question regading underground concrete duct bank ampacity thermal derating philosophy

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Dale001289

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Georgia
I'm struggling to understand a basic aspect of calculating ampacity for 'heat' derating. If for example, I have a 2100 foot circuit, with 2050ft in cable tray and 50 feet underground, buried at say 24" top of concrete ductbank, load of 68A (60hp, 460V, 3ph induction motor) how much derating should actually be applied? In other words how does the 50 feet of underground affect the rest of the circuit? Worth mentioning, there are surrounding ducts, mostly very low current control cables at 120VAC or 24VDC, and another motor branch circuit of the same size running within 7.5" of the other.

But this is more of a philosophical question.

Wouldn't there be only very minimal derating applied since there's only 50 feet of underground? I think this almost a rhetorical question, but I'd like to see some feedback just for my own personal 'comfort factor'

Thanks to all in advance.
 
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GoldDigger

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Dale001289 said:
The underground portion of this circuit is just over 2%.
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The rule in question is 10' or 10%, whichever is shorter. 50' is more than 10'. End of discussion with regard to that particular rule.

If thermal derating has to do with the ampacity of the 50' section, then the length of the rest of the circuit has no practical effect on the behavior of the 50' length of wire.

If (as does not seem to be the case here) you were worried only about voltage drop, then a short section could likely use a smaller wire than the long section and still keep voltage drop comfortably low.
 
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Dale001289

Senior Member
Location
Georgia
GoldDigger said:
The rule in question is 10' or 10%, whichever is shorter. 50' is more than 10'. End of discussion with regard to that particular rule.

If thermal derating has to do with the ampacity of the 50' section, then the length of the rest of the circuit has no practical effect on the behavior of the 50' length of wire.

If (as does not seem to be the case here) you were worried only about voltage drop, then a short section could likely use a smaller wire than the long section and still keep voltage drop comfortably low.
Click to expand...


You're right, voltage drop is not an issue in this case. Problem is our CARS program is giving us a much different result than ETAP and I'm trying to determine why (3/C #4/0 vs 3/C 500kcmil). I was told it was due to thermal derating, however using the IEEE Brown book even worst case scenario (9 surrounding ducts) does not result in 500kcmil. Goes without saying, its causing upset with construction. My bet is, 'operator error' using CARS.
 
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