Ampacity of Multiple Paralleled Conduits in Air

[kartracer087]

Hello,

In looking at the Neher McGrath calculations for ampacities of conductors routed in a conduit in air, I can't seem to find anything related to if you have multiple paralleled conduits with the same types of conductors in air, for example, if you had (3) 3" conduits spaced 2" apart in air each with 3-250kCMIL medium voltage cables. Is the effect of neighboring conduits suspended in air negligible versus if they were in earth? Or is there some kind of factor to account for multiple conduits in air which are in close proximity of each other? I would think there would be some mutual heating effect of having identical conductor conduits (parallel sets) in close proximity to the conduit which you are calculating.

Thanks

 

[9wearSuave]

At 2" apart I would think it would be negligible. I don't know of any derating systems for parallel conduits, just guidelines on how to install them for minimum inductive coupling.

 

[waross]

Each conduit must have a grounding conductor.
If grounding conductors are run separate from the phase conductors, inductive reactance will greatly reduce the fault current of phase to ground faults.
This effect may reduce fault current to below the tripping level.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

In EPRI EL 5036 V4 Wire and cable it is written that
in the old standard, ICEA P-46-426, it is tabulated ampacities for cables and conduits when the distance between them is one diameter and also derating factors for one-quarter of diameter also.
This standard is obsolated now and instead of this it is IEEE 835/1994. However in this standard it is no reference of distance between conduits-or cables- in air.
See also

https://www.nrc.gov/docs/ML2006/ML20065G719.pdf

 

[MKFPE]

Hi,

I believe both Okonite and Kerite engineering/catalog publications provide guidance for multiple conduits in air. These come to mind since I specified EPR cable exclusively for several decades with excellent reliability, in fact I know of no failures with many thousands or perhaps tens of thousands of feet of cable in service.

 

[7anoter4]

Indeed, in the last Engineering Handbook [ 2018] of Okonite there are tables [ 6-3 and 6-4] with recommended derating factors for more than one cable or conduit if the distance between it is from 1/4 to 1 cable or conduit diameter. In my opinion, these are the ICEA recommendation from obsolete standard ICEA 46-426.
Kerite presents recommendations only for underground cables,as I know.