Power Feeder

[speclt]

I have a dispute going with our design engineer over the location and feeder sizing for a new 6.9 kV 1200 amp switchgear. The gear will be feeding 10 loads which will run almost continuously and represent 80% of the bus sizing. He wants to locate the gear in the main power distribution center and I want to locate the gear near the loads.

The rub comes when we calculate the feeders size. We are both using 500mcm XHHW2 in a ductbank. I've derated the conductors by 40% and say we should be installing 5 - 500mcm cables per phase. He says we should derate the conductors by 80% and should be using 14 - 500 mcm cables per phase. Who’s right?
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[dpc]

The ampacity will depend on the duct bank configuration, insulation temperature rating (and whether it is copper or aluminum )

For a feeder this large, it would be advisable to run detailed ampacity calculations using the Neher-McGrath method. This will take into account the duct bank configuration, conduit material, thermal characteristics of the concrete and/or soil, etc.

If you don't have the means of doing this calculation, you can probably get one of the large cable manufacturers to do it for you. I've gotten Okonite to run these calcs in the past at no cost.

 

[ScottyUK]

If you locate the switchgear AND the distribution transformer associated with the switchgear near the loads you could be saving serious money on the cable installation alone. Depends how far away the two alternatives from each other to a large extent: a duct bank of that size is a serious undertaking when compared to a single cable a couple of voltage levels up. Obviously the availability of HV switchgear at the source of the supply is also a major concern.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[speclt]

dpc - The ductbank configuration will be will be a 4 tube 5" PVC run with 3' of fill over the ductbank. The 4th tube is a spare. I'm having a geotech run on the ground in the area of the ductbank for thermo conductivity. There are various natural thermo conductive materials available here, one of which I currently using for the relocation of two 138 kV and a 69 kV circuits. I had planned to backfill the ductbank with the same material.

ScottyUK – I’ll be feeding the new switchgear from an existing 6.9kV switchgear. There will not be a transformer in the circuit. The cable distance between the two switchgear units is approximately 600’ and the cost in wire could be considerable. Especially at today’s copper prices.