Aluminum Voltage Drop (k Factor) URD Cable Vs THHN/THWN2 Etc

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suicidelg

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Hello,

I feel like this may be a dumb question, but I cannot seem to find the answer for it. When calculating voltage drop for Aluminum URD cable, are we still using a k factor of 21.2? The reason that I ask is because these compact conductor cables have a significantly larger ampacity than lets say normal THHN/THWN-2 individual conductors. It looks. like 2/0,2/0,2/0 Aluminum URD has an ampacity of 225A vs 2/0 THHN/THWN-2 150A at 90c. Does it then follow that the compact Aluminum URD would have a smaller voltage drop over the same distance? If so, I cannot seem to find a number. Thanks!
 
The ampacity of the conductor does not change when you go from standard stranding to compact stranding. The total area of the conductor remains the same.
Where did you get the ampacities for the URD? Many manufacturers give you the 90°C free air ampacity in their specs. That ampacity for 2/0 aluminum is 235 amps. You can only use the ampacities found in the Article 310 ampacity tables for an NEC installation.
 
Hey Don,

Thanks for the response. The NEC Annex B Table B.310.15(B)(2)(9) references Detail 7 (Single Triplex) 2/0 USE Aluminum as being 205A under 75c. I'm assuming this is because we're allowed to increase the ampacity of the conductors due to the lower ambient temperature of the earth? The manufacturer states a 233A ampacity direct buried @ 90c.
 
suicidelg said:
Hey Don,

Thanks for the response. The NEC Annex B Table B.310.15(B)(2)(9) references Detail 7 (Single Triplex) 2/0 USE Aluminum as being 205A under 75c. I'm assuming this is because we're allowed to increase the ampacity of the conductors due to the lower ambient temperature of the earth? The manufacturer states a 233A ampacity direct buried @ 90c.
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Nothing in any of the annexes is enforceable code language. Yes, there can be some increase for a single set of conductors in the earth, but you will also find substantial decreases in ampacity for multiple sets run in a duct bank as a result of mutual heating.
Also remember that there are almost no cases where we can use the 90°C ampacity for anything other than ampacity adjustment and correction for any circuit operating below 1000 volts.
 
First, When you cal voltage drop, it has nothing to do with conductor ampacity, the more current push through conductor, the more voltage drop. Secondly, you can use 90c for under ground section only. When emerge and entering enclosure used different cables. As his example, he can use his cable rated 225A IAW manufacture for only UG section, when emerge use 4/0 cu (230A @ 75c) or 300 kcmil (AL). And his circuit will be rated at 225 A.
 
You can use exact K
R x CM/1000 for 75c
You can use other formulas for temp adjustment. See note at bottom of table.
Use 1.05 for 15 c change
 
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