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thin wires in substrate unsafe

     Recently George gave us some info on how to build your own undergravel 
     heating system including a posting from Uwe Behle which described 
     using (copper?) wires of very small diameter. The following table was 
     quoted:
     
      AWG  Dia/mm  Ohms/m  Ohms/1000ft
       26  0.405   0.136   41.62        <--- 6V
       27  0.361   0.172   52.48
       28  0.321   0.217   66.17
       29  0.286   0.274   83.44      \
       30  0.255   0.345   105.2      / ++++++ 9V
       31  0.227   0.435   132.7
       32  0.202   0.548   167.3        <---- 12V
       33  0.180   0.692   211
       34  0.160   0.873   266
     
     I would like to point out that the insulation on this small guage 
     wires is probably very thin and could easily be punctured by a sharp 
     or heavy stone. As an alternative I would like to suggest that people 
     interested in DIYS cables consider a heavier guage wire of a higher 
     resistance metal with correspondingly thicker insulation. I believe 
     someone had found a source for some type of teflon coated wires 
     previously mentioned on the APD. I don't know how thick the insulation 
     was on these wires. Another alternative would be to use longer and 
     heavier copper wires, coil them tightly around something like plastic 
     cladded clothesline wire and then wrap the whole thing with electrical 
     or duct tape to provide a mechanical protective barrier. The extra 
     insulation will increase the thermal resistance somewhat however the 
     actual heat output of the cable will remain exactly the same. A 
     concern would be the glue on the inside surface of the tape dissolving 
     into the water but I don't know if this is a cause for concern. 
     Another alternative would be to put the coil wrapped clothesline wire 
     inside PVC tubing. PVC tubing has a higher thermal resistance because 
     of its thickness but from a mechanical standpoint, it would be the 
     best. At the low heat outputs (about 4.7 watts per meter at 12 volts) 
     in the example, the internal temperatures would not be excessive.
     
     George, what is the wattage per meter of Dupla cables? Isn't this the 
     most important parameter for estimating the heat values required for a 
     given circulation (temperature gradient) rather than overall wattage 
     based upon tank volume as suggested by Uwe? Thicker hotter cables will 
     generate a higher temperature _gradient_ than a mass of very thin wire 
     distributed uniformly throughout the gravel.
     
     FWIW, I'm not a heating cable advocate but most of you probably 
     already know that. :-)
     
     Steve