Cable Sizing & Installation to NEC - Direct Buried 2

[rmullin]

I am sizing cables to NEC for an LNG Facility in Peru, following the rules from Article 310 of NFPA 70. The cables must be direct buried. From the main 13.8kV Switchboard there are 28 feeders to transformers (14 from bus A and 14 from bus B). Here, the probelm arises. From NEC Figure 310.60, it stipulates that there must be a 24 inch (600mm) centre-to-centre spacing between circuits. For this application we have numerous feeders in the same trench, and having to separate each circuit by 24 inches creates a huge problem in terms of trench size. Typically how would this problem be overcome? Is it possible to have multiple layers of MV cables, whilst complying with NEC? (Im used to sizing cables to British Standads or IEC, therefore I think I may be missing something in my interpretation of the NEC, as these standards do not require such a large separation distance). Thanks in advance.

 

[IanOwens]

Glad I'm not working on your project, sound like it is going to be an uphill battle !

 

[rmiell]

Rmullen, this sounds like a utility installation. Using the National Electrical Safety Code (C2-1993), in Section 35 (350. General (C)) it states if the cables meet the requirement of (B), they can be buried with no deliberate seperation.

You should also look at 354. Random Seperation - Additional Requirements, for more of the same language.

In one of our substations, the cables run from switchgear into a metal trench box, about 4' wide, with a removable cover. There is no seperation between the cables. In our other subs, the circuits are installed in conduits, which run next to each other until the point where they seperate to run in different directions to riser poles.

I don't think the NEC is requiring you to have the 24" seperation, it is just saying if you seperate them 24", your ampacity of the conductors in Table 310.77 & Table 310.86 is as listed (IE: Table 310.77: #1 AWG @ 13.8kv = 175amps). What is left unsaid is how much you lower the amp rating if the triplexed cables are seperated by a lesser distance.

IMHO

Rick Miell

 

[dpc]

NEC does not require 24 inch separation. The Tables for ampacities of direct-buried circuits are based on the 24" separation (for two three-conductor cables).

The NEC ampacity tables are based on Neher-McGrath method. Under engineering supervision, specific ampacities can be calculated for configurations not given in the NEC. See 310.60(D).

By doing exact calculations, you can account for actual soil temperature and use realistic thermal constant for your soil. But you will have to have some method of maintaining separation between the cables.

Having said that, my personal opinion is that direct burial of so many cables coming into a piece of switchgear is not a good design. I would put this cable in conduit in a duct bank system and encase it in concrete if at all possible.

 

[rmullin]

rmiell, referring to your first paragraph regarding separation, do you know where this is in the NEC 2005 Edition?

 

[rmiell]

It is in the same issue of the NESC (C2-1993), and not addressed in the NEC. My latest issue of the NESC is dated 1993, and there are newer issues. It is an IEEE and ANSI standard.

Rick Miell

 

[dpc]

NESC and NEC are two different standards. Unless a utility company will own this installation, forget about the NESC - it doesn't apply.

Latest version of NESC is 2007.

 

[apowerengr]

Duct bank is the only way to go for what you describe and the only way to construct a system which can be maintained.