Re: Duct bank ampacity calculation
Using AmpCalc or Etap or similar software is the best way to get a good handle on what the resulting duct and cable temperatures will be. We prefer the AmpCalc.
Appendix B in the NEC assumes the same wire size in each conduit and each wire has the same amp loading. This would apply for an UG feed to a large service or a feeder. Charlie?s explanation and suggestion is based on the same assumption. This is OK for a single feeder with parallel cables, but usually in industrial installations you are running a multiple conduit duct bank that contains many different feeders, circuits and controls, all with different loadings and wire sizes. If you use Appendix B, you will oversize the cable in most cases and under size it in others.
Get the software and try different configurations to get a feel for the best design.
Some suggestions:
? Put the heaviest loaded circuits in the top corners of the duct bank, then the top row and the side columns. These are the coolest conduits.
? Avoid power circuits in the middle conduits. Use these for the control and instrument cables or leave them empty.
? Try various configurations to come up with the worst-case temperature and try various size cables to optimize the cable sizing.
? Look at your load diversity carefully. If you have two pumps in the same service, they may not be running at the same time, or if they are both on they may only be able to run at 50-60% load. So don?t size the cable based on both pumps running at 100% load. Investigate the duct bank temperatures with loads set at 100%/0%, 60%60% and 0%/100%.
? Try various configurations. Find the cable with the highest temperature and either increase its size or relocate it to another conduit position and run the calculation again. Keep doing it until the design is optimized.
? Use 90C rated cable insulation. But verify that the resulting load amps are within the 60C or 75C limitation of the wire termination.
? The programs only look at a single cross section (slice) of the duct bank (2D modeling) like the examples in Appendix B. So if the duct bank is not the same through out the run due to conduits peeling out or joining in, select a representative point to do the analysis. The worst case is usually right at the electrical room where all of the duct banks and conduits come together. But if this worst case section is only 10 feet long, you may be able to ignore it and use the ampacity of adjacent sections.
? Assumptions on rho (thermal resistivity of the soil) can have a big impact on the temperature. Try to get some good information about the site soil and backfill.
? As Charlie suggested, consider the source of your load data so you know how realistic the load is.
? Run one of the examples from Appendix B on the program to see if the results match the Tables. This will give you confidence that you are running the program correctly.
Sometimes I think these programs are a bunch of hooey, because we never considered UG derating for years and had few problems. Then I saw water boiling from the concentrated cable heat in a manhole at a refinery.
Some clients now review these calculations to verify our cable sizing. We had to rewire a plant (cost $200K+) because our electricians did not install the cables per the calculations. The client was afraid of melting cables. So after a year of operation with the plant at full load, the cables we pulled out were as good as the larger cables we put back in. The duct bank calculations said they should have melted??
Bottom line ? I still don?t know if the calculations are correct. My gut feel is that we oversize the cables based on the calculations. But if I ever end up in court, I have calculations showing that the design was reasonable, met Code and meets the standard of care.
Good Luck and welcome to Engineering. You can?t look everything up in a book or table, that?s what makes this job fun.
