MV Ampacaties in a 8 circuit ductbank
MV Ampacaties in a 8 circuit ductbank
(OP)
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?
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?






RE: MV Ampacaties in a 8 circuit ductbank
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.
RE: MV Ampacaties in a 8 circuit ductbank
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:
htt
RE: MV Ampacaties in a 8 circuit ductbank
1) The link has to be:
htt
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:
RE: MV Ampacaties in a 8 circuit ductbank
RE: MV Ampacaties in a 8 circuit ductbank
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.
RE: MV Ampacaties in a 8 circuit ductbank
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.