Over here, I've just done few rural jobs with customers having already run sub-mains to out-buildings, before I get there. And they generally just want then hooked up at each end, so to speak. But, this is sort of got me worried, because 9 times out of 10, the underground cable(normally 2.5 or 4.0mm 2C+E)is run through a length of black alkathene water pipe(it's used for irrigation systems on farms)and buried and the trench back-filled. What worries me is that, some where I heard that alkathene is known to react with the chemical composition of PVC. In some cases I have even asked that the cable be dug up and run in PVC conduit and this does not normally go down too well!. Have any of you guys ever heard of this chemical problem?.
Trumpy, I've never heard of a chemical reaction between PVC sheathed cable & alkathene pipe. The method you describe is very common here, where people want to 'save on the expense of steel wire armoured cable' buried direct. It does seem to be preferable to electrical conduit, as alkathene pipe is much heavier gauge (better mechanical protection) & available in longer lengths. In the UK plastic conduit is sold in 3Mtr lengths, so would require jointing underground.
Could the chemical reaction you are thinking of be between polystyrene & PVC sheathed cables. That is very well known in this country. Plasticiser migrates from the PVC to the polystyrene & can result in the PVC becoming less flexible & sticky on the surface. I have seen a lot of examples of this in installations carried out in the late 70's, early 80's, where expanded polystyrene sheets were used to insulate the walls. You pull out a cable to do an alteration & it comes out of the wall sticky & soft on the sheath. Nowadays we have to enclose cables in UPVC conduit(which is unaffected by contact with polystyrene) where polystyrene insulation is used. However there are still thousands of buildings contructed before it was widely known about the PVC/polystyrene reaction, that could be a time bomb waiting to happen.
On the subject of supplies to outbuildings, I usually install steel wire armoured cable (sometimes in an alkathene duct) as it is not really expensive, & can take a lot of abuse. Do you not use SWA for underground supplies in NZ?
Generically, Alkathene appears to be low-density polyethylene tubing. [In the US, polypropylene in coils is a common material for water lines.] Polyethylene is quite resistant to a lot of organic-chemical reactivity. Other than satisfying Code requirements, I don’t think it would be a big issue for use with PVC-jacketed cable. It may be wise not to reduce the buried-raceway depth from that mandated for directly-buried cable. If splices and terminations for Alkathene are of smaller than that of the tubing itself, fill calculations should be based on the fitting inside diameter. http://www.southwire.com/wc/catalog/sec19/19-1.pdf and similar to 1999 NEC Article 343 — Nonmetallic Underground Conduit with Conductors
David, I was aware of the PVC/Polystyrene interaction. A new sort of TPS cable has just been released over here that has been especially formulated for contact with Polystyrene foam. SWA cable never really caught on over here, if you do want to use it for anything, it is horrendously expensive. We use concentric wound Neutral-Screened PVC or XLPE cables for underground services here, with XLPE being used exclusively above 3.3kV. Paul, I've seen the garden hose installation and the direct burial too!. One guy even laid all the wires under the concrete slab in his house and just threw the concrete over the top. This was all well and good, until the plumber went to drill the holes to bolt down the toilet pan.
Re: Alkathene+PVC#137076 05/31/0305:47 AM05/31/0305:47 AM
Paul, I'm not entirely sure what we would pay for SWA over here, but I do know that it would be a lot more than $82 for a 50m drum of 2.5mm 2C. Add to that, the frieght costs, as it would probably have to be transported from Auckland, if they even have it in stock there.