provides the adjustment factor for more than three current carrying conductors in a raceway or cable with load diversity. An accurate Neher-McGrath calculation is required under these conditions.

Simple conclusion - generally parralell installations(phases A-B-C-N in each conduit) for a service/feeder circuit would not have load diversity due to each parralell run (installed properly) should equally share the same load.
Definitions can drive me nuts. does the diversity also deal with total load of service/feeder circuit and a major percentage of load on each paralell?

 

[mwm1752]

Tried to edit last comment apparently I took to long --- the definition is from a previous Post
?Load Diversity? is defined as, ?The difference between the sum of the maximum of two or more individual loads
generally parralell installations(phases A-B-C-N in each conduit) for a service/feeder circuit would not have load diversity due to each parralell run (installed properly) should equally share the same load.
and the coincident or combined maximum load, usually measured in kilowatts.?
does the diversity also deal with a load percentage on the paralells max current rating and/or a major percentage of load on each paralell -- if so what would the percentage threshold be?

 

[mwm1752]

Tried to edit last comment apparently I took too long --- the definition is from a previous Post
?Load Diversity? is defined as, ?The difference between the sum of the maximum of two or more individual loads
generally parralell installations(phases A-B-C-N in each conduit) for a service/feeder circuit would not have load diversity due to each parralell run (installed properly) should equally share the same load.
and the coincident or combined maximum load, usually measured in kilowatts.?
does the diversity also deal with a load percentage on the paralells max current rating and/or a major percentage of load on each paralell -- if so what would the percentage threshold be?

 

[GoldDigger]

Tried to edit last comment apparently I took too long --- the definition is from a previous Post
?Load Diversity? is defined as, ?The difference between the sum of the maximum of two or more individual loads
generally parralell installations(phases A-B-C-N in each conduit) for a service/feeder circuit would not have load diversity due to each parralell run (installed properly) should equally share the same load.
and the coincident or combined maximum load, usually measured in kilowatts.?
does the diversity also deal with a load percentage on the paralells max current rating and/or a major percentage of load on each paralell -- if so what would the percentage threshold be?

Within a single duct or raceway (therefore adjustment) the diversity factor will have nothing to do with parallel conductors.
It also would not apply to three conductors in a three phase circuit, since with balanced loads all three will be conducting their max current at the same time.
An example of diversity (on a smaller scale) would be six circuits feeding six washing machines in one laundry room. You may be allowed to assume that not all washers will be drawing max load (spinning?) at the same time.