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Medium Voltage Ductbanks

Medium Voltage Ductbanks

Medium Voltage Ductbanks

I am looking for some specific information on the design of underground duct banks.  The information should cover multiple ductbanks running parallel to each other, as well as what percentage of the total duct bank, in a parallel situation, does the spacing need to be maintained.

For example: a ductbank calculation shows that to maintain a 400 amp load on 5KV cables the 2 ductbanks be maintained at a 36" spacing.  If you need to transition from an underground system to an overhead system how do you handle the 36" spacing coming out of the ground and onto a stanchion while maintaining a cost effective installation?
Does 100% of the ductbank need to be maintained at the 36" spacing or is there latitude for transitioning and other purposes?

RE: Medium Voltage Ductbanks

BJC has some good points here.  The ampacity is highly dependent upon the thermal resistivity of the concrete or backfill used to form the duct bank, but usually your limiting factor is the riser (where you transition from UG to OH) since the air temperature in the riser conduit can be significantly higher than what you will find in the duct bank.
The thermal resistivity values and ambient temperatures upon which the NEC's ampacities are based should be included in the tables that BJC is referencing.  I don't have a copy of the NEC near me right now, so I can't quote table numbers, but it should be possible to find them if you're looking for them.

RE: Medium Voltage Ductbanks

1. The previous posting responsibly refers to the NFPA 70-1999 National Electrical Code. The following is useful:
a. 310-60. Conductors Rated 2001 to 35,000 Volts.
b. Figure 310-60 Cable installation dimensions for used with Tables 310-77 through 310-86
Note 1 indicates that the maximum depth to the top of electrical duct banks shall be 30 inches. The buried two circuits have a 24 inch spacing. This spacing can suffice for electrical duct banks since they have some air circulation around conductors in the conductor electrical duct bank conduits.
2.  Fink and Beaty "Standard Handbook for Electrical Engineers" 14th Ed., McGraw-Hill, 2000 includes a good section 14.2 "Underground Power Transmission" with many useful References.
3. Electric Power Research Institute (EPRI) Volume 4 "Wire and Cable" is also informative with many useful References.
4. For ampacities see
IEEE 835-1994 (IPCEA Pub. No. P-46-426) Standard  Power Cable Ampacity Tables. Also, NEMA stds could be used.

RE: Medium Voltage Ductbanks

1.- OKONITE is good place to start with ampacity values for "typical" configurations. Typical meaning a common set of values.
2.- In addition you should use the IEEE books Volume 1 & 2 ( Copper and Aluminum ) 70's vintage and/or the new book by Anders ( find it in the IEEE Online bookstore ).
3.- Over the years IEC 287 Standard has proven a better solution for me, as it basically gives you the main formula to calculate ampcity and then works with each module that makes up the equation. The modules make up for the configuration and installation details selected along the way.
4.-The NEC requires that if the U/G cable lenght is > than the O/H lenght by 25' the ampcity for the U/G prevails.
5.- Please remember that a cable installed U/G has a lower ampacity availability than the same cable installed overhead. This is due to the fact that in order for the cable to dissipate it's heat it will have a series of "thermodynamic resistances " to overcome:

a) All resistance units part of the components that make up a cable ( insulation,shield,armour,jacket,etc )
b) Resistance of the installation components ( concrete,conduit,air in conduit,fill,sorrounding fill).
c) Installation components such as the depth on the duct bank,the deeper the worst dissipation model.

Several techniques are use to improve ampcities:

a) Separation between circuits.
b) Leaving empty ducts between circuits.
c) Using the outer conduits always for circuit installations.
d)The installation of chemically treated sands in lieu of air for all conduits that hold current carrying conduits.
e) Cooling water pipes.
f) In Europe the use of direct buried cables is very popular.

RE: Medium Voltage Ductbanks

Do you have any references or examples on using the forced air and water circulation for cooling duct banks?  
I keep running into situations where clients want to use 2500 KVA xfmrs and run them at 133% ( when one side of a double eneded sub is down). They think bus duct is to ugly and whine a lot when it's the only way to go. A duct bank for 4000 amps is not practical.
I have considered using empty conduits and forcing air through them using a fan.  The fan would be controlled using current sensors,  I would use flow switches and temperature sensors on the output air to assure things are working OK.
Any thoughts?

RE: Medium Voltage Ductbanks

The separation and configuration of a duct bank is permitted to be different under engineering supervision than the one suggested in the NEC.

Using a good software package rather than hand or table from NEC let the engineer use recognised methods such as the Neher-McGrath calculations to size the cable more efficient. Please consider in your application similar to the following software:


For transition from UG to please consider the following statement from the NEC 310-15

"Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables or under engineering supervision......

Exception No. 4: Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit, or rigid nonmetallic conduit having a length not exceeding 10 ft (3.05 m) and the number of conductors does not exceed four.

RE: Medium Voltage Ductbanks

Do any of you have or know of software or spreadsheets in the low cost or free price range for computing ductbank cable capacities?  Calcware has been mentioned above, any other candidates out there?   Your user comments would be appreciated on Calcware and any other programs that you have experience with.


Harry Wynne

RE: Medium Voltage Ductbanks

I suppose the cost of software packages is in the eye of the beholder, but there are costjustifications that can be made.  
CYME (www.cyme.com) makes a package called CymCap for UG cable ampacity that has some features that I like and some that I feel need some work.  It has a price tag that starts in the $7,000-$9,000 (the company I used to work for had Cymcap) with a yearly maintenance contract cost of about 10% of the original purchase price.

This thread is the first time I have seen the calcware product, so I don't know much about that one.

Underground Systems, Inc. (USI) has a package that has just recently been upgraded to a Windows app, and it looks pretty good to me.  Their price is comparable to Cyme's.  They also told me a few months ago that they were not planning on charging customers any maintenance fees.  I may have to get back with you on the contact information for USI (I have recently changed jobs, and all of my references are in storage).  If we're lucky, someone out there has the information and can post it in this thread.

The best thing I can suggest is to get ahold of some of these companies and ask for literature (or maybe even a demo version of the product).
  Good Luck!

RE: Medium Voltage Ductbanks

USI HQ is located in Armonk NY, phone #: 914-273-8727, or usi-power.com

RE: Medium Voltage Ductbanks

I have also used CYMCAP.  I am continually frustrated by CYME charging $1000's for thinly disguised DOS programs with marginal Windows interfaces and no finite-element analysis.  If they could at least step into the 1990's I would feel better.

CYME is a Canadian company, yet you will go crazy trying to model a TECK cable (used by virtually all industrials in Canada) in CYMCAP.  The cable library that comes with the program is crap.  Editing cable types is a nightmare.

Sorry to rant about CYME, but they don't impress me.  I have used CYMCAP, CYMTCC, and CYMGND, and they all offer far less than what you would expect for the big dollars you pay.  Hopefully the USI software is better.

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