Above Ground Conduit Grouping Derating?

[Flapjack]

I have 13 conduits that will be routed together above ground on a pipe rack to feed a 4000 A switchboard. I was modeling this in ETAP. In the ampacity tab of the cable editor in ETAP, I selected NEC for Standard and A/G Conduit for Type. I noticed below a note saying "Base Ampacity is from NEC Tables. Derating for multiple Rows and Columns is based on IPCEA Standard."

I have never derated for grouping above ground conduits, but ETAP is reducing the ampacity by a little over 1000 amps. I contacted ETAP and asked for the standard they are referencing. They told me it was ICEA P-46-426 Table IX. Does anybody have access to that standard to confirm this referenced table is for above ground conduit grouping? And is it true that groups of above ground conduits should be derated?

 

[ron]

First off, I've had this question in my head from a theoretical basis forever. I've also thought about it for derating of lots of conduit lined up in a wall. BTW I do not derate for either in my normal practice, just think about it a lot.

ICEA is the Insulated Cable Engineers Asosc, and does not make codes.


310.15 says you can use the tables of 310.15(B) OR engineering supervision.



INSULATED CABLE ENGINEERS ASSOCIATION

 

[Flapjack]

I too have thought about it from time to time, but this was the first time I have seen a reference. I don't know why ETAP has the option to apply the derating if it has no validity.

I did a google search of the standard that ETAP referenced, which led me to the IHS standards store. It says the ICEA P-46-426 is superseded by IEEE 835-2012.

 

[Julius Right]

IPCEA Publication P-46-426 it is now IEEE-835/1994 [2012?], indeed.
NEC recommends IEEE Standard 835-1994, Standard Power Cable Ampacity Tables. See NEC 2014 Annex B art. B.310.15(B)(2) Typical Applications Covered by Tables.
However, I agree with Ron: it is not a code.
As I already said I have only 1994 edition. I see no "table IX" and no derating is stated any where in here.
A calculation way for pipe-or cable-in air is in ch.3.5.6 Horizontal cables in air.
On NEC art. 310.15 Ampacities for Conductors Rated 0–2000 Volts. (B) Tables. (3) Adjustment Factors. (b) More Than One Conduit, Tube, or Raceway. is stated:
“Spacing between conduits, tubing, or raceways shall be maintained”
How much space?
Our experience: if the clearance between cables-or conduits-horizontally laid in air is two diameters or more no derating is required.
German standard [DIN VDE 276-1000] requires 2 diameters apart-for instance.

 

[Paul1955]

IPCEA Publication P-46-426 it is now IEEE-835/1994 [2012?], indeed.
NEC recommends IEEE Standard 835-1994, Standard Power Cable Ampacity Tables. See NEC 2014 Annex B art. B.310.15(B)(2) Typical Applications Covered by Tables.

Here's an example from one table in IEEE-835 (1994)(R2006) to shed some light on derating factors. I see nothing in the standard to indicate further derating for conduits grouped above ground.

Assume conduit in free air, 40 degree C air ambient, triplexed 75 degree CU wire, 500 MCM:

With no sun and wind speed of 0 FT/s, ampacity is 334A.
With no sun and wind speed of 2 FT/s, ampacity is 350A.
In full sun, with wind speed of 0 FT/s, ampacity is 183A.
In full sun, with wind speed of 2 FT/s, ampacity is 241A.

Paul

 

[Julius Right]

In my opinion, in the "old" IPCEA 46-426 Table IX it could be as per Okonite Table 6-4. See:
http://okonite.com/okonite-wire-and-cable-product-brochures/Engineering-Handbook.pdf
The clearance between conduit has to be from 1/4 to 1 diameter. However, the maximum number of conduits is only 6.
It seems to me, if distance between conduits is more than 1 diameter no derating is required.