UG Cable Ampacity

[tdpatel]

I have following situation and need some code clarification:

NEC Figure 310.60, Detail 4

The first row - 1st and 2nd Duct contains (3) Single Conductor 750 MCM 15 KV Cable (Third Duct is spare)
The second row - 1st and 2nd Duct contains (3) Single Conductor 750 MCM 15 KV Cable (Third Duct is spare)
The third row - 1st and 2nd Duct contains (3) Single Conductor 500 MCM 15 KV Cable (Third Duct is spare)

In essence I have (3) breakers feeding load via underground duct banks - 1200 A, 1200 A, 800 A.
I am using NEC Table 310.77 one circuit, 90 Degree C, 5001-35,000 Volts ampacities for cable listed above to meet 1200 amp and 800 amp full load requirements. Is this true ? OR do I have to go to six circuit table of Table 310.77 to define the ampacity of each cable used ?

I have 3 breakers feeding load via parallel sets. Is parallel sets in duct bank regarded as separate circuit or single circuit ?. i.e. the first row is really a 1200 amp circuit from a 1200 amp breaker feeding same load.

Your response will be greatly appreciated.

 

[charlie b]

Re: UG Cable Ampacity

First of all, how do you figure that a pair of 750 MCM cables (15kV, 90C) is enough for a 1200 amp load? The table you plan to use says that this cable is worth 565 amps per conductor, or 1130 amps for the pair. You are above 800 amps, so you cannot use the next higher standard size OCPD ? per 240.4(C).

Secondly, I think that what you are describing is essentially a ?Detail 3.? Don?t confuse the presence of six ducts with the notion of feeding six separate loads. A conductor carrying current will be a heat source, as far as the other conductors surrounding it are concerned. It does not matter if all six ducts form one parallel feeder to a single load, nor if you feed six separate loads, nor if you feed three circuits in the manner that you describe. You will have six conduits housing current-carrying conductors, and therefore six heat sources.

Unfortunately, this puts you into a bind. If you have to use the six circuit table, you cannot get to 1200 amps with a pair of conductors, even if you use 1000 MCM rated at 105C. Therefore, you need to use three sets conductors. But that means that you will need more than 6 of the available 12 ducts that show up in Detail 4. But Table 310.77 has no entries for Detail 4. So you are stuck.

My advice is to have someone calculate the ampacity using one of the programs that run the Neher-McGrath equation.

[ August 06, 2004, 03:36 PM: Message edited by: charlie b ]