This too seems realisticįinally, what is the meaning of the NEC wording "shall be permitted"? why isn't it stated "shall be required"? Is it due to the fact that an earlier paragraph allows a ampacity determination based upon "engineering supervision"? In this case the rather simple formula stated in the NEC just before Para. I might mention that certain European cable companies supply tables for derating due to depth and typically the factor becomes larger as depth increases up to 10 meters at which point the factor becomes constant, about 10-15%. For example, the 35' case would be 0.94^35 = 11.5% deratng at least something realistic. I'm certainly not sure what the NEC means but it could be a "compound interest" type of calculation where the figure (1-6%)or 0.94 is raised to the power of the depth. Put in another way, if a cable is installed in a duct 16.6-ft deep it will have no rating whatsoever therefore not even a 1 Amp load would be permitted using this concept not realistic either.
Based on this, cvirgil calculates a derating factor of 2.1 which indeed doesn't sound right. RE: Electrical Duct Bank Calcs cod (Electrical) 17 Mar 05 15:33ī310.60.C.(2)b states that for deeper portions an ampacity derating factor of 6% per foot shall be permitted. DO NOT USE HOSE CLAMPS!!!!!!!!!!!!!! Hose clamps will break the tubing. Polybutylene tubing is very reliable as long as the connector uses an elestomeric gland such a a nylon or rubber gland. Your chillers should be hooked up with electrical plugs and hydraulic quick connects so that you can swap in a spare chiller. You will need chillers at each end and you should use a total of 6 cooling circuits, 2 for each row with a total 12 chillers. polybutylene tubes that form 2 cooling circuits in each conduit. Your ducts are theoretically big enough to contain four 0.625 inch O.D. This ONLY applies to the part that has no more than 4 feet of dirt on top.īecause of the deep portion and because of the puny capacity of each cable, you need artificial cooling.
For 15 ducts you MUST use concrete encasement because that is the only kind of backfill that will not break the ducts and you need the thermal conductivity of the concrete. Since for 6 or more ducts the main thermal bottleneck you can figure 500 KCM at 168.75 amps per cable for natural cooling. In Annex B of National Electrical Code there is a 15 duct bank that uses 2,000 KCM conductors. William RE: Electrical Duct Bank Calcs cuky2000 (Electrical) 10 Mar 05 14:24 To figure Vd for a given wire size, use a worst-case power factor (I use 0.7) and the corresponding X and R values from table 9. Find the required wire size from the value in the proper column in the proper table. Divide the ampacity of the wire determined in step 1 by each of the derating factors in turn to get a final ampacity requirement ģ. Figure a minimum wire size for a maximum Vd (I use 3% for feeders) at full load amps Ģ. You don't exactly "derate" for voltage drop. Don't forget that the conductor jacket temperature rating cannot exceed the temperature rating of the conduit (usually 90 deg C, so MV-105 is out for PVC). If medium voltage, you also need to derate for induced shield currents. If medium voltage, go to B.310-77 or somewhere close by. Use ambient derating factors at the bottom of the table, also. You did not indicate the system voltage, ampacity, and number of circuits, but assuming you are at 480 VAC, then table B.310-7 includes different colums for derateing based on the number of circuits. The code book says that is good for 90% of USA. I would use rho = 90 as a starting point.
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