PVC VS. RMC
PVC VS. RMC
(OP)
I am installing 15kV cable (duct bank consisting of 5 inch conduits encased in concrete) and pad-mounted Cooper oil-switches. My specs call for using PVC schedule 40 except at elbows (conduits that will stub up into pad-switch) and when entering into a building. At these locations, I called for Galvanized Rigid Metal Conduit (RMC). My contractor suggest using PVC at elbows but I am concerned about stress at the elbows so I am requiring RMC. What would you use at the elbows. RMC or PVC?






RE: PVC VS. RMC
RE: PVC VS. RMC
I have heard the argument for GRS elbows for years. The most vocal advocates for steel elbows are the same people who use steel cable or some other poor installation practice.
If your shredding PVC elbows you have a basic problem with the system. You may hide it with a steel elbow ( "There wern't any conduit parts on the pull rope so it must be OK") but the bad design/pull practice is still there and you can damage cable.
Pull ropes are important. You want a large diameter "zero" stretch line.
I have seen people use "aircraft cable" (nylon coated steel) cable thinking that the nylon will protect the duct system. I also have some pictures of steel 90s sliced neatly open with steel cable. Small diameter hard polyethelyne rope will also cut into steel ( and cuts very well into PVC).
A pull rope should have linear fibers so as not to twist you cable. Any twisted rope will undwind under tension and twist your cable as it does.
RE: PVC VS. RMC
Utilities typically run one phase per conduit in distribution circuits. This is not normally seen in industrial applications.
RE: PVC VS. RMC
DPC also makes a very good point about whether the circuit is one phase per conduit.
If DPC's comment is not a consideration though I would still use the GRC elbows. Unless you are the one that is going to be pulling the cables I would prefer having the added duarability. What was the contractors rational for wanting to use PVC elbows? Economic or something else?
RE: PVC VS. RMC
Our standard spec is steel elbows in PVC runs, but where it makes for a better installation, I don't hesitate to change to all PVC.
RE: PVC VS. RMC
The real problem with steel or any pull line that stretchs is stored energy. A long pull with an elastic rope is dangerous. When the rope breaks or the pulling eye comes undone, bad thinks can happen. Not only can they happen they have. There are plenty of documented cases of people beingg injured or killed by snapping ropes and cables.
The other scenario is where the guy at the feed end of a pull gets his hand or foot caught in a coil or wedged in a duct yells on the radio "stop". With an elastic rope the pull end doesn't stop right away. There is at least one one handed electrican resulting from this kind of accident.
If your on a job where you see this do your best to get it stopped. Most people understand stored energy and the potential danger.
RE: PVC VS. RMC