Posted by david - OR on August 02, 2013 at 09:04:17 from (208.67.204.213):
In Reply to: Tri-plex posted by TGIN on August 02, 2013 at 03:11:33:
This is unlikely to meet code because overhead triplex is not listed for use in raceways (conduit). If you plan to get the work inspected, you need to ask the inspector first.
Overhead triplex uses a bare "messenger wire" (ACSR) which is a multi-stranded aluminum conductor wrapped around a single steel strand. The steel strengthens the messenger wire so that it can support the entire cable assembly. Running it in conduit presents several practical problems, whether you care about inspections or not. The most immediate practical one is that the cable assembly is bulky and stiff. You'll need a large diameter conduit to be able to pull the wire through it, especially with a 4th neutral conductor added. You are going to have to make at least two 90 degree bends to get from the service entrance to the sub panel. This is going to require some long gentle sweeps to pull such stiff wire. There may not be room in the attic to get the sweeps in place. Getting out of the service entrance and up the wall won't be fun either. You really don't want to make the turns with junction boxes and splices, as aluminum wire is demanding about keeping oxidation out, and every splice is one more hidden fire hazard in your house.
The ampacity of the wire is based on free air at a reasonable temperature (40 degrees C), not a hot attic in a raceway. You can be conservative about the actual ampacity and get around this.
The wire insulation is thinner than typical indoor wire, and may not be able to stand up to the friction of being pulled through conduit. This would be harder to get around.
An 80 foot run of PVC conduit in an attic is going to need an thermal expansion joint of some sort.
Because you will need to buy the conduit, and supply the 4th wire, the savings aren't as big as you might have first thought, even though the wire is "free".
The installation you propose is so much more trouble than using SER cable without conduit that the savings don't seem worth it.
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