PVC VS. RMC

[mmt019]

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?

 

[umrpwr]

In response to your question regarding the use of PVC verses GRC elbows. I would at a minimum recommend using GRC elbows. Having experience in both engineering and contracting I am sure the contractor would like to use PVC elbows. There is a significant cost savings. Even if you were to get a credit back for the difference I would still not recommend using PVC elbows. They just don't stand up to pulling stresses put on them depending on what types of cables you are installing. I have seen PVC elbows shreaded from having cables pulled in. GRC elbows for PVC runs is a standard practice that I have seen in most electrical specifications. It is not a case of over engineering but rather just a matter of good installation and construction practices.

 

[BJC]

If your damaging your cable or PVC elbows your problem is in your cable pulling practice and/or duct bank design.
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&quot 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.

 

[dpc]

Just a reminder that steel elbows should not be used if single conductor cable is run with one phase per conduit. This can result in severe heating in the steel elbow due to induced current and subsequent cable damage.

Utilities typically run one phase per conduit in distribution circuits. This is not normally seen in industrial applications.

 

[umrpwr]

In response to BJC I was not trying to sugguest that steel elbows are the solution for poor cable pulling practice or duct bank design. Both of those are critical factors in installing cables without experiencing any damage. Other considerations might include using long sweep 90s to reduce stress, using lesser degree bends (ie minimize the degree of bend) and limiting runs to less than 270 degrees of bend. This is probably particularly true of 15kV cable. If there any doubt it is certainly wise to do cable pulling calculations. Pull location and set-up can are also important factors in eliminating damage.

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?

 

[davidbeach]

PVC can be used to get longer radius bends, and use of steel rope, rather than nylon, will actually reduce the likelihood of burn-through on the elbows. Isolated pieces of steel in an otherwise PVC conduit can create bonding issues, and as pointed out absolutely can not be used in one phase per conduit installations.

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.

 

[BJC]

steel cable and some abrasive grit make a great rock cutting system.
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.

 

[jbartos]

Suggestion: If the conduit installation uses the maximum bend angle allowed and maximum cable tension pull force, it is better to use rigid conduit elbows. The above posted experiences are easily reconstructable. If the installation uses less of bend angles, e.g. 180deg only and reasonably long pulls only, then there is a good chance that the sidewall stress within the PVC elbows will be acceptable and harmless. The answer is: It depends on the conduit installation geometry and pull box spacings. Small cable pull tensions will work in favor of PVC elbows and large cable pull tensions will work in favor of RGS elbows.