Multiple Sets of 4.16kV in a Single Conduit

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zclawz

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I am reviewing an existing installation of a 3500ft run of (3) sets of #6 4.16kV conductors (airfield lighting cable) installed in a single 4" conduit. There is a short along the conductors length that is bringing up the review of the existing conditions. The owner has a recieved a proposal to replace the existing conductors with new appropiate conductors or reuse the existing conductors and provide repairs where needed. Both proposals utilize a single conduit with multiple sets of conductors.

Reviewing 2017 NEC 310.60 the code does not expressly dictate if multiple conductors can share the same conduit. Reviewing the work with a local contractor they have installed a set of (3) 4.16kV conductors in a single conduit on previous projects but never multiple sets. They were not confident the installation of (3) sets of conductors would even be code compliant. Is it possible to have multiple sets of conductors at 4.16kV sharing the same conduit? If so what deration factor should be assumed (a table like 310.15(B)(3)(a) is not provided for 2001-35000V conductors)?

To replace the conductors with new and replace underground pullboxes along the route would cost the owner 250k. Adding additional parallel conduits along the 3500ft run into extremely rocky soil would cost the owner multiple hundreds of thousands of dollars. If possible I would like to limit the additional cost on the owner. Thanks
 
Is this a series lighting system? A three phase parallel lighting system with or without a neutral return path? The cable manufacturer will have all of the relevant thermal data for the cable and should be able to help you. If not Southwire offers a pretty good guide based on their cables that may be of help to you. You will of course need load data, load factor or duty, soil resistivity, etc. Hope this helps.
 
All 3 sets of conductors serve residences at 208Y/120V ( (1)800A and (2) 400A), the 3 sets of conductors do not serve lighting directly. The developer installed step up transformer to 4160V to counter volt drop and most likely price.

Pullboxes are proposed to be replaced due to being undersized.

The code does not explicitly state if medium voltage conductors can share a raceway (or electrical duct as the NEC calls it) and the diagrams are noting a single conductor. Conversing with our local contractors who have worked with this type of installation in the past, leads me to believe a set of conductors serving the same load could share an electrical duct, but not necessarily multiple sets of conductors serving multiple loads.
 
9 cables in 4" conduit [one single steel conduit in 11*11" concrete bank 30" depth in 90 rho 20oC earth] approx. 60A per cable]
 
I am sorry: for 9 cables in one conduit has to be 48 A [75oC conductor temperature only]
 
Julius Right said:
I am sorry: for 9 cables in one conduit has to be 48 A [75oC conductor temperature only]
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In your experience, you find it code compliant to install multiple sets of conductors (9 in this case) in a single conduit as long as it is in a 11" x 11" backfilled bank (code says earth or concrete). I haven't reviewed conductor sizing yet.
 
The NEC [nor IEEE 835] is considering not more than 3 cables [single core] per duct [or conduit] but in any case, permits ampacity calculation as per Neher and McGrath.
 
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Julius Right said:
The NEC [nor IEEE 835] is considering not more than 3 cables [single core] per duct [or conduit] but in any case, permits ampacity calculation as per Neher and McGrath.
Click to expand...
Just to be clear, yes you would likely get into derating with multiple conductors /sets, but there is no general prohibition against multiple circuits/sets in a single conduit.
 
electrofelon said:
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Just to be clear, yes you would likely get into derating with multiple conductors /sets, but there is no general prohibition against multiple circuits/sets in a single conduit.
Click to expand...
Would you apply the Table 310.15(B)(3)(a) for more than 3 current carrying conductors even though it is listed for 0-2000V conductors?
 
In my opinion, NEC allows to use table 310.15 (3)( a) only connected with table 16,18 only.
For information, for low voltage cables in underground duct, Table B.310.15(B)(2)(6), or Table B.310.15(B)(2)(7) , Annex B, using Formula [B.310.15(B)(7)a] that included reference to the actual number loaded conductor and a diversity of 50%.
So, for medium voltage cable in underground duct bank it is not indicated.
 
There are still two problems:
1-Neher did not indicate what is the factor for more than 4 cables in order to calculate Ds' [equivalent cable bunch diameter in the duct].
2-Since the cables are parallel supply cables an unbalance between parallel cables of the same phase is expected. In this case, it is advisable to tighten at first a 3-phase cables as a separate cable of 3 conductors, and then tighten these 3 new cables again.
 
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