LV Cable Sizing

[FaradayFF]

Hi Guys,

I need to size some feeders to MCCs. The parallel sets of cables will be run in underground ductbank to supply MCCs. I would be sizing the said cables using NEC guideless.
I know there are more stringent guidelines, such as provided by EPRI books that take into account configuration of the cables, depth of burial, soil resistivity and other factors.
What warrants using these guidelines as opposed to more simple NEC ampacity sizing tables. I don't envision any applying derating factors in my calculation.

Thanks,
EE

 

[pv_n00b]

The NEC is conservative so you usually get larger conductor sizes than with other methods. The other methods are not as "rule of thumb" easy though so have to be done under engineering supervision. But if you get a smaller conductor size out of it you can save money, and that's the bottom line.

 

[Julius Right]

If the duct bank depth is not more than 30 inches [750 mm] and the values of thermal resistivity (Rho) are as indicated in Annex B [B2] , number of ducts and number of cables per duct correspond to one of the details presented in NEC then no further calculation is required.

 

[petersonra]

How much engineering time are you willing to spend uo front vetsus how much it might save at install time is the real question.

My guess would be most situations do not warrant anything special beyond what the code says you can already do without having to think about it.

You want to save money? Aluminum wire costs less than copper for the same ampcity and is safe