design requirements for trench with low voltage and medium voltage
design requirements for trench with low voltage and medium voltage
These specifications are not listed in the NEC book to my knowledge, however, this may help. We can run medium voltage cables or any other voltages in pull boxes just as long as the voltages are labeled and the pull box is separated between one voltage and the other.
I do NOT recommend that high voltage, medium voltage and low voltages of any kind are installed into any metallic conduit together, or in any metallic raceway because these voltages at just a few amps have massive amounts of wattage capabilities and they will absolutely without a doubt disintegrate any sized bonding wire that would bond the pipes, or pull boxes, rendering the now non grounded enclosure the ability to become live and lethal if the high voltage, or medium voltage cleared itself from ground but did not blow the fuse and remained energized. This would mean that the box is now live and the low voltage would be connected to the high voltage feeds inadvertently.
This may sound impossible, but I did watch this happen at an old plant in CT where cables were run in the same metallic box as the secondaries. The bonds got blown off because they used #6 solid grounds to bond the bonding bushings, which disintegrated and
was not enough to take out the fuse. The circuit was an open delta on the high voltage side. You could ground one phase completely and it will remain energized on the other phases at full rated voltage. If a partial short to ground happens, the open delta will just burn the circuit clear if the fusing is large enough.
High voltages are not to be played with and the NEC should eliminate any means of conductive duct banks and boxes sharing the different voltages. The NEC should force raceways, ducts, and LB's to be plastic in the event duct banks need to be shared, or vice versa.
I do know there are limitations in distance from other things, such as direct buried MV cables and communications, gas, and other flammables, but I am uncertain of this myself.. I didn't see any part of the new NEC book that has this and it should be specified. MV is way too dangerous if it doesn't have protective measures such as high speed relays controliing them, etc.
Given medium voltage and utilization voltage, all in dedicated conduits, underground in a trench, are there any design requirements of which I should be aware?
Such as:
1. don't even think about it. Better off with dedicated trenches.
2. make sure you separate the conduits by X inches.
3. make sure to bury the medium voltage to a greater depth (which I'm already familiar with)
