Underground Ductbank Ampacity Calc. - Very Limiting

[Designer_82]

I ran an underground ductbank ampacity calculation in ETAP.

Although the ductbank is very large with about 46 conduits, the calculation results are crippling.
It shows a 1200 Amp base ampacity cable derated to only about 400 Amps. A 67% derate.
We're already running 4 sets of 500Kcmil for 1200A feeders.

Are these ductbank ampacity calculations too conservative?


Any experience with this? Do these results seem reasonable?

 

[Gr8blu]

Designer 82 I suggest you do an internet search for Neher-McGrath calculations. The NEC tables you are probably familiar with are NOT intended (or designed) to be used with large conductor counts in enclosed spaces - such as modern duct banks. Having a derate of 60-70 percent for the "middle conductors" is not unheard of.

You might do better to run several smaller duct banks instead of the single large one.

Converting energy to motion for more than half a century

 

[7anoter4]

If RHO=90 and 20oC-as per NEC- four 500 MCM copper EPR/XLPE insulated in 46 filled ducts duct bank 12" apart 100% loaded 7*7 ducts[46 ducts 3 lives+1 neutral] 3 ducts in the middle empty] 170.6 A [each live conductor] calculated according to IEEE 835/1994 [following Neher and McGrath ].
If you may use only 75% LF [according to Neher and McGrath] you'll get 211 A in same conditions.

 

[dpc]

The Neher-McGrath calculation is dependent on the assumptions made for the heat transfer constants for all of the material involved. I haven't used the ETAP version, but since you're talking about a lot of money, it might be worthwhile to check the results using AmpCalc. This software does only Neher McGrath calcs and has been around a long time.

Also, FWIW, if this is a low voltage cable run, the NEC does not explicitly require derating of the cable in an underground duct bank as it does for medium voltage, provided the conductors are sized based on NEC load calculations (which are extremely conservative). Unless something has changed recently. There is a long history on this topic as it relates to the NEC.

Cheers,

Dave

 

[AppleJaxJap]

We run these calculations all the time, and while there is not enough information in your post to determine if your particular situation is "reasonable," it certainly could be. You would need to know the load factor, RHO values, and other values to assess the accuracy of your calculations.

Also, As dpc correctly stated "Also, FWIW, if this is a low voltage cable run, the NEC does not explicitly require derating of the cable in an underground duct bank as it does for medium voltage"

This is true, but it is also true that NEC requires you to run the conductors below the insulation temperature of the conductor.

Scott C.
Neher-McGrath.com
Neher-McGrath.net