MV Ampacaties in a 8 circuit ductbank

[RKyleM]

NEC Table 310.60.c.77 lists ampacities of MV copper conductors in an electrical ductbank in accords with figure 310.60. i am working on a project where 9 circuits will be used in a 3x3 square ductbank arrangement. i don't see ann NEC table for this?

is there a similar NEC table or publication address addressing more than a 6 circuit ductbank? is there a linear degradetion of ampacity between the 6 and 9 circuit ductbank the same as between a 3 and a 6?

 

[cuky2000]

Significant derating factor is expected for this application. For non standard NEC ductbank arrangement, the Neher-McGrath method could be used as an alternative to calculate the allowable cable ampacity in compliance with the NEC NEC 310.15(C) allow per code as "Engineering Supervision Calcs".

Your could try hand calc. or using homemade application template with Excel, MathCad or similar tools. However, there are commercially available programs such as AmpCalc that could help to facilitate this task (see link enclosed).

http://www.calcware.com/cwnmcalc2.htm

Other option is to find a qualify engineer or firm that have the knowlege and software available to provide this service.

 

[7anoter4]

First of all I agree with cuky2000 an approved software based on Neher-McGrath method [or IEC 60287] is recommended.
If the cable shield is grounded in one point only, for cable up to 8 kV you may use the same NEC article as for up to 2 kV. Usually for cables of 500-1000 mcm if you want to use 6 ducts per bank to calculate the maximum permitted current in 9 ducts bank you have to reduce the current by 12.5-13.5%. See:

http://www.calcware.com/NEC Tables/NEC B310-5.pdf

 

[7anoter4]

Corrections:
1) The link has to be:

http://www.calcware.com/NEC Tables/NEC 310-77.pdf

2) Actually you have to divide the ampacity for 500 mcm by 1.16 and for 1000 mcm by 1.115
that means only 10-13.8% reduction.
3) Shield grounded in one end only
For more information see:

http://www.calcware.com/NEC Table Verifications.htm

 

[jghrist]

Cables in a 3x3 ductbank with all ducts filled will have significantly lower ampacity because one of the cables is completely surrounded by other cables. The center cable will get a lot of heating from the surrounding cables.

 

[7anoter4]

I agree with you, jghrist. In the case of 3*1/c 500 MCM 8kV [100% insulation] in each of 3*2 PVC ducts RHO=90 and 3 ft depth of duct bank top, for I=290 A [as per NEC 310.60(c)77] the temperatures of conductor per duct are[in Celsius,of course]:
83 86 83
84 88 84
In 3*3 duct bank for I=250 A the temperature per duct will be as follows:
80 83 80
85 88 85
83 86 83
Then 290/250=1.16 or 250/290=86.2%
Remark:
This is not an actual situation as none could be sure the RHO=90[may be 80 or 100?], also the load is never the same in all duct and more: in an actual case in every duct could be a different cable type. The calculation is done for the reference conditions and that is theoretical only.

 

[BJC]

A 3x3 duct is useless. Adding the center duct and conductor deminishes the ampacity of the whole.
I need 4,000 amp through a ductbank on a job. Proposed using a 3x3 using 8 of the 9. The 8 outer were PVC the center steel. I had a mechanical engineer help with a system that blew air through the center duct. With the air in the center duct at or near ambient I reasoned Detail 2 in Figure B.310.2 was applicable. That and table B.310.7 got it to almost 4,000 amps. That and Figure B.310.1 got it up to a LF of .85.
The fan system had a filter, a flow switch and a temperature switch at the air outlet. The fan start was loss of one end of a main-tie-main switchgear lineup.
It never had to work. It was complicated enough to convince the project manager that there is a practical limit to duct banks.
We switched to Precast trench with cable tray. That worked cheaper, faster and better.