Duct bank ampacity calculation

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jonesjimbo

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Hello,

My name is Jim and I am a rookie electrical engineer. One of my first assignments is to find out the ampacity of duct bank conductors, and I need some help from some more knowledgable and experienced people.


I am working on finding the solution to an ampacity calculation of conductors in a ductbank. Do I need to buy the duct bank calculator software to simulate the ductbank to achieve a credible solution? Are these ductbank calculators reliable?

For example, how much derating needs to be done in a ductbank of 18 or 40 conduits? Other than annex B, is there anything in the code that gives guidelines for engineering large ductbanks? Annex B is not very helpful in this case. It only gives values for ampacity up to 6 ducts.

My main question is: Do I need the ampacity calculation software to find a solution for the ampacity, and is it a reliable solution?

Thank you for any help you can provide.

Jim
 
Re: Duct bank ampacity calculation

Jim
The caculations for cable ampacity in duct banks is too complex to be done by hand. There are several programs that will do the job for you.
There is nothing in the code that can help you. This is an engineering problem not covered by the code.
Maybe some of the guys can advise you of the best product to buy.
 
Re: Duct bank ampacity calculation

Let's talk about the 18 duct system. If you can design the ductbank system so that you have three sets of 6 conduits, with each set of 6 conduits in its own concrete, and the three concrete banks separated from each other by, let us say, 5 feet edge-to-edge (the 5 feet is a guess, not a code requirement), then you can use the tables that apply to the voltage level of the distribution system. For example, 18 conduits in three sets of 6, 500 MCM THHN, each set arranged per Figure B.310.2 Detail 3, 90 RHO, then per Table B.310.7 each of the conductors is good for 273 amps.

But if you cannot match a figure in the NEC, then you have to turn to other means.

The simple approach is to use the ampacities straight out of 310.16. But this is only good if the load is calculated per article 220. There is a good deal of conservatism built into that calculation process. Many AHJs will allow you to essentially apply some of that conservatism against the uncertainty of the impact of mutual heating. In other words, if the load calculation says you need 2200 amps, the load that you will really see once the facility is in operation is probably closer to half of that number. So you can use 6 sets of 500 MCM THHN (380 amps each, per 310.16), and say that their ampacity is 2280 amps. You can do this even if the conduits are in a ductbank, because the bad effects of mutual heating at 1100 amps of load will not be as bad as they would be if you really were putting 2280 amps through them.

HOWEVER, if the load is determined by any other means (such as measuring an existing facility to which you plan to add load), then 310.16 is not applicable to underground installations.

I have used several software packages that perform Neher McGrath calculations. The least expensive one that I have used comes from "calcware." I have found it to be very easy to use. Here is a link. AmpCalc
 
Re: Duct bank ampacity calculation

Thanks guys,

Charlie, I read some of your other, previous posts. Do you like Calcware, and can you customize it to any type of ductbank that you would like?

The ductbank that I stated previously is existing and we do not know the exact configuration. Would I be able to plug in any arrangement of a duct bank with spare conduits included?

Is the etap calculator better?

If my company decides to purchase one of these software titles, I want to make sure it can do everything we could need it to do.

Thank you again,

Jim
 
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.
 
Re: Duct bank ampacity calculation

I want to be careful to avoid a product endorsement. I can say that AmpCalc is far less expensive than ETAP, but ETAP does many more things. The "TA" in "ETAP" stands for "transient analysis." Another analysis program that includes ductbanks is offered by SKM. I have used all three.

AmpCalc can do nothing more than calculate ampacity for underground ductbanks. If that is the only type of power system analysis your company will ever do (i.e., no fault coordination, no load flow, no short circuit analysis, no transient analysis), then AmpCalc might answer your needs. It does let you model any configuration of conduits you like. Different sizes, different cable sizes in each conduit, and spare conduits are easy to model.
 
Re: Duct bank ampacity calculation

Good point, Charlie. Amp Calc is a single use product. The other software programs do a lot more, as well as UG cable ampacity. (Etap, SKM, Captor/Dapper, EDSA, plus others.)

All of them are good products. In our particular situation it worked out better to mix and match software instead of upgrading some older versions. It had more to do with our budget, existing license agreements, anticipated number of users for each program and our projected near term work load than with the relative technical merits or costs of the programs.


Bob Wilson
 
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