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smokey1561 (Electrical) (OP)
26 Sep 05 16:25
Does anyone know of a code or standard that applies to installing electrical wire & cable in a concrete trench?  The trench was saw cut into a floor and the owner wants to just lay the cable and wire into the bottom of the cut-out and cover it with steel plate.  There is some 480vac and 120 control power in there.  I don't like it but don't see a code or standard to back up my opinion.  I didn't see anything in NEC but sometimes that's like trying to find a needle in a haystack.
dpc (Electrical)
26 Sep 05 17:32
I don't think you will find anything specific in the NEC.  I went through this many years ago and eventually we characterized the trench as a "wireway" to satisfy the local inspector.  This subjects you to a lot of restrictions if running power circuit in the trench.

These folks make a pre-cast concrete trench system, so they may have looked into this in more detail:  www.trenwa.com
alehman (Electrical)
26 Sep 05 20:58
I've done that as well. All wiring that's not separated by metallic barriers (raceway, shield, or other physical separation) will need to be rated for the highest voltage in use. If the 480V cables are not in conduit or armor shield, the 120V wiring will need to have insulation rated to 480V.

If the runs are very long, you may be well to do that anyway to reduce interference with the control circuits.
rcwilson (Electrical)
27 Sep 05 16:43
We've done it in industrial buildings and were stopped by one local inspector. He accepted it when we installed tray in the bottom of the trench.  The tray also lifted the cables off the junk in the bottom of the trench and allowed the trench to drain.

Cable trenches can interfere with the design of a floor drain system, especially if trench drains are used.  The floor should slope away from the cable trench and the trench should have some means for draining any collected water.
BJC (Electrical)
27 Sep 05 18:23
I haven gotten inspectors to accept it as " continous ridig cable support". Which it is in a way.  
I have also installed tray in trench and allowed to use the ampacity table for cable in tray.  
If you install a 2500 KVA transformer at 480 volts a combination of trench and tray works fairly well.  It almost impossible to run 4000 amps ( 133% overload ) in a duct bank.
davidbeach (Electrical)
27 Sep 05 18:54
BJC, careful with tray ampacities in trenches.  The tray ampacity is based on free flow of air around the tray and cables, the trench probably won't provide nearly as much air circulation.

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